Discussion about the Korg PolySix synthesizer

Hi Folks,

Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.

Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:

http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html

Am I getting it right? What is this thing actually doing? Is it just a noise gate?

I'd love your thoughts.

Chip

Looking at the LM13600 data sheet, down through all the example circuits, this use of the LM13600 does NOT match the circuit shape of a VCA. The negative feedback and the capacitor between the OTA and the darlignton pair actually make this look like a voltage-controlled low-pass filter.

The app notes say that each stage of the LM13600 configured as a VCF generates a -6 dB/octave filter. So, in the Polysix, the two stages cascaded together would give a -12 dB/octave filter.

Looking at the values for the resistors and capacitors, and knowing the control currect (Iabc), one can calculate the cutoff frequency of the filter. The circuit values are clear from the schematic. It's Iabc that's trickier.

For Iabc, there's a tiny barely-legible note in the schematic shows that the IE is 1.2 mA, which is then knocked down to 1/3*IE (which is 0.4 mA) which is then divided between the two halves of the LM13600. As a result, the maximum (?) control current to each 13600 is 0.2 mA. Running this through the F_cutoff equation in the LM13600 datasheet, you get:

Fc = (Ra*gm)/((R+Ra)*2*pi*C) where gm = 19.2*Iabc
Fc = (220*19.2*0.2e-3)/((10E3+200)*2*pi*470e-12)
Fc = 28 kHz

This suggests that if the Polysix really drives this LM13600 to the 0.2 mA level suggested by the note in the schematic, that this element will open up to just beyond the range of human hearing (20 kHz). That's pretty reasonable.

With less current drive, the 13600-based VCF will close down, cutting out the high frequencies (often noisy and hissy) first. Since the current drive appears to be based on the magnitude of the input audio signal, it suggests that the filter opens when the input is strong (loud) and closes when weak (quiet).

Is this the behavior that the designers intended? Is this the right interpretation for this portion of the "Compressor/Expander" circuit? If so, I'm still troubled by the fact that the current drive circuit (the level detection circuits) seem to be railed at +15V. How are they doing their job?

Thoughts?

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Hi Folks,
>
> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
>
> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
>
> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
>
> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
>
> I'd love your thoughts.
>
> Chip
>

The circuit is a dBx-style noise shaper: the more amplitude the dry
signal provides, the wider the response of the filter. This squelches
MN3005 noise at low levels and allows more high frequency content at
higher levels.

The 2SA798 is a standard V/I converter that is "locked on" (filter
wide open) if the "OFF" signal voltage appears.

The timbre change is of course due to the VCF changing vs. amplitude.

Crow
/**/

On 2/25/2013 11:10 AM, chipaudette wrote:
> Hi Folks,
>
> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
>
> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
>
> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
>
> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
>
> I'd love your thoughts.
>
> Chip
>

With respect to my interpretation that the input stage to the gain control is railed at 15V...

Well, by examining the actual circuit that's in my Polysix, I just found that R107 is actually 4.7M, not the 4.7K value shown in the schematic. That factor of 1000 makes a pretty big difference!

Now, that first gain stage won't rail at 15V -- the circuit operates without going into saturation. The half-wave rectifier and low-pass filter that follow now at least have a chance of doing something useful.

Looking at the output of that first stage (Pin 7) with no key pressed shows a DC value of ~1V. This is also the value predicted by doing a DC analysis on this op-amp if R107 is 4.7M instead of 4.7K. Good.

It makes more sense now!

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
>
> Looking at the LM13600 data sheet, down through all the example circuits, this use of the LM13600 does NOT match the circuit shape of a VCA. The negative feedback and the capacitor between the OTA and the darlignton pair actually make this look like a voltage-controlled low-pass filter.
>
> The app notes say that each stage of the LM13600 configured as a VCF generates a -6 dB/octave filter. So, in the Polysix, the two stages cascaded together would give a -12 dB/octave filter.
>
> Looking at the values for the resistors and capacitors, and knowing the control currect (Iabc), one can calculate the cutoff frequency of the filter. The circuit values are clear from the schematic. It's Iabc that's trickier.
>
> For Iabc, there's a tiny barely-legible note in the schematic shows that the IE is 1.2 mA, which is then knocked down to 1/3*IE (which is 0.4 mA) which is then divided between the two halves of the LM13600. As a result, the maximum (?) control current to each 13600 is 0.2 mA. Running this through the F_cutoff equation in the LM13600 datasheet, you get:
>
> Fc = (Ra*gm)/((R+Ra)*2*pi*C) where gm = 19.2*Iabc
> Fc = (220*19.2*0.2e-3)/((10E3+200)*2*pi*470e-12)
> Fc = 28 kHz
>
> This suggests that if the Polysix really drives this LM13600 to the 0.2 mA level suggested by the note in the schematic, that this element will open up to just beyond the range of human hearing (20 kHz). That's pretty reasonable.
>
> With less current drive, the 13600-based VCF will close down, cutting out the high frequencies (often noisy and hissy) first. Since the current drive appears to be based on the magnitude of the input audio signal, it suggests that the filter opens when the input is strong (loud) and closes when weak (quiet).
>
> Is this the behavior that the designers intended? Is this the right interpretation for this portion of the "Compressor/Expander" circuit? If so, I'm still troubled by the fact that the current drive circuit (the level detection circuits) seem to be railed at +15V. How are they doing their job?
>
> Thoughts?
>
> Chip
>
>
>
> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >
> > Hi Folks,
> >
> > Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> >
> > Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> >
> > http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> >
> > Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> >
> > I'd love your thoughts.
> >
> > Chip
> >
>

Thanks for the added insight!

Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.

I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.

Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.

If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?

Chip

--- In PolySix@yahoogroups.com, The Old Crow <oldcrow@...> wrote:
>
> The circuit is a dBx-style noise shaper: the more amplitude the dry
> signal provides, the wider the response of the filter. This squelches
> MN3005 noise at low levels and allows more high frequency content at
> higher levels.
>
> The 2SA798 is a standard V/I converter that is "locked on" (filter
> wide open) if the "OFF" signal voltage appears.
>
> The timbre change is of course due to the VCF changing vs. amplitude.
>
> Crow
> /**/
>
> On 2/25/2013 11:10 AM, chipaudette wrote:
> > Hi Folks,
> >
> > Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> >
> > Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> >
> > http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> >
> > Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> >
> > I'd love your thoughts.
> >
> > Chip
> >
>

Hi,

The part of the schematic you posted in you blog clearly is a voltage
controlled low pass filter, 2 stages, therefore 12dB/oct. They had
such circuits in the 80s in tape decks and other Hi-Fi equipment. It was
then called "Dynamic Noise Reduction" or similar.
The analog delay lines used for the effects introduce quite some noise
and additionally the clock frequency. Apparently it was necessary to
put in substantial amount of circuitry to make the noise acceptable.

Another measure for noise reduction is the compressor/expander system
built from NE571 (IC7, IC8). The NE571 consists of two identical
stage which can act, depending on the external circuitry, as compressor
or expander. The topmost circuit on the KLM-368 schematics (one half
of IC8) is the compressor. After that follows the input anti-aliasing
filter (Q13, Q25). Then the signal is split into three paths and enters
a BBD delay line for each phase. The BBDs are clock with a modulated
clock, which is built out of CMOS inverters used in linear amplfifier
mode (IC22). The modulating LFO is of the multivibraotr type and built
around IC18. DEpending on the type of effect the modulation depth,
frequency and audio paths are switched (4066 switch IC14). After going
through the torture of the BBDs the signals are again anti-alias
filtered (same type of filter, two transistors) and fed to the output
mixers and swsitches.
Then the sum signal goes through the discussed controlled low pass
filter and goes to KLM-369. OF course all this filtering and processing
will (potentionally) have a big impact on frequency response.
--
Thre three effect types Chorus, Phaser, and "Ensemble" result from
different delay times and different signal paths and modulations.
E.g. the third phase is only used in the "Ensemble" setting. It would
be interesting to hear how a flanger would sound, if they had provided
the possibilty for it. Has anybody on the list done that? The circuit
changes should not be too dramatically.
--
I always wondered why the Resonance control is part of this. Similar
to the output LPF there is a controlled (by the Resonance knob) LPF
at the input.

All in all the KLM-368 offers a lot analog processing circuitry and
some unusual circuits (e.g. the modulated oscillator made of 4069
inverters). If you look at the board size it is larger than the
KLM-367 board; and this is just for the effects. But this was a
distinguishing factor at that time.
Today this all just a piece of software...

Johannes

On 2013-02-26 18:02, chipaudette wrote:
> Thanks for the added insight!
>
> Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
>
> I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
>
> Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
>
> If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
>
> Chip
>
>
> --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@...> wrote:
>>
>> The circuit is a dBx-style noise shaper: the more amplitude the dry
>> signal provides, the wider the response of the filter. This squelches
>> MN3005 noise at low levels and allows more high frequency content at
>> higher levels.
>>
>> The 2SA798 is a standard V/I converter that is "locked on" (filter
>> wide open) if the "OFF" signal voltage appears.
>>
>> The timbre change is of course due to the VCF changing vs. amplitude.
>>
>> Crow
>> /**/
>>
>> On 2/25/2013 11:10 AM, chipaudette wrote:
>>> Hi Folks,
>>>
>>> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
>>>
>>> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
>>>
>>> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
>>>
>>> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
>>>
>>> I'd love your thoughts.
>>>
>>> Chip
>>>
>>
>
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>

just have to say I am quite amazed by the technical know-how of list members and time and care they spend in sharing it.

I salute you! :0)

On 27 Feb 2013, at 20:32, "Johannes Hausensteiner" <johau@gmx.net> wrote:

> Hi,
>
> The part of the schematic you posted in you blog clearly is a voltage
> controlled low pass filter, 2 stages, therefore 12dB/oct. They had
> such circuits in the 80s in tape decks and other Hi-Fi equipment. It was
> then called "Dynamic Noise Reduction" or similar.
> The analog delay lines used for the effects introduce quite some noise
> and additionally the clock frequency. Apparently it was necessary to
> put in substantial amount of circuitry to make the noise acceptable.
>
> Another measure for noise reduction is the compressor/expander system
> built from NE571 (IC7, IC8). The NE571 consists of two identical
> stage which can act, depending on the external circuitry, as compressor
> or expander. The topmost circuit on the KLM-368 schematics (one half
> of IC8) is the compressor. After that follows the input anti-aliasing
> filter (Q13, Q25). Then the signal is split into three paths and enters
> a BBD delay line for each phase. The BBDs are clock with a modulated
> clock, which is built out of CMOS inverters used in linear amplfifier
> mode (IC22). The modulating LFO is of the multivibraotr type and built
> around IC18. DEpending on the type of effect the modulation depth,
> frequency and audio paths are switched (4066 switch IC14). After going
> through the torture of the BBDs the signals are again anti-alias
> filtered (same type of filter, two transistors) and fed to the output
> mixers and swsitches.
> Then the sum signal goes through the discussed controlled low pass
> filter and goes to KLM-369. OF course all this filtering and processing
> will (potentionally) have a big impact on frequency response.
> --
> Thre three effect types Chorus, Phaser, and "Ensemble" result from
> different delay times and different signal paths and modulations.
> E.g. the third phase is only used in the "Ensemble" setting. It would
> be interesting to hear how a flanger would sound, if they had provided
> the possibilty for it. Has anybody on the list done that? The circuit
> changes should not be too dramatically.
> --
> I always wondered why the Resonance control is part of this. Similar
> to the output LPF there is a controlled (by the Resonance knob) LPF
> at the input.
>
> All in all the KLM-368 offers a lot analog processing circuitry and
> some unusual circuits (e.g. the modulated oscillator made of 4069
> inverters). If you look at the board size it is larger than the
> KLM-367 board; and this is just for the effects. But this was a
> distinguishing factor at that time.
> Today this all just a piece of software...
>
> Johannes
>
> On 2013-02-26 18:02, chipaudette wrote:
> > Thanks for the added insight!
> >
> > Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
> >
> > I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
> >
> > Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
> >
> > If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
> >
> > Chip
> >
> >
> > --- In PolySix@yahoogroups.com, The Old Crow wrote:
> >>
> >> The circuit is a dBx-style noise shaper: the more amplitude the dry
> >> signal provides, the wider the response of the filter. This squelches
> >> MN3005 noise at low levels and allows more high frequency content at
> >> higher levels.
> >>
> >> The 2SA798 is a standard V/I converter that is "locked on" (filter
> >> wide open) if the "OFF" signal voltage appears.
> >>
> >> The timbre change is of course due to the VCF changing vs. amplitude.
> >>
> >> Crow
> >> /**/
> >>
> >> On 2/25/2013 11:10 AM, chipaudette wrote:
> >>> Hi Folks,
> >>>
> >>> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> >>>
> >>> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> >>>
> >>> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> >>>
> >>> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> >>>
> >>> I'd love your thoughts.
> >>>
> >>> Chip
> >>>
> >>
> >
> >
> >
> >
> > ------------------------------------
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>


[Non-text portions of this message have been removed]

Hi All,

I measured the current control and frequency response of this LM13600 VCF. If you're interested in data and graphs, check out:

http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects.html

At the end of the post, I do a simple mod to force the filter to be way open. Man, is sure does make the sawtooth buzz. I wouldn't call it a sweet sound, but it sure is lively and invigorating!

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Thanks for the added insight!
>
> Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
>
> I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
>
> Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
>
> If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
>
> Chip
>
>
> --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@> wrote:
> >
> > The circuit is a dBx-style noise shaper: the more amplitude the dry
> > signal provides, the wider the response of the filter. This squelches
> > MN3005 noise at low levels and allows more high frequency content at
> > higher levels.
> >
> > The 2SA798 is a standard V/I converter that is "locked on" (filter
> > wide open) if the "OFF" signal voltage appears.
> >
> > The timbre change is of course due to the VCF changing vs. amplitude.
> >
> > Crow
> > /**/
> >
> > On 2/25/2013 11:10 AM, chipaudette wrote:
> > > Hi Folks,
> > >
> > > Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> > >
> > > Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> > >
> > > http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> > >
> > > Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> > >
> > > I'd love your thoughts.
> > >
> > > Chip
> > >
> >
>

Hi Johannes,

Thanks for the outstanding reply! I hadn't yet done much snooping into the effects circuits themselves (just the VCF following the effects)...since, most of the time, I play with the effects off. This information that you provided will be incredibly helpful as I dig into those parts.

As for the resonance-following VCF...I had seen that part of the circuit but, once again, I had assumed that it was a VCA instead of a VCF. I see that it is a one-stage VCF (6 dB per octave), so it is pretty gentle.

As a theory for *why* it's there, perhaps it is there to counter-balance the small boost to the very high frequencies that comes with the resistor-cap network at the input to IC21, which is at the end of KLM-368. At high resonance, when the filter is self-oscillating (squeeling!), I find that the Polysix often has the squeeling occur at very high frequency. Maybe the resonance-controlled VCF is there to tame the amplitude of the high-frequency squeeling prior to it being re-boosted by IC21.

Or perhaps there's some other good reason that I'm not seeing.


As for another one of your suggestions...I, too, am interested in a Flanger mod. Anyone?

And, to contribute my own mod ideas...I might decide that I don't want to keep the Post-Effects VCF (IC15) because I don't like how it cuts the sizzle off my sawtooth. So, I could just wire it so that it's permanently open (like I tried already with clip leads). Or, another option would be to wire it as a High-Pass Filter! Or, I could route other modulation signals to it and get some crazy FM or something! Has anyone tried anything here?

Chip

--- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@...> wrote:
>
> Hi,
>
> The part of the schematic you posted in you blog clearly is a voltage
> controlled low pass filter, 2 stages, therefore 12dB/oct. They had
> such circuits in the 80s in tape decks and other Hi-Fi equipment. It was
> then called "Dynamic Noise Reduction" or similar.
> The analog delay lines used for the effects introduce quite some noise
> and additionally the clock frequency. Apparently it was necessary to
> put in substantial amount of circuitry to make the noise acceptable.
>
> Another measure for noise reduction is the compressor/expander system
> built from NE571 (IC7, IC8). The NE571 consists of two identical
> stage which can act, depending on the external circuitry, as compressor
> or expander. The topmost circuit on the KLM-368 schematics (one half
> of IC8) is the compressor. After that follows the input anti-aliasing
> filter (Q13, Q25). Then the signal is split into three paths and enters
> a BBD delay line for each phase. The BBDs are clock with a modulated
> clock, which is built out of CMOS inverters used in linear amplfifier
> mode (IC22). The modulating LFO is of the multivibraotr type and built
> around IC18. DEpending on the type of effect the modulation depth,
> frequency and audio paths are switched (4066 switch IC14). After going
> through the torture of the BBDs the signals are again anti-alias
> filtered (same type of filter, two transistors) and fed to the output
> mixers and swsitches.
> Then the sum signal goes through the discussed controlled low pass
> filter and goes to KLM-369. OF course all this filtering and processing
> will (potentionally) have a big impact on frequency response.
> --
> Thre three effect types Chorus, Phaser, and "Ensemble" result from
> different delay times and different signal paths and modulations.
> E.g. the third phase is only used in the "Ensemble" setting. It would
> be interesting to hear how a flanger would sound, if they had provided
> the possibilty for it. Has anybody on the list done that? The circuit
> changes should not be too dramatically.
> --
> I always wondered why the Resonance control is part of this. Similar
> to the output LPF there is a controlled (by the Resonance knob) LPF
> at the input.
>
> All in all the KLM-368 offers a lot analog processing circuitry and
> some unusual circuits (e.g. the modulated oscillator made of 4069
> inverters). If you look at the board size it is larger than the
> KLM-367 board; and this is just for the effects. But this was a
> distinguishing factor at that time.
> Today this all just a piece of software...
>
> Johannes
>
>
>
> On 2013-02-26 18:02, chipaudette wrote:
> > Thanks for the added insight!
> >
> > Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
> >
> > I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
> >
> > Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
> >
> > If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
> >
> > Chip
> >
> >
> > --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@> wrote:
> >>
> >> The circuit is a dBx-style noise shaper: the more amplitude the dry
> >> signal provides, the wider the response of the filter. This squelches
> >> MN3005 noise at low levels and allows more high frequency content at
> >> higher levels.
> >>
> >> The 2SA798 is a standard V/I converter that is "locked on" (filter
> >> wide open) if the "OFF" signal voltage appears.
> >>
> >> The timbre change is of course due to the VCF changing vs. amplitude.
> >>
> >> Crow
> >> /**/
> >>
> >> On 2/25/2013 11:10 AM, chipaudette wrote:
> >>> Hi Folks,
> >>>
> >>> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> >>>
> >>> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> >>>
> >>> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> >>>
> >>> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> >>>
> >>> I'd love your thoughts.
> >>>
> >>> Chip
> >>>
> >>
> >
> >
> >
> >
> > ------------------------------------
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>

To clarify my own post...of course one could never get the LM13600 VCF to do FM effects. I was getting a little too excited when I said that. I should have said something more like "crazy AM-like effects". Yes, you can already route modulation to each voice's VCF, but this would be a global effect. Plus, you could modulate this Post-Effects VCF at a different rate than the per-voice VCFs. This might make some really interesting evolving textures!

If one wanted to do something like this, the arpeggiator LFO is ripe for wiring into this VCF. I'm not sure what kind of waveform it is (square, triangle, something else?), but it could sure be used to do *something* fun via the Post-Effecs VCF. The only thing that one would need is an "amount" knob to control how much modulation to induce.

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Hi Johannes,
>
> Thanks for the outstanding reply! I hadn't yet done much snooping into the effects circuits themselves (just the VCF following the effects)...since, most of the time, I play with the effects off. This information that you provided will be incredibly helpful as I dig into those parts.
>
> As for the resonance-following VCF...I had seen that part of the circuit but, once again, I had assumed that it was a VCA instead of a VCF. I see that it is a one-stage VCF (6 dB per octave), so it is pretty gentle.
>
> As a theory for *why* it's there, perhaps it is there to counter-balance the small boost to the very high frequencies that comes with the resistor-cap network at the input to IC21, which is at the end of KLM-368. At high resonance, when the filter is self-oscillating (squeeling!), I find that the Polysix often has the squeeling occur at very high frequency. Maybe the resonance-controlled VCF is there to tame the amplitude of the high-frequency squeeling prior to it being re-boosted by IC21.
>
> Or perhaps there's some other good reason that I'm not seeing.
>
>
> As for another one of your suggestions...I, too, am interested in a Flanger mod. Anyone?
>
> And, to contribute my own mod ideas...I might decide that I don't want to keep the Post-Effects VCF (IC15) because I don't like how it cuts the sizzle off my sawtooth. So, I could just wire it so that it's permanently open (like I tried already with clip leads). Or, another option would be to wire it as a High-Pass Filter! Or, I could route other modulation signals to it and get some crazy FM or something! Has anyone tried anything here?
>
> Chip
>
> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@> wrote:
> >
> > Hi,
> >
> > The part of the schematic you posted in you blog clearly is a voltage
> > controlled low pass filter, 2 stages, therefore 12dB/oct. They had
> > such circuits in the 80s in tape decks and other Hi-Fi equipment. It was
> > then called "Dynamic Noise Reduction" or similar.
> > The analog delay lines used for the effects introduce quite some noise
> > and additionally the clock frequency. Apparently it was necessary to
> > put in substantial amount of circuitry to make the noise acceptable.
> >
> > Another measure for noise reduction is the compressor/expander system
> > built from NE571 (IC7, IC8). The NE571 consists of two identical
> > stage which can act, depending on the external circuitry, as compressor
> > or expander. The topmost circuit on the KLM-368 schematics (one half
> > of IC8) is the compressor. After that follows the input anti-aliasing
> > filter (Q13, Q25). Then the signal is split into three paths and enters
> > a BBD delay line for each phase. The BBDs are clock with a modulated
> > clock, which is built out of CMOS inverters used in linear amplfifier
> > mode (IC22). The modulating LFO is of the multivibraotr type and built
> > around IC18. DEpending on the type of effect the modulation depth,
> > frequency and audio paths are switched (4066 switch IC14). After going
> > through the torture of the BBDs the signals are again anti-alias
> > filtered (same type of filter, two transistors) and fed to the output
> > mixers and swsitches.
> > Then the sum signal goes through the discussed controlled low pass
> > filter and goes to KLM-369. OF course all this filtering and processing
> > will (potentionally) have a big impact on frequency response.
> > --
> > Thre three effect types Chorus, Phaser, and "Ensemble" result from
> > different delay times and different signal paths and modulations.
> > E.g. the third phase is only used in the "Ensemble" setting. It would
> > be interesting to hear how a flanger would sound, if they had provided
> > the possibilty for it. Has anybody on the list done that? The circuit
> > changes should not be too dramatically.
> > --
> > I always wondered why the Resonance control is part of this. Similar
> > to the output LPF there is a controlled (by the Resonance knob) LPF
> > at the input.
> >
> > All in all the KLM-368 offers a lot analog processing circuitry and
> > some unusual circuits (e.g. the modulated oscillator made of 4069
> > inverters). If you look at the board size it is larger than the
> > KLM-367 board; and this is just for the effects. But this was a
> > distinguishing factor at that time.
> > Today this all just a piece of software...
> >
> > Johannes
> >
> >
> >
> > On 2013-02-26 18:02, chipaudette wrote:
> > > Thanks for the added insight!
> > >
> > > Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
> > >
> > > I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
> > >
> > > Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
> > >
> > > If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
> > >
> > > Chip
> > >
> > >
> > > --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@> wrote:
> > >>
> > >> The circuit is a dBx-style noise shaper: the more amplitude the dry
> > >> signal provides, the wider the response of the filter. This squelches
> > >> MN3005 noise at low levels and allows more high frequency content at
> > >> higher levels.
> > >>
> > >> The 2SA798 is a standard V/I converter that is "locked on" (filter
> > >> wide open) if the "OFF" signal voltage appears.
> > >>
> > >> The timbre change is of course due to the VCF changing vs. amplitude.
> > >>
> > >> Crow
> > >> /**/
> > >>
> > >> On 2/25/2013 11:10 AM, chipaudette wrote:
> > >>> Hi Folks,
> > >>>
> > >>> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> > >>>
> > >>> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> > >>>
> > >>> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> > >>>
> > >>> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> > >>>
> > >>> I'd love your thoughts.
> > >>>
> > >>> Chip
> > >>>
> > >>
> > >
> > >
> > >
> > >
> > > ------------------------------------
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >
> >
>

Oh, and as another reply to my own reply...

Can one make an LM13600-based VCF be a *resonant* filter? If I wanted to use it as some sort of global VCF for cool modulation effects, it would be way more useful if it were resonant.

I haven't done much research on LM13600-based filters. Can they be made resonant? Got any pointers or links?

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> To clarify my own post...of course one could never get the LM13600 VCF to do FM effects. I was getting a little too excited when I said that. I should have said something more like "crazy AM-like effects". Yes, you can already route modulation to each voice's VCF, but this would be a global effect. Plus, you could modulate this Post-Effects VCF at a different rate than the per-voice VCFs. This might make some really interesting evolving textures!
>
> If one wanted to do something like this, the arpeggiator LFO is ripe for wiring into this VCF. I'm not sure what kind of waveform it is (square, triangle, something else?), but it could sure be used to do *something* fun via the Post-Effecs VCF. The only thing that one would need is an "amount" knob to control how much modulation to induce.
>
> Chip
>
> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >
> > Hi Johannes,
> >
> > Thanks for the outstanding reply! I hadn't yet done much snooping into the effects circuits themselves (just the VCF following the effects)...since, most of the time, I play with the effects off. This information that you provided will be incredibly helpful as I dig into those parts.
> >
> > As for the resonance-following VCF...I had seen that part of the circuit but, once again, I had assumed that it was a VCA instead of a VCF. I see that it is a one-stage VCF (6 dB per octave), so it is pretty gentle.
> >
> > As a theory for *why* it's there, perhaps it is there to counter-balance the small boost to the very high frequencies that comes with the resistor-cap network at the input to IC21, which is at the end of KLM-368. At high resonance, when the filter is self-oscillating (squeeling!), I find that the Polysix often has the squeeling occur at very high frequency. Maybe the resonance-controlled VCF is there to tame the amplitude of the high-frequency squeeling prior to it being re-boosted by IC21.
> >
> > Or perhaps there's some other good reason that I'm not seeing.
> >
> >
> > As for another one of your suggestions...I, too, am interested in a Flanger mod. Anyone?
> >
> > And, to contribute my own mod ideas...I might decide that I don't want to keep the Post-Effects VCF (IC15) because I don't like how it cuts the sizzle off my sawtooth. So, I could just wire it so that it's permanently open (like I tried already with clip leads). Or, another option would be to wire it as a High-Pass Filter! Or, I could route other modulation signals to it and get some crazy FM or something! Has anyone tried anything here?
> >
> > Chip
> >
> > --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@> wrote:
> > >
> > > Hi,
> > >
> > > The part of the schematic you posted in you blog clearly is a voltage
> > > controlled low pass filter, 2 stages, therefore 12dB/oct. They had
> > > such circuits in the 80s in tape decks and other Hi-Fi equipment. It was
> > > then called "Dynamic Noise Reduction" or similar.
> > > The analog delay lines used for the effects introduce quite some noise
> > > and additionally the clock frequency. Apparently it was necessary to
> > > put in substantial amount of circuitry to make the noise acceptable.
> > >
> > > Another measure for noise reduction is the compressor/expander system
> > > built from NE571 (IC7, IC8). The NE571 consists of two identical
> > > stage which can act, depending on the external circuitry, as compressor
> > > or expander. The topmost circuit on the KLM-368 schematics (one half
> > > of IC8) is the compressor. After that follows the input anti-aliasing
> > > filter (Q13, Q25). Then the signal is split into three paths and enters
> > > a BBD delay line for each phase. The BBDs are clock with a modulated
> > > clock, which is built out of CMOS inverters used in linear amplfifier
> > > mode (IC22). The modulating LFO is of the multivibraotr type and built
> > > around IC18. DEpending on the type of effect the modulation depth,
> > > frequency and audio paths are switched (4066 switch IC14). After going
> > > through the torture of the BBDs the signals are again anti-alias
> > > filtered (same type of filter, two transistors) and fed to the output
> > > mixers and swsitches.
> > > Then the sum signal goes through the discussed controlled low pass
> > > filter and goes to KLM-369. OF course all this filtering and processing
> > > will (potentionally) have a big impact on frequency response.
> > > --
> > > Thre three effect types Chorus, Phaser, and "Ensemble" result from
> > > different delay times and different signal paths and modulations.
> > > E.g. the third phase is only used in the "Ensemble" setting. It would
> > > be interesting to hear how a flanger would sound, if they had provided
> > > the possibilty for it. Has anybody on the list done that? The circuit
> > > changes should not be too dramatically.
> > > --
> > > I always wondered why the Resonance control is part of this. Similar
> > > to the output LPF there is a controlled (by the Resonance knob) LPF
> > > at the input.
> > >
> > > All in all the KLM-368 offers a lot analog processing circuitry and
> > > some unusual circuits (e.g. the modulated oscillator made of 4069
> > > inverters). If you look at the board size it is larger than the
> > > KLM-367 board; and this is just for the effects. But this was a
> > > distinguishing factor at that time.
> > > Today this all just a piece of software...
> > >
> > > Johannes
> > >
> > >
> > >
> > > On 2013-02-26 18:02, chipaudette wrote:
> > > > Thanks for the added insight!
> > > >
> > > > Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
> > > >
> > > > I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
> > > >
> > > > Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
> > > >
> > > > If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
> > > >
> > > > Chip
> > > >
> > > >
> > > > --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@> wrote:
> > > >>
> > > >> The circuit is a dBx-style noise shaper: the more amplitude the dry
> > > >> signal provides, the wider the response of the filter. This squelches
> > > >> MN3005 noise at low levels and allows more high frequency content at
> > > >> higher levels.
> > > >>
> > > >> The 2SA798 is a standard V/I converter that is "locked on" (filter
> > > >> wide open) if the "OFF" signal voltage appears.
> > > >>
> > > >> The timbre change is of course due to the VCF changing vs. amplitude.
> > > >>
> > > >> Crow
> > > >> /**/
> > > >>
> > > >> On 2/25/2013 11:10 AM, chipaudette wrote:
> > > >>> Hi Folks,
> > > >>>
> > > >>> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
> > > >>>
> > > >>> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
> > > >>>
> > > >>> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
> > > >>>
> > > >>> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
> > > >>>
> > > >>> I'd love your thoughts.
> > > >>>
> > > >>> Chip
> > > >>>
> > > >>
> > > >
> > > >
> > > >
> > > >
> > > > ------------------------------------
> > > >
> > > > Yahoo! Groups Links
> > > >
> > > >
> > > >
> > > >
> > >
> >
>

Hi!

Yes it is possible to build resonant filters with LM13600. With
the current 2-stage design this probably will not sound what you are
expecting. The rolloff is not so steep so the resonance frequency
cannot be controlled so exactly. I remember they had 12dB/oct filters
on the MS-20. I think they were based on a CA3080 or similar, which
basically is one half of the LM13600. I once made a modification to
an MS-20 combining the two 12dB/oct filters to one 24dB/oct with
switchable characteristic (HP/LP/BP). When you cranked up the resonance
it would produce an absolute phantastic sreaming sound. I just checked
the schematics, they actually used LM13600s ...
The question for the Polysix is how would the single filter on the
sum signal of all six voices will sound. And how would you control it,
dynamically, statically, etc.

The ARP "LFO" is not a real modulating LFO. Its output is just a short
pulse of variable repetition rate used to trigger periodic interrupts
to the key assigner CPU.

If you like a crisp sawtooth sound then take the signal directly from
the KLM-366 and feed it to an external effects unit. You can save a
lot of noise as well. I can imagine quite impressive Hammond-like
sounds when processed with a good-quality leslie effect.


Keep the work going!

Johannes

On 2013-02-28 13:02, chipaudette wrote:
> Oh, and as another reply to my own reply...
>
> Can one make an LM13600-based VCF be a *resonant* filter? If I wanted to use it as some sort of global VCF for cool modulation effects, it would be way more useful if it were resonant.
>
> I haven't done much research on LM13600-based filters. Can they be made resonant? Got any pointers or links?
>
> Chip
>
> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>>
>> To clarify my own post...of course one could never get the LM13600 VCF to do FM effects. I was getting a little too excited when I said that. I should have said something more like "crazy AM-like effects". Yes, you can already route modulation to each voice's VCF, but this would be a global effect. Plus, you could modulate this Post-Effects VCF at a different rate than the per-voice VCFs. This might make some really interesting evolving textures!
>>
>> If one wanted to do something like this, the arpeggiator LFO is ripe for wiring into this VCF. I'm not sure what kind of waveform it is (square, triangle, something else?), but it could sure be used to do *something* fun via the Post-Effecs VCF. The only thing that one would need is an "amount" knob to control how much modulation to induce.
>>
>> Chip
>>
>> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
>>>
>>> Hi Johannes,
>>>
>>> Thanks for the outstanding reply! I hadn't yet done much snooping into the effects circuits themselves (just the VCF following the effects)...since, most of the time, I play with the effects off. This information that you provided will be incredibly helpful as I dig into those parts.
>>>
>>> As for the resonance-following VCF...I had seen that part of the circuit but, once again, I had assumed that it was a VCA instead of a VCF. I see that it is a one-stage VCF (6 dB per octave), so it is pretty gentle.
>>>
>>> As a theory for *why* it's there, perhaps it is there to counter-balance the small boost to the very high frequencies that comes with the resistor-cap network at the input to IC21, which is at the end of KLM-368. At high resonance, when the filter is self-oscillating (squeeling!), I find that the Polysix often has the squeeling occur at very high frequency. Maybe the resonance-controlled VCF is there to tame the amplitude of the high-frequency squeeling prior to it being re-boosted by IC21.
>>>
>>> Or perhaps there's some other good reason that I'm not seeing.
>>>
>>>
>>> As for another one of your suggestions...I, too, am interested in a Flanger mod. Anyone?
>>>
>>> And, to contribute my own mod ideas...I might decide that I don't want to keep the Post-Effects VCF (IC15) because I don't like how it cuts the sizzle off my sawtooth. So, I could just wire it so that it's permanently open (like I tried already with clip leads). Or, another option would be to wire it as a High-Pass Filter! Or, I could route other modulation signals to it and get some crazy FM or something! Has anyone tried anything here?
>>>
>>> Chip
>>>
>>> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@> wrote:
>>>>
>>>> Hi,
>>>>
>>>> The part of the schematic you posted in you blog clearly is a voltage
>>>> controlled low pass filter, 2 stages, therefore 12dB/oct. They had
>>>> such circuits in the 80s in tape decks and other Hi-Fi equipment. It was
>>>> then called "Dynamic Noise Reduction" or similar.
>>>> The analog delay lines used for the effects introduce quite some noise
>>>> and additionally the clock frequency. Apparently it was necessary to
>>>> put in substantial amount of circuitry to make the noise acceptable.
>>>>
>>>> Another measure for noise reduction is the compressor/expander system
>>>> built from NE571 (IC7, IC8). The NE571 consists of two identical
>>>> stage which can act, depending on the external circuitry, as compressor
>>>> or expander. The topmost circuit on the KLM-368 schematics (one half
>>>> of IC8) is the compressor. After that follows the input anti-aliasing
>>>> filter (Q13, Q25). Then the signal is split into three paths and enters
>>>> a BBD delay line for each phase. The BBDs are clock with a modulated
>>>> clock, which is built out of CMOS inverters used in linear amplfifier
>>>> mode (IC22). The modulating LFO is of the multivibraotr type and built
>>>> around IC18. DEpending on the type of effect the modulation depth,
>>>> frequency and audio paths are switched (4066 switch IC14). After going
>>>> through the torture of the BBDs the signals are again anti-alias
>>>> filtered (same type of filter, two transistors) and fed to the output
>>>> mixers and swsitches.
>>>> Then the sum signal goes through the discussed controlled low pass
>>>> filter and goes to KLM-369. OF course all this filtering and processing
>>>> will (potentionally) have a big impact on frequency response.
>>>> --
>>>> Thre three effect types Chorus, Phaser, and "Ensemble" result from
>>>> different delay times and different signal paths and modulations.
>>>> E.g. the third phase is only used in the "Ensemble" setting. It would
>>>> be interesting to hear how a flanger would sound, if they had provided
>>>> the possibilty for it. Has anybody on the list done that? The circuit
>>>> changes should not be too dramatically.
>>>> --
>>>> I always wondered why the Resonance control is part of this. Similar
>>>> to the output LPF there is a controlled (by the Resonance knob) LPF
>>>> at the input.
>>>>
>>>> All in all the KLM-368 offers a lot analog processing circuitry and
>>>> some unusual circuits (e.g. the modulated oscillator made of 4069
>>>> inverters). If you look at the board size it is larger than the
>>>> KLM-367 board; and this is just for the effects. But this was a
>>>> distinguishing factor at that time.
>>>> Today this all just a piece of software...
>>>>
>>>> Johannes
>>>>
>>>>
>>>>
>>>> On 2013-02-26 18:02, chipaudette wrote:
>>>>> Thanks for the added insight!
>>>>>
>>>>> Specifically with regards to the impact of the "Effect Off" signal, I did not find that necessarily forced the LM13600 VCF to be wide open.
>>>>>
>>>>> I've yet to actually do a measurement of the frequency content, but I did measure the current being sent to the LM13600 control input. When "Effect Off" signal is high (+15V), the current to the control input is indeed a little higher, implying that the filter is a little more open.
>>>>>
>>>>> Even with the "Effect Off" signal high, the control signal still changes in response to which key is being pressed. When playing C1, for example, the control current implies (via the equation in the datasheet) that the cutoff frequency is down at 8-9 kHz. On C6, the measured control current implies that it opens up much wider...to about 15-16 kHz.
>>>>>
>>>>> If the "Effect Off" signal is supposed to force the filter to be all the way open, it doesn't seem to be doing that on my Polysix. I'm planning on posting details of my measurements tonight. But until then, do you think that something might be wrong with this part of my circuit?
>>>>>
>>>>> Chip
>>>>>
>>>>>
>>>>> --- In PolySix@yahoogroups.com, The Old Crow <oldcrow@> wrote:
>>>>>>
>>>>>> The circuit is a dBx-style noise shaper: the more amplitude the dry
>>>>>> signal provides, the wider the response of the filter. This squelches
>>>>>> MN3005 noise at low levels and allows more high frequency content at
>>>>>> higher levels.
>>>>>>
>>>>>> The 2SA798 is a standard V/I converter that is "locked on" (filter
>>>>>> wide open) if the "OFF" signal voltage appears.
>>>>>>
>>>>>> The timbre change is of course due to the VCF changing vs. amplitude.
>>>>>>
>>>>>> Crow
>>>>>> /**/
>>>>>>
>>>>>> On 2/25/2013 11:10 AM, chipaudette wrote:
>>>>>>> Hi Folks,
>>>>>>>
>>>>>>> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
>>>>>>>
>>>>>>> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
>>>>>>>
>>>>>>> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
>>>>>>>
>>>>>>> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
>>>>>>>
>>>>>>> I'd love your thoughts.
>>>>>>>
>>>>>>> Chip
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> ------------------------------------
>>>>>
>>>>> Yahoo! Groups Links
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>

Hi Johannes,

A few posts ago, you said:

> I always wondered why the Resonance control is part of this.
> Similar to the output LPF there is a controlled (by the
> Resonance knob) LPF at the input.

Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.

Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.

Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.

Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...

Chip

Well, I still think that it's a VCA, but I found that the gain goes *up* with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.

More analysis still forth-coming...

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
>
> Hi Johannes,
>
> A few posts ago, you said:
>
> > I always wondered why the Resonance control is part of this.
> > Similar to the output LPF there is a controlled (by the
> > Resonance knob) LPF at the input.
>
> Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
>
> Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
>
> Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
>
> Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
>
> Chip
>

OK, I took some measurements of the resonance-controlled VCA. You can see my data and analysis here...

http://synthhacker.blogspot.com/2013/03/polysix-resonance-controlled-vca.html

My conclusion is that this VCA compensates for the natural drop in volume from the voice board that occurs when you start turning up the resonance knob. On my Polysix, as I turn the knob from "1" to "3", I get an increase in gain up to 6 dB, which almost perfectly components for the drop in volume from the voice board.

When I turn the resonance knob past "3", there appears to be no further change in gain from this VCA.

I found this very interesting!

In doing this analysis, I really struggled with trying to predict the behavior of the LM13600. In the end, I just accepted its behavior based on the data. I would prefer to have good working equation for the gain of the LM13600 given the values of the circuit elements and given the value of the control current. The datasheet wasn't helpful enough. Does anyone have a good quantitative understanding of the gain of the LM13600?

Thanks,

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
>
> Well, I still think that it's a VCA, but I found that the gain goes *up* with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.
>
> More analysis still forth-coming...
>
> Chip
>
>
> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >
> >
> > Hi Johannes,
> >
> > A few posts ago, you said:
> >
> > > I always wondered why the Resonance control is part of this.
> > > Similar to the output LPF there is a controlled (by the
> > > Resonance knob) LPF at the input.
> >
> > Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
> >
> > Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
> >
> > Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
> >
> > Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
> >
> > Chip
> >
>

The LM13600 as VCA works without (negative) feedback. If you think about
it this makes sense. The operational amplifier with negative feedback
forms a control loop: the output voltage is increased or decreased until
the two inputs have zero voltage difference - as long the circuit can
reach and maintain this condition. The purpose of the LM13600 (and
similar circuits) is that the gain can be controlled via a control
signal (current in this case). You would not be able to control the
gain and hence the output level of a closed-loop amp. (you would just
be controlling the loop gain, which is not what is desired). So the
classical VCA application of the LM13600 must not have a negative
feedback. In order to not overdrive the input stages the signal must be
rather low. Therefore you see a voltage divider at the LM1360' input.
In the case of the schematic snipped it is about the factor 100 (if we
neglect the pre-emphasis R156/C80).
Furthermore you may have noticed that the LM13600 is dubbed as
"Operational Transcoductance Amplfier" in the datasheet (OTA), not as
simple "Operational Amplifier". "transconductance" means the ability
of conducting current. It is the reciproc (1/x) value of the electrical
resistance. Ohm's Law is well known to mankind; it says R=U/I. This is
the resistance. We get the transconductance as I/U (reciproc value). In
the case of an amplifier this denotes a voltage-type input and a
current source output. The transfer function (output / input) becomes
Iout / Vin. Normally we are used to a simple voltage rational transfer
function Vout / Vin, which commonly is denoted as amplification or
(voltage) gain. Btw. to achieve a low-source resistance voltage output
the LM13600 features am emitter follower which transofrms the high
impedance current output to a low-impedance voltage output. It does so
by means of a parallel resistor. The output current creates a
proportional voltage on that resistor and the voltage follower makes
this low (source-) impedance.

Conclusion:
To be able to calculate the voltage gain of an LM13600 stage both the
input divider and the parallel output resistor have to be known and
put into the equation.

Here at the KLM-368 input stage the LM13600 injects the current
directly into the following op amp IC17. R164 does not matter for the
DC amplification, I expect the LM13600 source impedance one or two
magnitudes higher than 4.7kOhm. Probably R164 together with C75 form
an additional low pass characterstic of the stage.


Everthing clear ;-)?

Johannes

On 2013-03-02 22:31, chipaudette wrote:
> OK, I took some measurements of the resonance-controlled VCA. You can see my data and analysis here...
>
> http://synthhacker.blogspot.com/2013/03/polysix-resonance-controlled-vca.html
>
> My conclusion is that this VCA compensates for the natural drop in volume from the voice board that occurs when you start turning up the resonance knob. On my Polysix, as I turn the knob from "1" to "3", I get an increase in gain up to 6 dB, which almost perfectly components for the drop in volume from the voice board.
>
> When I turn the resonance knob past "3", there appears to be no further change in gain from this VCA.
>
> I found this very interesting!
>
> In doing this analysis, I really struggled with trying to predict the behavior of the LM13600. In the end, I just accepted its behavior based on the data. I would prefer to have good working equation for the gain of the LM13600 given the values of the circuit elements and given the value of the control current. The datasheet wasn't helpful enough. Does anyone have a good quantitative understanding of the gain of the LM13600?
>
> Thanks,
>
> Chip
>
> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>>
>>
>> Well, I still think that it's a VCA, but I found that the gain goes *up* with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.
>>
>> More analysis still forth-coming...
>>
>> Chip
>>
>>
>> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
>>>
>>>
>>> Hi Johannes,
>>>
>>> A few posts ago, you said:
>>>
>>>> I always wondered why the Resonance control is part of this.
>>>> Similar to the output LPF there is a controlled (by the
>>>> Resonance knob) LPF at the input.
>>>
>>> Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
>>>
>>> Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
>>>
>>> Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
>>>
>>> Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
>>>
>>> Chip
>>>
>>
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>

What I find so difficult about the LM13600 is my inability to calculate what the transconductance should be for a given circuit configuration.

For the VCA shown in the datasheet, the datasheet says that:

Iout = Iin*(2*Iabc)/Idiode

Seems simple enough, except I can't seem to use this equation to get anything reasonable. For example, the datasheet says that their example circuit results in Vout/Vin = 940 x Iabc. I can't see how they got that value from their circuit.

Then, when i try to generalize to slightly different circuit setups (like the one that we're discussing in the polysix), I can't get any reasonable answer that comes close to the experimental result.

Part of the reason could be that the assumptions in the datasheet, while accurate for the specific circuit that they drew, has important (critical) differences with how people (such as the Polysix designers) actually end up implementing their VCA. This page has a very interesting discussion...

http://lushprojects.com/blog/2012/08/lm13700-missing-forumla/

...but it doesn't have quite enough detail to help me through all the way so that I can compute the gain vs Iabc relationship that I'm looking for.

Bummer.

Johannes, are you able to correctly calculate the gain of the LM13600 for the example in the datasheet or for the setup in the Polysix? It thwarted me....

Chip

--- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@...> wrote:
>
> The LM13600 as VCA works without (negative) feedback. If you think about
> it this makes sense. The operational amplifier with negative feedback
> forms a control loop: the output voltage is increased or decreased until
> the two inputs have zero voltage difference - as long the circuit can
> reach and maintain this condition. The purpose of the LM13600 (and
> similar circuits) is that the gain can be controlled via a control
> signal (current in this case). You would not be able to control the
> gain and hence the output level of a closed-loop amp. (you would just
> be controlling the loop gain, which is not what is desired). So the
> classical VCA application of the LM13600 must not have a negative
> feedback. In order to not overdrive the input stages the signal must be
> rather low. Therefore you see a voltage divider at the LM1360' input.
> In the case of the schematic snipped it is about the factor 100 (if we
> neglect the pre-emphasis R156/C80).
> Furthermore you may have noticed that the LM13600 is dubbed as
> "Operational Transcoductance Amplfier" in the datasheet (OTA), not as
> simple "Operational Amplifier". "transconductance" means the ability
> of conducting current. It is the reciproc (1/x) value of the electrical
> resistance. Ohm's Law is well known to mankind; it says R=U/I. This is
> the resistance. We get the transconductance as I/U (reciproc value). In
> the case of an amplifier this denotes a voltage-type input and a
> current source output. The transfer function (output / input) becomes
> Iout / Vin. Normally we are used to a simple voltage rational transfer
> function Vout / Vin, which commonly is denoted as amplification or
> (voltage) gain. Btw. to achieve a low-source resistance voltage output
> the LM13600 features am emitter follower which transofrms the high
> impedance current output to a low-impedance voltage output. It does so
> by means of a parallel resistor. The output current creates a
> proportional voltage on that resistor and the voltage follower makes
> this low (source-) impedance.
>
> Conclusion:
> To be able to calculate the voltage gain of an LM13600 stage both the
> input divider and the parallel output resistor have to be known and
> put into the equation.
>
> Here at the KLM-368 input stage the LM13600 injects the current
> directly into the following op amp IC17. R164 does not matter for the
> DC amplification, I expect the LM13600 source impedance one or two
> magnitudes higher than 4.7kOhm. Probably R164 together with C75 form
> an additional low pass characterstic of the stage.
>
>
> Everthing clear ;-)?
>
> Johannes
>
>
>
>
>
>
> On 2013-03-02 22:31, chipaudette wrote:
> > OK, I took some measurements of the resonance-controlled VCA. You can see my data and analysis here...
> >
> > http://synthhacker.blogspot.com/2013/03/polysix-resonance-controlled-vca.html
> >
> > My conclusion is that this VCA compensates for the natural drop in volume from the voice board that occurs when you start turning up the resonance knob. On my Polysix, as I turn the knob from "1" to "3", I get an increase in gain up to 6 dB, which almost perfectly components for the drop in volume from the voice board.
> >
> > When I turn the resonance knob past "3", there appears to be no further change in gain from this VCA.
> >
> > I found this very interesting!
> >
> > In doing this analysis, I really struggled with trying to predict the behavior of the LM13600. In the end, I just accepted its behavior based on the data. I would prefer to have good working equation for the gain of the LM13600 given the values of the circuit elements and given the value of the control current. The datasheet wasn't helpful enough. Does anyone have a good quantitative understanding of the gain of the LM13600?
> >
> > Thanks,
> >
> > Chip
> >
> > --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >>
> >>
> >> Well, I still think that it's a VCA, but I found that the gain goes *up* with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.
> >>
> >> More analysis still forth-coming...
> >>
> >> Chip
> >>
> >>
> >> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >>>
> >>>
> >>> Hi Johannes,
> >>>
> >>> A few posts ago, you said:
> >>>
> >>>> I always wondered why the Resonance control is part of this.
> >>>> Similar to the output LPF there is a controlled (by the
> >>>> Resonance knob) LPF at the input.
> >>>
> >>> Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
> >>>
> >>> Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
> >>>
> >>> Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
> >>>
> >>> Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
> >>>
> >>> Chip
> >>>
> >>
> >
> >
> >
> > ------------------------------------
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>

Hi Chip,

I just finished watching the movie "The Cincinnati Kid". Whereas in
former years I would identify more with Steve McQueen nowadays I tend
more to the Edward G. Robinson character. It seems a little an
allegory to this LM1600 matter.
I am perfectly confident that I am able to understand and calculate
this VCA stage. But it is late today now (11 p.m.) and I hear my bed
calling my name :-)

I take the challenge and I'll come back later, hopefully with some
figures!

Johannes


P.S. I am not sure whether to answer to the list or your blog. What do
you prefer?

On 2013-03-03 21:41, chipaudette wrote:
> What I find so difficult about the LM13600 is my inability to calculate what the transconductance should be for a given circuit configuration.
>
> For the VCA shown in the datasheet, the datasheet says that:
>
> Iout = Iin*(2*Iabc)/Idiode
>
> Seems simple enough, except I can't seem to use this equation to get anything reasonable. For example, the datasheet says that their example circuit results in Vout/Vin = 940 x Iabc. I can't see how they got that value from their circuit.
>
> Then, when i try to generalize to slightly different circuit setups (like the one that we're discussing in the polysix), I can't get any reasonable answer that comes close to the experimental result.
>
> Part of the reason could be that the assumptions in the datasheet, while accurate for the specific circuit that they drew, has important (critical) differences with how people (such as the Polysix designers) actually end up implementing their VCA. This page has a very interesting discussion...
>
> http://lushprojects.com/blog/2012/08/lm13700-missing-forumla/
>
> ...but it doesn't have quite enough detail to help me through all the way so that I can compute the gain vs Iabc relationship that I'm looking for.
>
> Bummer.
>
> Johannes, are you able to correctly calculate the gain of the LM13600 for the example in the datasheet or for the setup in the Polysix? It thwarted me....
>
> Chip
>
> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@...> wrote:
>>
>> The LM13600 as VCA works without (negative) feedback. If you think about
>> it this makes sense. The operational amplifier with negative feedback
>> forms a control loop: the output voltage is increased or decreased until
>> the two inputs have zero voltage difference - as long the circuit can
>> reach and maintain this condition. The purpose of the LM13600 (and
>> similar circuits) is that the gain can be controlled via a control
>> signal (current in this case). You would not be able to control the
>> gain and hence the output level of a closed-loop amp. (you would just
>> be controlling the loop gain, which is not what is desired). So the
>> classical VCA application of the LM13600 must not have a negative
>> feedback. In order to not overdrive the input stages the signal must be
>> rather low. Therefore you see a voltage divider at the LM1360' input.
>> In the case of the schematic snipped it is about the factor 100 (if we
>> neglect the pre-emphasis R156/C80).
>> Furthermore you may have noticed that the LM13600 is dubbed as
>> "Operational Transcoductance Amplfier" in the datasheet (OTA), not as
>> simple "Operational Amplifier". "transconductance" means the ability
>> of conducting current. It is the reciproc (1/x) value of the electrical
>> resistance. Ohm's Law is well known to mankind; it says R=U/I. This is
>> the resistance. We get the transconductance as I/U (reciproc value). In
>> the case of an amplifier this denotes a voltage-type input and a
>> current source output. The transfer function (output / input) becomes
>> Iout / Vin. Normally we are used to a simple voltage rational transfer
>> function Vout / Vin, which commonly is denoted as amplification or
>> (voltage) gain. Btw. to achieve a low-source resistance voltage output
>> the LM13600 features am emitter follower which transofrms the high
>> impedance current output to a low-impedance voltage output. It does so
>> by means of a parallel resistor. The output current creates a
>> proportional voltage on that resistor and the voltage follower makes
>> this low (source-) impedance.
>>
>> Conclusion:
>> To be able to calculate the voltage gain of an LM13600 stage both the
>> input divider and the parallel output resistor have to be known and
>> put into the equation.
>>
>> Here at the KLM-368 input stage the LM13600 injects the current
>> directly into the following op amp IC17. R164 does not matter for the
>> DC amplification, I expect the LM13600 source impedance one or two
>> magnitudes higher than 4.7kOhm. Probably R164 together with C75 form
>> an additional low pass characterstic of the stage.
>>
>>
>> Everthing clear ;-)?
>>
>> Johannes
>>
>>
>>
>>
>>
>>
>> On 2013-03-02 22:31, chipaudette wrote:
>>> OK, I took some measurements of the resonance-controlled VCA. You can see my data and analysis here...
>>>
>>> http://synthhacker.blogspot.com/2013/03/polysix-resonance-controlled-vca.html
>>>
>>> My conclusion is that this VCA compensates for the natural drop in volume from the voice board that occurs when you start turning up the resonance knob. On my Polysix, as I turn the knob from "1" to "3", I get an increase in gain up to 6 dB, which almost perfectly components for the drop in volume from the voice board.
>>>
>>> When I turn the resonance knob past "3", there appears to be no further change in gain from this VCA.
>>>
>>> I found this very interesting!
>>>
>>> In doing this analysis, I really struggled with trying to predict the behavior of the LM13600. In the end, I just accepted its behavior based on the data. I would prefer to have good working equation for the gain of the LM13600 given the values of the circuit elements and given the value of the control current. The datasheet wasn't helpful enough. Does anyone have a good quantitative understanding of the gain of the LM13600?
>>>
>>> Thanks,
>>>
>>> Chip
>>>
>>> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
>>>>
>>>>
>>>> Well, I still think that it's a VCA, but I found that the gain goes *up* with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.
>>>>
>>>> More analysis still forth-coming...
>>>>
>>>> Chip
>>>>
>>>>
>>>> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
>>>>>
>>>>>
>>>>> Hi Johannes,
>>>>>
>>>>> A few posts ago, you said:
>>>>>
>>>>>> I always wondered why the Resonance control is part of this.
>>>>>> Similar to the output LPF there is a controlled (by the
>>>>>> Resonance knob) LPF at the input.
>>>>>
>>>>> Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
>>>>>
>>>>> Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
>>>>>
>>>>> Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
>>>>>
>>>>> Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
>>>>>
>>>>> Chip
>>>>>
>>>>
>>>
>>>
>>>
>>> ------------------------------------
>>>
>>> Yahoo! Groups Links
>>>
>>>
>>>
>>>
>>
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>

Hi Johannes,

If you're willing to explain your calculation of LM13600 gain, you can give the explanation in whatever forum you'd like!

If you post it here, more people are likely to see it. And, if you don't mind, I can then copy select quotes over to my blog as necessary (with attributions, of course) when I go to use the information in future posts.

If you were looking for an example to explain, I cannot see how the LM13600 datasheet (page 8) got the equation: Vo/Vi = 940 x Iabc for the circuit in Figure 2.

If you can explain that one, then maybe I can take a crack at analyzing the one in the Polysix.

Thanks!

Chip

--- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@...> wrote:
>
> Hi Chip,
>
> I just finished watching the movie "The Cincinnati Kid". Whereas in
> former years I would identify more with Steve McQueen nowadays I tend
> more to the Edward G. Robinson character. It seems a little an
> allegory to this LM1600 matter.
> I am perfectly confident that I am able to understand and calculate
> this VCA stage. But it is late today now (11 p.m.) and I hear my bed
> calling my name :-)
>
> I take the challenge and I'll come back later, hopefully with some
> figures!
>
> Johannes
>
>
> P.S. I am not sure whether to answer to the list or your blog. What do
> you prefer?
>
> On 2013-03-03 21:41, chipaudette wrote:
> > What I find so difficult about the LM13600 is my inability to calculate what the transconductance should be for a given circuit configuration.
> >
> > For the VCA shown in the datasheet, the datasheet says that:
> >
> > Iout = Iin*(2*Iabc)/Idiode
> >
> > Seems simple enough, except I can't seem to use this equation to get anything reasonable. For example, the datasheet says that their example circuit results in Vout/Vin = 940 x Iabc. I can't see how they got that value from their circuit.
> >
> > Then, when i try to generalize to slightly different circuit setups (like the one that we're discussing in the polysix), I can't get any reasonable answer that comes close to the experimental result.
> >
> > Part of the reason could be that the assumptions in the datasheet, while accurate for the specific circuit that they drew, has important (critical) differences with how people (such as the Polysix designers) actually end up implementing their VCA. This page has a very interesting discussion...
> >
> > http://lushprojects.com/blog/2012/08/lm13700-missing-forumla/
> >
> > ...but it doesn't have quite enough detail to help me through all the way so that I can compute the gain vs Iabc relationship that I'm looking for.
> >
> > Bummer.
> >
> > Johannes, are you able to correctly calculate the gain of the LM13600 for the example in the datasheet or for the setup in the Polysix? It thwarted me....
> >
> > Chip
> >
> > --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@> wrote:
> >>
> >> The LM13600 as VCA works without (negative) feedback. If you think about
> >> it this makes sense. The operational amplifier with negative feedback
> >> forms a control loop: the output voltage is increased or decreased until
> >> the two inputs have zero voltage difference - as long the circuit can
> >> reach and maintain this condition. The purpose of the LM13600 (and
> >> similar circuits) is that the gain can be controlled via a control
> >> signal (current in this case). You would not be able to control the
> >> gain and hence the output level of a closed-loop amp. (you would just
> >> be controlling the loop gain, which is not what is desired). So the
> >> classical VCA application of the LM13600 must not have a negative
> >> feedback. In order to not overdrive the input stages the signal must be
> >> rather low. Therefore you see a voltage divider at the LM1360' input.
> >> In the case of the schematic snipped it is about the factor 100 (if we
> >> neglect the pre-emphasis R156/C80).
> >> Furthermore you may have noticed that the LM13600 is dubbed as
> >> "Operational Transcoductance Amplfier" in the datasheet (OTA), not as
> >> simple "Operational Amplifier". "transconductance" means the ability
> >> of conducting current. It is the reciproc (1/x) value of the electrical
> >> resistance. Ohm's Law is well known to mankind; it says R=U/I. This is
> >> the resistance. We get the transconductance as I/U (reciproc value). In
> >> the case of an amplifier this denotes a voltage-type input and a
> >> current source output. The transfer function (output / input) becomes
> >> Iout / Vin. Normally we are used to a simple voltage rational transfer
> >> function Vout / Vin, which commonly is denoted as amplification or
> >> (voltage) gain. Btw. to achieve a low-source resistance voltage output
> >> the LM13600 features am emitter follower which transofrms the high
> >> impedance current output to a low-impedance voltage output. It does so
> >> by means of a parallel resistor. The output current creates a
> >> proportional voltage on that resistor and the voltage follower makes
> >> this low (source-) impedance.
> >>
> >> Conclusion:
> >> To be able to calculate the voltage gain of an LM13600 stage both the
> >> input divider and the parallel output resistor have to be known and
> >> put into the equation.
> >>
> >> Here at the KLM-368 input stage the LM13600 injects the current
> >> directly into the following op amp IC17. R164 does not matter for the
> >> DC amplification, I expect the LM13600 source impedance one or two
> >> magnitudes higher than 4.7kOhm. Probably R164 together with C75 form
> >> an additional low pass characterstic of the stage.
> >>
> >>
> >> Everthing clear ;-)?
> >>
> >> Johannes
> >>
> >>
> >>
> >>
> >>
> >>
> >> On 2013-03-02 22:31, chipaudette wrote:
> >>> OK, I took some measurements of the resonance-controlled VCA. You can see my data and analysis here...
> >>>
> >>> http://synthhacker.blogspot.com/2013/03/polysix-resonance-controlled-vca.html
> >>>
> >>> My conclusion is that this VCA compensates for the natural drop in volume from the voice board that occurs when you start turning up the resonance knob. On my Polysix, as I turn the knob from "1" to "3", I get an increase in gain up to 6 dB, which almost perfectly components for the drop in volume from the voice board.
> >>>
> >>> When I turn the resonance knob past "3", there appears to be no further change in gain from this VCA.
> >>>
> >>> I found this very interesting!
> >>>
> >>> In doing this analysis, I really struggled with trying to predict the behavior of the LM13600. In the end, I just accepted its behavior based on the data. I would prefer to have good working equation for the gain of the LM13600 given the values of the circuit elements and given the value of the control current. The datasheet wasn't helpful enough. Does anyone have a good quantitative understanding of the gain of the LM13600?
> >>>
> >>> Thanks,
> >>>
> >>> Chip
> >>>
> >>> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >>>>
> >>>>
> >>>> Well, I still think that it's a VCA, but I found that the gain goes *up* with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.
> >>>>
> >>>> More analysis still forth-coming...
> >>>>
> >>>> Chip
> >>>>
> >>>>
> >>>> --- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@> wrote:
> >>>>>
> >>>>>
> >>>>> Hi Johannes,
> >>>>>
> >>>>> A few posts ago, you said:
> >>>>>
> >>>>>> I always wondered why the Resonance control is part of this.
> >>>>>> Similar to the output LPF there is a controlled (by the
> >>>>>> Resonance knob) LPF at the input.
> >>>>>
> >>>>> Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
> >>>>>
> >>>>> Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
> >>>>>
> >>>>> Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
> >>>>>
> >>>>> Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
> >>>>>
> >>>>> Chip
> >>>>>
> >>>>
> >>>
> >>>
> >>>
> >>> ------------------------------------
> >>>
> >>> Yahoo! Groups Links
> >>>
> >>>
> >>>
> >>>
> >>
> >
> >
> >
> > ------------------------------------
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>

For the LM13600 datasheet example, I can get: Vo/Vi = 1739 * Iabc

or I can get: Vo/Vi = 870 * Iabc

or, stretching, I can even get: Vo/Vi = 1948 * Iabc

But I can't get the Vo/Vi = 940 * Iabc that they claim is true.

I'd love to see how someone gets 940*Iabc.

Chip

--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Hi Johannes,
>
> If you're willing to explain your calculation of LM13600 gain, you can give the explanation in whatever forum you'd like!
>
> If you post it here, more people are likely to see it. And, if you don't mind, I can then copy select quotes over to my blog as necessary (with attributions, of course) when I go to use the information in future posts.
>
> If you were looking for an example to explain, I cannot see how the LM13600 datasheet (page 8) got the equation: Vo/Vi = 940 x Iabc for the circuit in Figure 2.
>
> If you can explain that one, then maybe I can take a crack at analyzing the one in the Polysix.
>
> Thanks!
>
> Chip
>
>
>
> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@> wrote:
> >
> > Hi Chip,
> >
> > I just finished watching the movie "The Cincinnati Kid". Whereas in
> > former years I would identify more with Steve McQueen nowadays I tend
> > more to the Edward G. Robinson character. It seems a little an
> > allegory to this LM1600 matter.
> > I am perfectly confident that I am able to understand and calculate
> > this VCA stage. But it is late today now (11 p.m.) and I hear my bed
> > calling my name :-)
> >
> > I take the challenge and I'll come back later, hopefully with some
> > figures!
> >
> > Johannes
> >
> >
> > P.S. I am not sure whether to answer to the list or your blog. What do
> > you prefer?
> >
> > On 2013-03-03 21:41, chipaudette wrote:
> > > What I find so difficult about the LM13600 is my inability to calculate what the transconductance should be for a given circuit configuration.
> > >
> > > For the VCA shown in the datasheet, the datasheet says that:
> > >
> > > Iout = Iin*(2*Iabc)/Idiode
> > >
> > > Seems simple enough, except I can't seem to use this equation to get anything reasonable. For example, the datasheet says that their example circuit results in Vout/Vin = 940 x Iabc. I can't see how they got that value from their circuit.
> > >
> > > Then, when i try to generalize to slightly different circuit setups (like the one that we're discussing in the polysix), I can't get any reasonable answer that comes close to the experimental result.
> > >
> > > Part of the reason could be that the assumptions in the datasheet, while accurate for the specific circuit that they drew, has important (critical) differences with how people (such as the Polysix designers) actually end up implementing their VCA. This page has a very interesting discussion...
> > >
> > > http://lushprojects.com/blog/2012/08/lm13700-missing-forumla/
> > >
> > > ...but it doesn't have quite enough detail to help me through all the way so that I can compute the gain vs Iabc relationship that I'm looking for.
> > >
> > > Bummer.
> > >
> > > Johannes, are you able to correctly calculate the gain of the LM13600 for the example in the datasheet or for the setup in the Polysix? It thwarted me....
> > >
> > > Chip
> > >