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
>>>
>>
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