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
> >>>
> >>
> >
> >
> >
> >
> > ------------------------------------
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