Monday, February 25, 2013

Mystery Circuit - Polysix Post-Effects VCA

Continuing my journey to understand the frequency response of my Korg Polysix, Johannes made a comment at the end of this post that that I should examine the effect of the "compressor/expander" circuit on the KLM-368 board .  He suggested that it would likely have a large effect on frequency response.  This circuit is definitely interesting and, through previous probings (the subject of a future post), I had indeed discovered that it has a pretty strong effect on the sound of the synth.  Here, I'm trying to figure out how this circuit works.  I'm looking forward to your help!

Post-Effects Voltage Controlled Amplifier (Click to Enlarge) of Korg Polysix
Above is an excerpt of the schematic for the "compressor/expander" circuit that I think that Johannes was referring to.  It is clearly some sort of voltage controlled amplifier (VCA) that is controlled in some way by the overall level of the audio that's being input.  What is it really doing?  How does it work?  Well, I'm not really sure, but I've labeled a few of the constituent blocks along with my guess at their function.  A brief discussion of each of these blocks is below:

(1) Amplify with High Frequency Emphasis:  Amplifies the input audio, with emphasis to the high frequencies.  Because of the connection to the -15V rail, though, the output looks like it would be slammed to +15V, regardless of the input.  I don't think that the input audio would ever be strong enough to pull the output of this off the +15V rail.  What is it doing? <Edit this part is wrong...see the comments below...the amplifier is not railing.  The schematic is wrong...the 4.7K resistor is actually 4.7M, which makes a big difference.>

(2)  Half-Wave Rectify with Low-Pass Filter:  If it weren't for the fact that the signal coming from the preceding block is railed at +15V, I would say that this block was an envelope follower, being composed of something that looks kinda like a half-wave  rectifier followed by a low-pass filter.  Because the input signal is railed at +15V, I don't know what this circuit is doing.

(3) Current Drive:  The output of the rectifier is controlling a BJT transistor.  Since the transistor is not configured like a typical voltage amplifier, I'm guessing that the transistor is controlling current flow.  I guess that it is setting up a current drive that will eventually control the VCA.

(4) Switch for the Current Control:  This block is a bit of mystery to me.  It appears to control the current flowing to the VCA.  Is it some sort of log or anti-log converter?  I don't know.  Is it merely acting as a switch to allow one behavior when effects are on versus when effects are off?  (Note that the input labeled "OFF" is at +15V when the Polysix effects are off and it is at -15V when any of the Polysix effects are on.)  What is it doing?

(5) Voltage Controlled Amplifier:   This appears to be a fairly standard (?) VCA circuit using a LM13600 transconductance amplifier.  The VCA is controlling the amplitude of the input audio (either the audio output by the effects section or the output of the dry un-effected signal).  There are a few 13600-based VCA circuits in the Polysix.  What I'm a little surprised about is the fact that this circuit uses both halves of the 13600.  Why both?  In earlier parts of the synth, it looked like one-half of the 13600 was sufficient.  What is this trying to do that it requires both halves?  <EDIT: This part is wrong.  It is not a VCA, but a VCF.  See the comments below and see this follow-up post>

If it weren't for the saturation against the +15V rail in the first element of this schematic, I would have said that this was a circuit that sensed the amplitude of the input audio and amplified itself in proportion to its amplitude.  Maybe even with the +15V saturation it is magically serving this function (though I don't understand how).  It appears to make the overall output louder when the input is louder.  Why?

Looking at it the other way, the circuit might make the overall output quieter when the input is quieter.  That sounds like a noise gate.  Is that what this is doing?

I'd love your thoughts...maybe I'm totally missing the point of this circuit...


  1. 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?



  2. By examining my circuit, I just found that R107 is actually 4.7M, not the 4.7K value shown in the schematic. This makes a pretty big difference! Now, that first gain stage won't rail at 15V. The circuit operates without going into saturation. It makes more sense now!

  3. On the Polysix Yahoo Groups, Johannes posted this very informative reply. I hope that he doesn't mind, but I'm posting it here for my own record keeping (when I want to find this later, it's a lot easier for me to find it on my own blog than to search the Yahoo Groups archive)...



    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...