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|
(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...