[sdiy] VCA distortion - bad

[Grant Richter]

> I have not been able to get rid of the distortion entirely. I have
> determined that the 3080 is definately the cause. here's something
> interesting, I changed the input resistor back to it's original value -100K
> and found that now the signal is only distorted when the CV is more than
> half maxed. attenuating the CV level only results in a weak (with
> distortion) or no output from the VCA. I suspect the problem lies in the
> Iabc, too much current. I think Scott mentioned that earlier. however I
> cannot seem to correct it by changing Rf. In Hutchins' design he has the
> negative pin of the 3080 (pin 4) going to the CV buffer amp (I think that's
> correct) output. then there is a diode from the buffer output to it's
> inverting input and another diode from that point to pin 5 on the 3080. is
> there a way to limit the current through these diodes?
> 

Referring to EN#63 page 5 Figure 5.

PC-1 the "Initial Amplitude" control should be from ground to +15, not -15
to +15.

All later designs show two diodes in series with pin 5 of the 3080.

R1 the 100K resistor to pin 2 of the op-amp (not OTA) should be changed to
200K for a 10 volt envelope.

If using a wide band op-amp (like TL07x) R4 (560K) may need 10pF to 33 pF
across it to suppress oscillation.

Using 220K for R7 with 560K for R4 gives a gain of 1.25 with PC-1 set to 15
volts. Increase R7 if you need unity gain.