[sdiy] Korg Volca Bass service manual

[Gordonjcp]

On Sun, Nov 01, 2026 at 05:55:44PM -0500, Aaron B. via Synth-diy wrote:
> On Tue, 27 Oct 2026 09:25:53 +0000
> "synth at schmitzbits.de" <synth at schmitzbits.de> wrote:
> 
> > The VCF is derived from the Korg 700 AFAIK. It's also similar in construction to the MS50 Filter.
> 
> Is there an analysis online anywhere explaining how this filter works? Having trouble wrapping my head around it.

It's just a Sallen-Key filter, same as in an MS20 or Steiner Synthacon. In fact, it's more like a Steiner Synthacon filter because it uses a balanced pair of diodes in each leg of the "variable resistor", driven with opposing bias voltages.

It looks funny because the diodes are drawn as a bridge rectifier. As an aside, the one in the VCA is meant to be the same except the other way up but the bridge is drawn wrong.

IC7a and b provide the opposing bias currents - IC7a is the control mixer and IC7b inverts it. The diodes on pins 3 and 4 and 1 and 9 provide the exponential part.

Then the diodes between pins 5, 6, and 8 have the input signal stuffed in on pin 6, and form the first "resistor" of the filter, and 5, 2, and 8 the second. You can see how feedback is applied to both pairs of diodes via C11 and C12 - this is the feedback tuning cap in a normal fixed S-K filter - and the output from pin 2 of the diode bridge has C13 to ground and off to the non-inverting amp.

There's obviously the diode chains to clip the output to something sane, and the gain programming resistors and capacitor R44/R45/C14, and a bit of business around the feedback pot to provide a "reverse log" taper to slow it down at the upper end. Bit of high frequency correction and some cleverness to keep the gain roughly constant as feedback increases, and there you go.

They sound pretty good.

-- 
Gordonjcp