Sunday, 10 January 2016

Buchlaized Korg Volca Bass

Some notes regarding incorporating a Korg Volca Bass into a Buchla 208 easel.
The aim is that the Easel's patch card provides all power, sync & CVs for the volca bass.
This is a progression from my earlier attempt of buchlaizing a Korg monotron Duo.
I'll update this as I do the mods.

Open from the back.

 Korg has as with the monotron labelled patch points.
VCO out, VCA out & VCF out.
Plus there are the outs for the Saw & square waves for all 3 VCOs
There are no schematics available. So there is a fair bit of circuit bending and guesswork here.
If someone out there finds the schematics could you please send me the link.
Maybe Korg will make them available over time.
A close up.

You will need to remove the battery compartment to fit the program card into the case.
And how do you sync both sequencers.???
This appears to be the point:
For it to work you need to fool the volca into thinking you have plugged in a sync cable.

These are my first patch points:
From left to right,
Orange: Sync in.
Blue: Ground
Red: filter (cut off) modulation in.
Green: Filter (Peak) mod in.

The filter cutoff seems to like the 208's random & envelope CVs.
The peak responds ok to the Envelope. Need to do more tests.
Fitting the card into the volca case .... very tight. Needed to remove pretty much everything including the speaker.... not a major loss.

The 4 resistors in the top left are simple voltage dividers.
The volca syncs nicely to 5V pulses and the Buchla outputs 10V pulses.
Though both sequencers do sync, the volca runs at twice the speed of the 208's sequencer.
One solution is to set the 208 to a 4 step sequence and use the 1st & 3rd pluses to clock the volca.

The components on the red circle are a simple DC mixer using a TL072  dual opamp.
The six 100k resistors are the inputs .

In the yellow circle are four 120k resistors for random 1 + 2 (black), sequencer (red), and envelope
generator (green) CV outputs .

Fitting the mixer into such a small box proved a challenge.
I've mounted the pots externally. The mixer is based around a Ken Stone DC mixer design.
DC mixer (CGS 04).

It's very simple - just 2 op-amps. The first opamp mixes the 6 input signals (inverting them in the process) and the second opamp reinverts them.

The mixer is being used for the 6 audio outs (3 from each VCO).
It seemed that the outputs were feeding back into the circuit so I added some diodes.

As I went along I discovered that the VCO, Filter and VCA outputs marked on the PBC also worked
quite nicely as modulation inputs.... So I decided to use these instead on the filter cutoff & peak inputs I marked out earlier (see above). Is this a mistake of the manufacturer? Are the outputs really inputs?

Useful links:
+ Ken Stone's DC mixer (CGS 04)

To be continued.


  1. I will just leave this here:
    It is volca bass service manual including schematics.

  2. Hi! Impressive work!
    I'm interested in just getting out the individual saws and squares and I'd like to understand how exactly you used the diodes to prevent the feedback. In fact if the Volca isn't playing a sequence, the oscillators output a cyclic repeating pattern