korg-35 apparently works

[tomg]

----------
> It works.
> 
> Couldn't await it, went to the breadboard right after I sent that mail.
> I made minor changes (forgot the 33u at the output, and a 4M7
> resistor parallel to opamp input).
> Test was very short so far, but it sounds very much like a MS-20
> filter. Typical interaction of self oscillation with high level input
> signal.
> Bingo !
> 
> It cannot be the whole Korg-35 chip, though. 
> PS-3200 uses it without external opamp, but still has resonance.
> Must at least have an internal buffer.
> 
> Someone prevent me from dreaming of a PS clone now !!
> 
> JH.

Yes indeed it does. I cloned your clone just to see. I though the diode
pair in the feedback loop caused too much distortion so I put in 6 like the
ms20. This raised the gain of the opamp so I compensated by lowering the
270K feedback resistor. It cleaned up nicely. but then the output was too
high and it started oscillating at about 10% res so I added a 56K resistor
to the loop. I changed a few other resistors to tighten up the response,
moved the res pot to the outside of the coupling cap, and added a 100K bias
resistor to the non-inverting input of the op-amp.

It sounds close to the 13600 version, but doesn't have quite the depth and
the self-oscillation doesn't sound quite the same. I guess I should put a
scope on it and look... tomorrow and maybe a short sample too. I though
there was something wrong with it but it turned out to be interference from
the TV set on my bench, it appears to be a lot more susceptible to RF. It
works! Good job! JH and while I'm at it.. Thank you for all of your
efforts.

http://mindspring.com/~vco/misc/pics/ms35_vcf.gif

-tg