[sdiy] 2164 Korgasmatron VCF

[Harry Bissell]

Zener diodes in an inverting feedback loop will clamp, not give
unity gain. They can essentially carry an unlimited amount of current
and so they completely oppose any change in current at the input. Of course
they will have ~some~ variation as the zener knee voltage changes.

H^) harry


----- Original Message -----
From: Stewart Pye <stewpye at optusnet.com.au>
To: David G. Dixon <dixon at interchange.ubc.ca>
Cc: 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Sun, 08 Aug 2010 04:05:56 -0400 (EDT)
Subject: Re: [sdiy] 2164 Korgasmatron VCF

Hi David,

Hopefully it will sound good!

Are the 3.3V zeners still in the feedback loop of the resonance amps?
 From my understanding once the threshold voltage of the diodes is 
reached the amp becomes unity gain, not clamping the signal as it would 
if the diodes were going to ground or in the feedback loop of an 
inverting op amp stage.


Regards,
Stewart.


David G. Dixon wrote:
>> Does your 2164 VCF sound like the LM13600 based Korg circuit?
>>     
>
> Hey, Stewart, I'll let you know as soon as I build it!  I'm planning on
> getting it done Monday night.
>
> It is essentially the same topology, only done with 2164 VCAs and
> integrators rather than 13600 OTAs and buffers.  This required some fresh
> thinking, and the actual schematic doesn't look very similar to the original
> at all.  However, the transfer functions are identical.  Also, I've retooled
> the resonance sections.  The originals had anti-parallel triple diodes in
> the feedback loops of the resonance amps.  I replaced these with
> back-to-back 3.3V zeners and panel pots.  These should give sine waves at
> self-oscillation which can be varied from about 3V to 15V (peak-to-peak).
> Also, at high resonance, one should be able to vary the volume of the
> ringing relative to the input signal.  I don't know how this will actually
> sound, but it's another parameter to play with.
>
> And, of course, the filter should be very quiet, with very low CV
> bleedthrough.  I'm hoping it's going to be better than the original, but
> then again it might totally blow.  We shall see!
>
>
>   

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-- 
Harry Bissell & Nora Abdullah 4eva