[sdiy] Later MS-20 filter with 2164

[David G. Dixon]

Concerning this reply, I've sent Antti a little document with my filter
concept and corresponding transfer function analysis in it, along with basic
analyses of several simple OTA- and VCA-based filters and integrators.  If
anyone else wants it, please email me privately...

> What do you use the fourth VCA for? Or did you clone both highpass and
> lowpass filter?

I cloned the whole filter.  The feedback gains are not voltage-controlled
(as per the original) so no VCA is required for that.  There is one VCA for
each of the four filter stages.  I have not cloned the output circuitry of
the original.  I would take the filter outputs directly to jacks and use a
separate VCA module.

> The pulldown resistors to negative rail are there only to bias the emitter
> followers. The ac coupling cap at the output of hpf filter shouldn't
> be strictly necessary. My guess is that it's there only to remove CV
> feedthrough due to input offsets of the OTAs in the HPF. The only floating
> capacitor is the HPF input capacitor. All others have either direct or
> opamp buffered ground reference.

I'm not really following you here, as there are no emitter followers in the
schematic I'm working off of (unless you are talking about the situation
inside the OTAs themselves).

> The two transistor (Q3 & Q4 in my copy of klm-307.gif) discrete VCA is
> likely to affect the sound, so you'd need to tap the output of the LPF
> directly for a valid comparison.

I haven't used any discrete transistors; only 2164s.  And, as mentioned
above, I will not be reproducing the output circuitry of KLM-307. 

> BTW, you might be interested in the analysis of the filter I did as part
> of my masters thesis. See pages 28-33 (39-44 in pdf):
> http://users.tkk.fi/~ajhuovil/dippa/Huovilainen_MastersThesis.pdf
> 
> "Who Needs a Degree?" indeed ... ;)

I am very interested in it indeed!  I'll definitely have a close look at it
and then probably pester you with questions.