[sdiy] Later MS-20 filter with 2164

[Antti Huovilainen]

On Tue, 29 Jun 2010, David G. Dixon wrote:

> I think I've figured out how to clone the later MS-20 filter (the 
> OTA-based one on the famous "KLM-307" board) with 2164 VCAs (and thus, 
> no expo converters).  The parts count is very similar to the original: 4 
> VCAs (supplied by one 2164 chip) and 6 opamps (4 for the integrators 
> (yes, integrators!), and 2 for the feedback amps, which are inverting in 
> my design).

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

> The only difference is that the output signal is inverted relative to that
> of the original filter.  I don't think that this matters at all, but I
> wanted to hear your opinions.  It would be easy enough to add an inverter
> between the two filters, although this would necessitate another chip, which
> I'd like to avoid.
>
> Another thing about this design is that the capacitors are no longer 
> floating, which should obviate the need for all those pulldown resistors 
> in the original, as well as the ac coupling cap between the two filters, 
> and probably much of the output circuitry as well (I'm guessing).  And,

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'd like to breadboard (or just etch a PCB and build) this design, but I 
> won't be able to for at least two weeks.  When I do, I'll be looking for 
> a volunteer with an MS-20 to do some comparison tests.

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.

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 ... ;)

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova