[sdiy] Buchla 295 10-band comb filter topology

[Donald Tillman]

> On Nov 22, 2022, at 10:23 AM, Donald Tillman <don at till.com> wrote:
> 
>> On Nov 22, 2022, at 2:01 AM, Lanterman, Aaron D via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
>> 
>> Does anyone recognize the topology Don is using here in the bandpass sections? 
>> http://fluxmonkey.com/historicBuchla/buchlaFiles/Buchla_2950_200.jpg <http://fluxmonkey.com/historicBuchla/buchlaFiles/Buchla_2950_200.jpg>
>> 
>> The low and high bands are 3-pole Sallen-Keys, but I’m not sure what’s going on in the middle filters.
> 
> 
> Hey Aaron,
> 
> It's a twin-tee filter, but connected in an unusual way.  
> 
> The input is directed into the twin-tee's "ground", and the twin-tee's normal notch filtering action is the opamp's feedback loop.

Next question; how does that work?

The opamp with a twin-tee notch in the feedback loop becomes a sharp peak filter.

The input signal comes into the underside of the twin-tee (which would normally be grounded) which doesn't seem to make sense.  But as far as the circuit goes, there's no ground reference other than the opamp's + input.  So the opamp + input is moving wrt to the input signal.

But why do that?

Well, if you had a standard inverting opamp configuration, the zero resistance summing node would load down the twin-tee.

And if you had a standard non-inverting opamp configuration, the gain could never go below unity.

So this is a way to get the non-inverting opamp configuration, and no load on the twin-tee, but with the + input still grounded, and thus no minimum gain of unity.

  -- Don
--
Donald Tillman, Palo Alto, California
https://www.till.com
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