[sdiy] Buchla's funky 4 pole

[mark verbos]

If anyone downloaded that, I just fixed an error. the FET is a P CHannel 
(U147) and I originally drew an N Channel. So make sure you have the 
correct version. sorry about the mix up.

Mark




mark verbos wrote:

> Hi,
> 
> I recently had the opportunity to work with a Buchla 191 Sharp Cuttoff 
> filter. This is a 4 pole lopass and Hipass in one module, with 
> facilities to run the two as separate filters or as one bandpass. The 
> Hipass section in particular sounds great, however I was not able to get 
> it to self oscillate, even with TONS of external feedback. My guess is 
> that it is a matching issue, but that is only semi-related to my question.
> 
> This configuration is unlike any other filter I've seen before. It is 
> totally discrete, each of four cascaded integrator stages is made up of 
> an NPN pair acting like diodes for the voltage control and 2 gain 
> stages, the first a FET/NPN follower and the second a discrete OP AMP 
> made from an NPN pair and a PNP (quite common in the Buchla 100).
> 
> I've drawn up one stage to check out here.
> http://www.simple-answer.com/191/Buchla-191-lopass-stage.jpg
> 
> the  functional block diagram is here.
> http://www.simple-answer.com/191/191-lopass-equiv.gif
> 
> the control ports (the top and bottom of the "diodes") are driven by 
> differential voltages.
> 
> I'm thinking of cloning this thing, but I'm wondering if it is necessary 
> to have all those trim pots. Particularly the ones to trim the 
> transistor pair connected as diodes. I mean, that pot runs thru a 22M 
> resistor! Were transistors MUCH more unmatched within a matched NPN pair 
> in the 60s, so much that they would no longer need trimming these days? 
> It is a TD101, which I have never heard of and can't find any info on. 
> Should I try it with plain ol' diodes? Did anyone else use diodes like 
> this? I know the Steiner filter uses diodes, but it's quite different. 
> Also, the Korg MS-50, but that's a diode ring.
> 
> 
> I think this thing is cool. but just because I have never seen something 
> before, does not mean that it is innovative. However, I figured you guys 
> would get a kick out of seeing this.
> 
> Mark
>