[sdiy] Aaron's take on the Buchla 258 (rough draft)

[Aaron Lanterman]

Before trying to tackle the oscillators in the Music Easel, I thought  
it would be wise to tackle something simpler to get some experience. I  
thought the 258 would be a good thing to try, since many folks have  
tackled it successfully:

http://users.ece.gatech.edu/~lanterma/b208_draft_schem.png

This is based primarily on:

http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2580_1_200.jpg

With some understanding gained by the notations of one of Mark V's  
versions:

http://www.simple-answer.com/258mod.jpg

I made a few tweaks:

1) The original 258 has some resistors that you change or leave in or  
out depending on if you want a square wave or sawtooth wave. After  
studying the schematic a bit, I figured you could switch between the  
modes using a three-pole, double-throw switch (which are available),  
so I went ahead and set it up to work that way.

2) The buffer at the integration cap in the original is a FET/BJT  
combo. I replaced that with a TL082 in a straight noninverting  
follower configuration.

3) Cynthia's Buchla version of her Zeroscillator has separate "Don"  
and "Bob" CV inputs, which I though was an insanely cool idea. My  
implementation of it works by putting the trimpots on the input  
instead of in the feedback loop of the summing amp. (I'm not sure if  
that's wise or not, since it means that you're also sort of trimming  
the input impedance.)

4) I bring out the raw triangle output - seemed easy to do, just an  
output protection resistor and another jack.

5) I buffer the weird spiky waveform at emitter of the comparator  
transistor pair, under the theory that it might sound interesting on  
its own, and bring that out. Mark marks this as swinging from -2.7 to  
1.3. I buffer this with an op amp and also stuck in some other  
resistors to level shift it. (Note the resistor values there around  
that op amp are just place holders, to be changed once I figure out  
what values I need.)

6) The original 258 uses the unobtainium mu726. I adapted the expo  
converter from the Bergfotron VCO, which I think itself is an  
Electronotes/ASM/etc. NPN-pair-based variant. To this I added a pot- 
changeable exponential CV input in addition to the linear CV and  
trimmed inputs.

7) There's a weird resistor on the lower right of the Buchla schematic  
that says "230K to 470K" as needed, so I made this a resistor with a  
trim pot to avoid having to actually swap resistors in and out.

8) Boosted the output resistance of the main mix output to 1K.

9) Specified J201 JFETs since I happen to have a bunch on hand. No  
idea how well they will work, I guess that's what testing is for.

Two questions:

A) Any other cool but easy hacks come to mind that I should put in  
while I'm at it?

B) Anything I'm doing seem blatantly stupid? Any obvious errors jump  
out at anyone? ;)

- Aaron