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

[David G. Dixon]

Aaron,

I haven't had a chance to analyze your circuit in detail.  I might get to it
over the next few days (I've got a circuit of my own to breadboard tonight).
However, I have a couple of questions:

1.  I really like your schematic.  What program did you use for this?

2.  Have you simulated the circuit?  If so, awesome.  If not, then I might
do it myself.  Do you have access to Multisim 10?


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Aaron Lanterman
> Sent: Monday, July 20, 2009 6:54 PM
> To: Synth DIY
> Subject: [sdiy] Aaron's take on the Buchla 258 (rough draft)
> 
> 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_2
> 00.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
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