[sdiy] Deciphering the Buchla 259's expo converter

Grant Richter grichter at asapnet.net
Tue Feb 5 18:36:54 CET 2008

Measurements from actual stuff.

Model 217: 4 Key section, bottom key = 0.290 volts, top key 14.75  
volts 17 key section, bottom key 0.287 volts, top key 14.8 volts.

Model 218 keyboard: Bottom Key = -0.118 Top Key 6.667 volts.

On Feb 5, 2008, at 1:03 AM, Aaron Lanterman wrote:

> Greetings! This is Aaron Lanterman, the GaTech professor that  
> taught a class called "Theory and Design of Music Synthesizers" a  
> couple of times a couple of times during 2006.
> I'm running the class again, somewhat modified, this semester,  
> under the name "Electronics for Music Synthesis." Alas, I don't  
> have the website for the class really set up yet. I will post here  
> when I've done so. In the meantime, I've been recording the  
> lectures with a cheap camcorder and posting them on blip.tv under  
> the username "abovenyquist." Stop by and enjoy. (It's blip.tv, not  
> blip.tv.com - I think you can get it from abovenyquist.blip.tv, or  
> go to blip.tv and search on the username abovenyquist.) You can  
> also find lectures from my Fall 2007 class, "Multicore and GPU  
> Programming for Video Games," if your interested in that sort of  
> thing - I've been spending many months knee deep in things like  
> shader programming and the Cell processor, which is why I'd dropped  
> out of looking at synths for a while.
> Anyway, last time I did the class, I analyzed the core of the  
> Buchla 259 in lecture as an example of a triangle-core VCO. I  
> talked about the discrete OTA, the comparitor, the integrator,  
> etc., and then handwaved and said "look, here's the expo generator."
> This time I'd like to actually do the expo generator. I'm pretty  
> well versed in Buchlaese by this point (and I teach Buchlaese, i.e.  
> how to read his schematics, to my students), but this one has me  
> confused.
> I'm looking here:
> http://rubidium.dyndns.org/~magnus/synths/companies/buchla/ 
> Buchla_2590_2_200.jpg
> So, I'm looking at R122, R123 and R123A. R122 is a 1K tempo, R123  
> appears to be 68K but it's hard to read from the schematic, and  
> R123A is 121K. R122 and R123 form a typical divide down ladder  
> before the base of an expo-converting differential pair. R123A is  
> tied up to +15 for reasons that befuddle me - I'm not sure why  
> you'd mix in something here instead of mixing it in at the  
> inverting op amp.
> Anyway, I'm guessing that 121K in parallel with 1K is sufficiently  
> close to 1K (i.e. it's 0.99K) that I could interpret the divide  
> down ladder as consisting of 1K and 68K, which gives me, if I want  
> to find the volts per octave,
> exp(-v*(1/69)/vt) = 2
> and assuming a vt of 26 mv, I get
> -v*(1/69) = 0.026*log(2)
> v = -1.2435
> The divide down ladder is fed by an inverting op amp, so I could  
> interpret this as being 1.2435 volts per octave. This, I suppose,  
> is pretty close to the 1.2 volts/octave I've heard that Buchla uses.
> But then the inverting op amp is confusing me. I see an octave  
> input - OK, I imagine thats from some kind of rotary switch, maybe  
> - that comes in through a 56K input resistor, and a "freq cv" input  
> that comes in through a 150K resistor. In any case, the feedback  
> resistor is 100K - so it doesn't seem there's any way I could  
> interpret either input voltage as giving 1.2 volts per octave. If I  
> saw something with a 100K input resistor that would make more sense  
> to me.
> Can someone help me with my confusion?  If I can't figure it out,  
> I'm going to handwave when I get to the expo converter part. (Last  
> week I showed how how expos work in a typical 1 volt/octave style  
> converter, as discussed on Rene Schmitz's website, but I wanted to  
> do another example of one since I didn't explain it very clearly. I  
> off my game that day.)
> - Aaron_______________________________________________
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