[sdiy] Tau VCO question

[nicolas]

Q5 and Q6 look like they are perhaps trying to be a back to back zener pair, but wired a bit different from usual.  If they are not well matched it would lead to the asymmetry you see.  Perhaps try replacing them with real zener diodes (of around 6V maybe) and see if it behaves better.  Or just try and get some transistors that are a good match for each other (and the originals).

Nicolas


--- On Thu, 21/4/11, lanterma at ece.gatech.edu <lanterma at ece.gatech.edu> wrote:

> From: lanterma at ece.gatech.edu <lanterma at ece.gatech.edu>
> Subject: [sdiy] Tau VCO question
> To: synth-diy at dropmix.xs4all.nl
> Received: Thursday, 21 April, 2011, 8:17 PM
> Thanks to the many folks who
> responded to our questions about the  
> MS-20 VCO!
> 
> Another of my students built the Tau VCO (using typical
> TL08x op amps  
> instead of the ones shown).
> 
> www.oldcrows.net/~patchell/TAU1010/tau.html
> 
> This one oscillates! It gives nice triangle and square
> waves.
> 
> The one curiosity is the voltage swings - the triangle wave
> is shown  
> on yh schematic as going from -5 to 5 volts, but on the
> scope it goes  
> more like -2 volts to a little under 6 volts. And the
> square wave is  
> like -13 to +14 volts.
> 
> Of course we could throw on some DC blocking caps at the
> output bur  
> we'd like to understand what is going on better. Anything
> come to mind?
> 
> Thanks for y'alls help, as always! :)
> 
> Sent from my iPhone
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