VCOs for home-built modulars - update

Haible_Juergen#Tel2743 HJ2743 at
Thu Aug 31 18:20:00 CEST 1995


     I got the Exar 4151's in the mail (thanks Ric!) and made a dual
     sawtooth VCO per the Chroma schematic. This dude has a sweep range
     from around 60 hertz (try to go lower and oscillation stops) to
     just above 30 khz. Waveshape ranges from a very jittery
     rounded-off rising saw at the low end, to a fairly normal-looking
     saw in mid-frequencies, to a half-sawtooth half-square-wave mutant
     thing around 15 khz, to a train of narrow pulses around 25-30 khz
     before they dwindle away into thin spikes.

     What do I think about this design? It's so similar to the
     Electronotes sawtooth VCO that the slightly lower parts count does
     not justify the drop in performance and waveshape. This VCO may be
     good enough for the bowels of a polysynth, but I'm after
     studio-quality modular specs here. Besides if I'm gonna spend the
     time, energy, and all that to sit down and burn my fingers with a
     soldering iron making some analog monster from hell, at least I
>     want a QUALITY WAVEFORM analog monster from hell.

What causes You to stay away in disgust might be the cause for
someone elses delight ...
My attraction is caught, for sure now. Owning lots of Quality Monsters,
I might like to add some zombies for special effects. Well, the Chroma
*has* a very special sound!

>     The 748 is an old, old, uncompensated 741. You may wonder what such
     an old non-ideal device is doing here but when you make a Schmidt
     trigger with hysteresis out of it I dare you to see the vertical
     edges on the square wave it produces with a scope. Once again here
     goes my personal belief that the extra extra huge slew rate which
     makes for the extra extra sharp edges on the square waves makes a
>     really profound hollow-sounding square wave. Just my opinion.

Good point. But might a 748 produce steeper edges than a common
comparator like the LM311 ?


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