[sdiy] Henry Walmsley's page

[Ian Fritz]

>Walmsley's VCO current source is at the following link:
>
>http://homepage.ntlworld.com/henry01/waveform_synth/waveform02-02.pdf
>
>It is a standard matched-pair-and-tempco arrangement based on LM394.  From
>this one current source, he is tapping three separate currents for various
>uses:

Four, actually -- AP1 also feeds the HFT circuit.  Does anyone understand 
the purpose of doing the HFT the way he does?  It seems to me to do the 
same thing as the standard Franco compensation, which could easily be done 
with a resistor in series with the timing cap.


>"INT-CC" drives the interpolator, "HF-CC" drives the high-frequency
>clock oscillator (this is basically the "core" of this oscillator, as it is
>what defines the frequency of the VCO output), and "TRACK-CC" drives the
>cutoff frequency of the tracking filter.
>
>My inclination would be to replace this current source with the 2164 Expo,
>but probably leave the rest of the circuit more or less as is.  Of course, I
>can never leave well enough alone, so I'd probably simulate the whole mess
>and go looking for tweaks, but based on Mr Walmsley's write-up, he seems to
>have thought the circuit through exceedingly well, and the proof is in the
>scope pictures.

Since the VCO core is just used as a clock, it seems to me that it would 
make *much* more sense to use a F-V chip (charge balance oscillator) rather 
than a switched integrator.  I used the LM331 for my DoubleDeka VCO, and it 
has phenomenal tracking up to well over 100kHz, and rock steady temperature 
compensation using the Pease scheme.

For the OTA-based VCO it will be difficult to get the core frequency over 
about 40kHz with decent tracking -- and even that will take quite a bit of 
effort. That will leave you with a max frequency of ~2.5kHz.

As far as needing an interpolator, well I guess that's a matter of 
taste.  I actually like the steps -- it gives the sound its own special 
character, and it's easy enough to filter down to a smoother waveform.  (A 
single pole filter changes a step into a ramp, no?)

   Ian