[sdiy] CS-80 Sub Osc Speed modulation

[jhaible at debitel.net]

You never stop learning - last night I read a CS-80 review at
www.amazona.de , and suddenly learned that the smaller CS-synths
are missing another fine detail of the CS-80: Sub Osc (LFO) speed
modulation via aftertouch!

Surprised that I never had noticed this before, a question 
was immediatly arising: The CS-80 has polyphonic aftertouch, but
there is only one Sub Oscillator. So the CS-80 must also emulate
monophonic aftertouch internally. I consulted the schematics, and 
found that there is in fact a set of diodes which creates the
*maximum* of all individual pressure sensors, and uses this to
control the LFO speed. As expected.

But there's more, and this is where it may become interesting:
The diodes cause a voltage drop of approx. 0.5V, and there is
no compensation circuit for this. So I expect the LFO speed
modulation to come *later* (at a higher threshold) than VCF / VCA
modulation when you gradually increase the pressure on a key.
This would allow some brilliance and volume modulation by
aftertouch without LFO speed modulation at low pressure
values, and LFO speed modulation joining in at higher pressure
values. Which would be another very sensible choice for a great
performance-type instrument.

Can anybody confirm (or correct) this behavior?

Now I know another mod I have to do for my CS-50 (;->)

JH.

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