AW: exponential VCO problems

[Haible Juergen]

	>It is more difficult than you think! The major problem is with
	>"motorboating"
	>oscillation, due to a low-frequency peak caused by the
interstage
	>coupling
	>capacitors. The only way to do it RELIABLY is with a
direct-coupled
	>circuit, probably
	>using tube op-amps. A basic state-variable filter would require
a
	>minimum of
	>8 tubes, 

How do you build a tube opamp?
I guess you'd either need a p-channel tube (only joking !), 
or some severe level shifting from the differential stage
to the output valve. How would this be done without silicon
(zener) diodes ?

Now, couldn't we build
the integrators of a state variable with a simple cascode
stage, with capacitive feedback ? 

	>more if tubes are used as voltage-variable transconductances.
	>This
	>is workable

I have no idea how to do this. Just out of curiosity: How would you
do it, and how many decades of cutoff frequency would you expect
to control that way ?

JH.

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