Korg-35, first try

[jhaible]

Hi again,

normally I don't publish untested circuits, but the discovery of the
(possible) working principle of the Korg-35 VCF got me excited.

I added a pnp emitter follower to the 2-transistor circuit for 1st
order temperature compensation and put it into a simulation
with the external components of the MS-20 LPF.
Simulation results are encouraging, both AC and transient.
I could even verify the use of the 1n cap across the 5k6
feedback resistor: It compensates for the limited bandwidth of
the 4556 opamp. With a TL071, resonance would be increased
at high cutoff frequency, with a (slower) LF353 it's nearly even.

Here comes the circuit and the AC plot. I won't find the time 
for breadboarding it next week, because I'll be on hollidays in
London. Maybe someone else is as curious as I am ... (;->)

V1 is signal input, V3 is CV input (-15 ... 0V)

JH.

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