scream & pull hair out

[Paul Perry]

At 12:00 PM 16/04/99 -0700, Sean Costello  wrote:

>P.P.S. What do the diodes in the Q feedback loops of some state-variable
>filters do? 

The diodes have lower impedance at higher voltages (that is, a non-linear
current vs voltage curve, in this case exponential).
So, since they are part of the negative feedback path of the amplifier in
the Q feedback loop, as the signal feeding back increases, they let more
current through, DECREASING the gain of the amp in the feedback loop,
thereby stabilising the feedback path overall.

Sounds complicated, but just remember there are 2 feedback loops here:
the filter with its Q feedback amplifier, and also:
the amplifier in the Q feedbak section with the diodes.

As to how it sounds....
well there are a lot of variables to tweak, diodes to try etc, but in the
case of the MS20 type filter there is strong intermodulation in the 
diodes, most obvious when the system is actually oscillating.

paul perry Frostwave P/L Melbourne Australia
(manufacturer of the MS20 clone filter box)