MOTM

Found a nifty patch which will allow for variable resonance on the 
410 filters.  If this has been covered before in this forum, one 
thousand pardons.

In order to do this correctly, you'll need a voltage processor or 
attenuator that's not AC coupled which can pass AC signals and 
facilitate phase inversion of the signals passing through it.  The 
Blacet Mixer/Processor, Cynthia Octal Attenuator and Doepfer 
A-138 polarizing Mixer are perfect examples.  For ease of 
reference, these instruments (or any others you have that serve 
the same function) this will be referred to as the 'Processor' in 
the patch below:

Patch:

Signal into Processor channel 1

Processor out into 410 signal input

Take any single filter output (out 1, out 2 or out 3) of the 410 into 
Processor Channel 2 and to speakers.

By increasing the level pot of the Processor channel 2 in the 
positive direction (positive feedback), the set resonance will 
decrease.  By increasing in the negative direction (negative 
feedback), the resonance increases up to and into the poitn of 
self-oscillation.

To test this, pull out  the Processor channel 1 input , crank up the 
negative feedback and bang - you're there. My scope's not by my 
system right now, but it sounds like a clipped sine.  Pretty  
certain a resistor value change or two would easily remove this 
clipping.

Obviously putting a VCA in line with the feedback loop will allow 
for VC resonance.

You can also use the Mix out, but  things tend to get a bit 
comples, real fast.  ADDING the Mix Out into the Processor 
channel 3 along with a separate filter output seems a bit less 
chaotic and will yield some interesting harmonic node points 
when sweeping the 410 into higher resonance.

If you don't have an inverting mixer or processor BUT if your 410 
has been fitted with Tony Allgood's 410 companion circuitry, 
simply use a standard audio mixer and run the feedback through 
Tony's inverter that comes with that mod before it leads its way 
back into the 410.  This set up won't allow for a decrease of 
resonance, but it will let you increase it up to and past the point 
of self-oscillation.

Enjoy (or not!),

- P