[sdiy] A diode-ring SVF

[Aaron Lanterman]

On Tue, 2 May 2006, [ISO-8859-1] René Schmitz wrote:

> Watch out for the biassing, the compliance of the -Icon is ~0V, so there is 
> not enough room for the lower diodes, since their other end is at a virtual 
> GND. Either rebias the filter core upawards so that the virtual GND is 
> somewhat positive. Or rework the current sink so that it sinks to -Vcc 
> instead of GND.

Rene, thanks for your help - they wound up putting an op amp in front 
that just added a bias onto the input, and then putting a DC block on the 
output, as the efforts to rework the current sink failed (mostly because 
their professor doesn't actually understand transistors very well.)

The diode bridge is really interesting.

They found they got a usable range of 100 kh to 1 Mh, by changing the 
voltage on the base of the current sinking transistors from 0.4 to 0.6 
volts, and then they played with various cap values until the found 
something that seemed cool.

The filter is insane. It's just a one-pole, but it's so interesting 
sounding, and they spent so much time getting it to work, I told them to 
go with that.

I never thought a one-pole could sound so interesting. Put a sinewave 
through it. Turn down the resonance. Does it turn down the volume of the 
sinewave? Well sort of, but it also turned it into an asymmetric triangle, 
then a triangle, and then a weird piece wise thing. Put in a sawtooth? 
Lord, what insanity! Square wave? All sorts of strange piecewise linear 
looking waves come out as you change the feedback. And that's just with a 
single pole.

This is consistent with what people tell me about the GX-1/MOTM-485 when 
you look at it on a scope...

- Aaron