[sdiy] A diode-ring SVF

[harry bissell]

when I was in college (and they did NOT have synth classes :^)

I wanted to build a 'phase shifter' ... I found the basic opamp stages 
in an engineering
text. It did not work... (It did not mention you need to sum the wet/dry 
mix :^)

I found that my simple lag circuit based on the stage gave a pleasant 
sound with audio through it
... a single stage filter that worked as an accidental resonant bandpass 
filter and nailed the sound
of the current (then) Pink Floyd album "Wish you were Here"... (the part 
with the guitar intro to the title tune)

Lucky accidents are a BIG part of progress !!!

H^) harry


Aaron Lanterman wrote:
>
> 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
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