[sdiy] Unusual BP filter - Voltage Control

[jhaible at debitel.net]

> hey folks - i've used this filter in my studio for
> several years now. great sound. talking with eric
> i'm debating adding photocells for each gang
> of the pots (2 pots) and a some leds to do this.
> 
> anyone else have any better ideas?

No better idea. (Although I'd live to see a VCF built from
remote cutoff tubes connected a variable gm differential
amps - but *not* in this circuit. (;->) )

Just make sure that you match these LDRs for good tracking.
You might want to buy a bunch of complete Vactrol devices.
You still have to match these, but at least the geometry
won't change during the selction then.

Of course the ultimate "Vintage" solution would be using
an electroluminiscence foil instead of a LEDs.
(See http://www.oldcrows.net/~jhaible/opto_compressor/jh_opto_comp.html
for some inspiration.)
Remaining question is how to feed that EL element.
I doubt DC would be a good thing here. So either
go ultrasonic, or amplify and clip the filter input signal,
and make a VC clipper to shape the (ac) control voltage for
the EL element. You can do it all with tubes. 8-)

JH.


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