Buchla 192 Lowpass Filter

(click on image for larger view)

It is necessary to shield the photocells from stray light, otherwise the filter will not close down completely. Heat-shrink tubing works best for this (and was used in the original). The light bulb should be close to the photocell, but I left a very small gap (~ 1mm) so it isn't quite touching. I was afraid that the photocell would get too hot if there was a control voltage applied for a long time! After shrinking, the tubing seals everything so that is light-tight. The picture above shows the tubing before shrinking.

Here is the original 192 schematic (actually from the 410 at ebolatone, thanks Mike Peake!)	This is the same schematic with a regeneration, or resonance modification. The 10k resistor R7 is replace by a panel mounted linear 10k pot, the wiper of which goes to a resistor and capacitor (so as not to upset the DC bias on the base of Q1), and fed back to the base of Q1.

I built a 191 with the resonance modification on a home etched PCB, and mounted it behind a dotcom (MU) panel. One of these days I'll silkscreen a proper Buchla format panel for it.