995A Reversible Attenuators

 

My latest module is a Eurorack implementation of the rare Moog 995A reversible attenuators, or “attenuverters” in modern terminology. This was suggested by a customer.

As far as I am aware, this was produced as a “special” for the Keith Emerson Moog Modular, and therefore only included in the recent "Emerson" reproductions.

The module uses my interpretation of the Moog op-amp based attenuverter circuit based on the  CP3A PCB.

912 Envelope Follower development

 

Debugging of the 912 envelope follower is proceeding. I found a couple of minor errors in my PCB design (missing tracks!) that are easily corrected.

Once adjusted using the published Moog procedure, it seems to work OK - mostly.

The one issue I need to resolve is that althought the control output is fine down to 1mV RMS input signal, it becomes unstable when there is no input signal present.

 

Below are a couple of oscilloscope traces of the "control output" (upper) when the input is a repeating 250Hz sine wave envelope (lower).

 

I believe there is an error in the published Moog Schematic - the CL-1 diode in the trigger generator is the wrong way round ? With the schematic, the diode simply provides a path from +12V to 0V via R9A (330 ohm) - R9A just gets hot!

I believe the diode needs to be reversed, this is supported by pictures of actual Moog 912 units, where the diode cathode is clearly connected to R9A.