True CP3A Mixer


Moog CP3A

Overview The Behringer CP35A-M is actually a clone of the CP3 mixer from the Moog IIIP, an all transistor design from the 1960’s. This is a great design which can add some colour to the sound when levels are pushed up.

However I wanted one of my CP35A-M’s to be the “clean” version from the System 55 that used Op Amps. A clean (no distortion) version is useful for audio mixing but also for CV mixing, as there are no clamping of a CV signals at -6V. The Moog CP3A uses 15V power rails, providing tons of headroom.

CP3-A Mixer PCB

AMSynths CP3A I have converted a Behringer CP35A-M into a true CP3A System 55 mixer by designing a small daughter board that holds an Op Amp mixer with a gain of two. I have used a TL072 Op Amp and added bypass caps to ensure they don’t oscillate. You can also use the original LM1458 Op Amps or precision LT1013’s with no bypass caps.

Modifications

The PCB fits onto the rear of  Behringer PCB using PCB spacers and the original screws.  Some desoldering of SMD resistors and wiring changes are needed. Here is the installation process;

  • Build the AMSynths CP3A mixer.
  • Desolder the power socket on the Behringer PCB and discard.
  • Carefully desolder the dual 10KA GAIN pot.
  • Solder in a replacement 5KB single gang pot but bend pins 1 and 2 to a horizontal position.
  • Wire Pin 1 and 2 of this pot to the Pots connector on the PCB
  • Wire the two PCB Outputs to the two Pin 2’s on the Behringer PCB, this connects the output jacks.
  • Wire Pin 3 of the new GAIN pot to the GND pin on the new power connector.
  • Wire the new PCB Inputs 1-4 to Pin 2 of the four Input Pots.
  • Connect the new PCB to the rear of the Behringer PCB with 5mm PCB standoffs, reusing the screws.

Signal LED (Optional) I have also added a signal output level LED driven by an Op Amp buffer. The bi-polar green/red 3mm LED is mounted between the input pots and the output jacks as there are no PCB traces here to get in the way. A 5mm diameter hole needs drilling through the Behringer PCB and a 4.5mm hole needs to be drilled through the front panel.

I mark the position of the LED pins through to some masking tape on the rear of the panel and then drill a 1mm pilot hole from the rear of the panel. Then I drill the final holes. I have used a clear LED lens to cover the panel hole burring, for a neater appearance.

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