True CP3 Mixer

Moog CP3 Click Filter

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.

One feature missing from the Behringer module is the click filter. which I have added with a small toggle switch. It connects a 1uF Tropical Fish capacitor (just like the original) to the signal input from GND.

Switching this on removes downstream clicks from fast moving CV’s or acts as a low pass filter to cut the top end of audio signals. This is useful when the mixer is being used in feedback resonance circuits (like on the 904B).

The installation requires patching onto a 0402 SMD resistor (R18) which is a tricky soldering job. A piece of veroboard can be used as a transition from the switch wiring to R18, to avoid the attached wire ripping out the resistor.

CP3 Click Capacitor

Components and Installation You need a 1uF tropical fish capacitor in good NOS condition and a sub miniature SPDT toggle ON/OFF switch, I used one from Thonk here.

The fitting instructions are as follows:

  • Remove the knobs and unscrew the PCB from the panel.
  • Carefully drill two 2mm holes on the left side of the PCB for the large 1uF capacitor with 27mm lead spacing.
  • Mount and glue the capacitor to the rear of the PCB, with insulated plastic sleeve over the leads.
  • This is important as the PCB has ground planes that will short the capacitor leads.
  • Carefully drill a 5mm diameter hole in the front panel between the Input 3 and 4 jacks.
  • Bend the PCB tabs on the switch so they are at right angles pointing to the left.
  • Mount the SPDT toggle switch onto the panel, using the washer and nut.
  • Solder two thin flying wires from the center and lower pole position.
  • Connect the lower pole position to the lower end of the capacitor with a thin wire, allow lots of wire length.
  • Solder a thin wire from the switch center position, allow lots of wire length, to the top of R18.
  • I tied the wire in a knot over the capacitor lead to ensure the SMD capacitor is not ripped off.
  • Solder the upper capacitor lead to the left hand pin of VR4, front of the PCB.
  • Trim the capacitor leads and check the work.
  • Put the PCB back on the panel with the screws and knobs.

CP3 Click Switch

Outcome This is a simple but careful modification to the Behringer CP3A-M which makes it a “True CP3” from the Moog IIIP, and adds a useful original feature and expands the usefulness of the module. The low pass filter is noticeable as it cuts out the top end and is not a subtle effect.

Try using it in the feedback loop of a HP filter to provide resonance. I have modified three of my CP3’s and upgraded one to a True CP3A.

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