TCP3A Control Panel Mixer and Control Voltage Router Clone


This is my first clone of a so called control panel module of a System 55. In System 55 modulars these units have a 2/3rd front panel height and a quad width (4 times the single standard width of a moog module).
The control panel modules often have interfacing functions. There are trigger converter, audio signal detectors and mixer modules like the TCP3A.

The TCP3A consists of two independant subunits:

The first one works as control voltage router which connects selected control voltages to cv inputs of one (or more) oscillators. Three of the selectable cvs are internally wired, the fourth one can be acessed and attenuated by the front panel. The control voltages are selected by switches on the front panel. A selected cv is marked by a lighted LED. One can select more than one cv at a time. The selected cvs are mixed and provided at an internal terminal to be wired internally to an oscillator.
Sources of cvs can be a keyboard, a sequencer, a ribbon conroller or whatever. Advantage of this form of cv connection to an oscillator is saving external patch cords and patch activities.

The second unit is a 4-channel audio / cv - mono mixer and amplifier. It can be used as a kind of final output stage of a modular system, as long as no stereo effects like reverb or other effect modules with stereo output are in use. The output is provided two times, one as phase inverted signal.
Other Functions of the mixer section are the adapter connections between 6.3mm and 3.5mm connectors. These enable the 3.5mm world of my Formant modular and the 6.3mm world of the System 55 to work together :-)

Circuit: (Please refer to the original Moog documentation for the circuit diagram)

So this is really nothing special. The three internally wired cvs are connected to the dual switches 1 - 3 of the front panel. The cv 4 plugged in at the front panel is leveled by the attenuator and connected to the dual switch 4. If switched on the coresponding cvs are mixed by a passive (and incomplete) 100k resistor network and provided at the internal cv output terminal. The missing 100k resistor on output side has to be the input resistor of the cv input of the connected oscillator(s).
The other switch functions of the dual switches switch on the corresponding LEDs. The LEDs are current limited by 1k resistors.

The mixer unit works with a dual opamp. The leveled input signals are mixed by a 22k resistor network and one half of the dual opamp. The feedback resistor of 47k does an amplifiaction of factor 2. The (inverted) output (OUT -) is leveled by the GAIN pot and inverted again afterwards by the second half of the dual opamp (OUT +).
Two multiturn trim potentiometers add small voltages to OUT - and OUT + for zero output adjustment.

The 6.3mm <=> 3.5mm interfaces are just sockets which are hard wired at the backside of the front panel. Normally I don't do this, but in this case there is really no need for wires and board connections.

Please refer to the original schemos of the Moog documentation for further details.

Part replacements:

The following parts I HAD to replace (see Circuit changes section below for additional changes I WANTED to do :-)): Circuit changes:

This time I changed several things in my clone:


The human interface of the CP3A shows the partitioning in two subunits: The control voltage router (CONTROLS TO OSCILLATOR) with EXTERNAL INPUT socket snd ATTENUATOR pot and the 4 channel mixer with MASTER GAIN and the phase inverted outputs OUT + and OUT -.
In the center one can find the INTERFACE sockets which are adapters between 6.3 and 3.5 mm plugs.

Board of the TCP3A:

  • Board dimensions: cute little 95 x 50 mm (Well, just an opamp and some extra parts, so what did you expect :-))
  • Upper side: Terminal strip for internal connections, cv router connections, opamp of mixer section
  • Lower side: 4 channel mixer

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Complete module:

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As you can see the board has to be mounted separately in the cabinet and does not have any fix connection with the front panel like the other modules have. This is typical for System 55 instruments, where the control panel modules are mounted on the cabinet bottom.
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Update 25.09.2021:

As I sold two of my CP3A clones, I decided to clone its predecessor, the CP3 (without an "A") as well, which contains a mixer section as pure transistor solution, which makes the module IC free totally. I kept the CV routing of the TCP3A though, so I just replaced the mixer part. And what do we see: this solution also works perfectly, the module sounds good, and I can add another hack to my tally stick of Moog Modular modules to clone.

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Four transistors, some resistors and capacities build this mixer, not more. The rest is IO and CV routing.


This is the newly drawn schematic of the original circuit of the Norlin service manual. It works typical for Moog Modular modules with 12V/-6V and amplifies with about factor 2. The attenuated input signals are processed by two difference amplifiers, consisting of an outer NPN stage and an inner PNP stage. This form of a transistor mixer can be found within several Moog modules. With T1 a null offset can be done. C1 suppresses an (extraordinary!) affinity to oscillation of the amplifier. The amplified output signal is provided in phase at the "+ OUTPUT" and 180 degrees shifted at the "- OUTPUT" connectors.

Please send questions or remarks to:
Carsten Toensmann


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