Mix and Attentuate


Introduction In August 2020 I bought a set of Behringer System 55 modules, as my first journey into Moog modular. My plan is to recreate the Klaus Schulze IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years.

Rather than rack up a set of Behringer modules and ride away making music, I thought it would be useful to explain how modules work, how to calibrate them, and how I have set them up and augmented them. I will also check their specifications, to see if they replicate the originals accurately and so I can understand them better.

Moog CP3A

Signal Control The orginal Moog modular design keeps mixing of audio and control signals largely away from the core VCO, VCA and VCF modules, and uses a set of separate mix and attenuate modules. Behringer have repliced these as;

  • CP3A-M – 4 Input Mixer
  • CP35 – 4x Attenuators
  • 995 – 3x Attentuators

Behringer CP3A – M This is the right hand side of the Moog CP3A, which is a four input mixer with a gain of two. What would appear as a simple module needs some explanation, and I have added a video from Learning Modular to help. Moog originally produced the CP3 mixer which uses transistors and a +12V and -6V rail. This version can overload with signals larger than 5V and is known for its warm distortion.

Moog created an updated version as the CP3A with a LM1458 Dual Op Amp and +/15V power rails, which you see in the System 55. This version has a lot of headroom for 10V p-t-p signals, can drive many modules and has a cleaner output at higher levels.

CP3 Transistor Pairs

The Behringer version is labelled as the CP3A and looking at the CP3A-M Quick Start Manual it seems like an active design consuming 30mA and having +14dBu input and output capability, which is over 10V. Opening up the module shows it does NOT have an Op Amp but two dual transistors (MMBT3904, MMBT3906) and a dual 10K pot for the MASTER level. This confirms the statements made in the Behringer CP3A intro video, so why not call it a CP3?

Once back in the rack I tested the clipping level of the mixer. Patching in a 921B VCO, which produces a 1.6V signal, there was no clipping and the maximum gain was x1.8. With a higher output signal of 5V from a Emu Systems VCO the mixer did start asymmetric clipping, once the input pot was increased above 7. The peak of the waveform is pulled up to +6v in the shape of a spike, adding high-end fizz to the sound.

There are two multi-turn trimmers on the rear of the PCB which can be adjusted by the customer but are not mentioned in the manual. There is a GAIN trimmer as well as an OFFSET trimmer, neither of which needed any adjustment from the factory alignment. I managed to reduce the offset from 40mV down to 20mV and the Gain seems to be pretty constant around x1.8.

Behringer 995

Behringer 995 This is a replica of the Moog module and consists of three “passive circuits”, each made of a 25k potentiometer between input and output. The signal input to the top attenuator is chained to the bottom two in series. Inserting a jack socket into the middle or bottom input breaks the normalizing system: 1 to 1-2-3, 1-2 to 2-3, 1-2-3 to 1-2-3.

The module has no setup, drops straight into your rack and requires no power and has no power connector! Here is the user manual if you need it 995 Quick Start Manual. The signal input to the top attenuator is chained to the two below in series. Inserting a jack socket into the middle input breaks the normalling system. Attenuators reduce the gain or amplitude of any applied input signal, control or audio.

Moving clockwise from zero gain to unity with input, these attenuators can be used for reducing the effect of a control upon a voltage controlled module, providing up to three variable outputs from a single source input or reducing the gain of an entire signal complex.

Behringer CP35 This module is a replica of the Moog CP35 and consists of four “passive circuits” with no transistors or Op Amps, very useful +/-6V reference voltages, and a couple of 4-way multiples. The Moog module has 4 trunk lines instead of the multiples, and as mentioned before Euro Rack does not have these internal connections, so they have been omitted from the Behringer module.

The module has no setup, drops straight into your rack and consumes just 1mA to power the reference voltages. The signal input to the left attenuator is chained to the right in series.Inserting a jack socket into the middle two or right input breaks the normalizing system.

Summary A nice set of utility modules which replicate the Moog System 55 versions very well. Do you need them? Yes you do need the mixer as soon as you get a few modules to play with, and the attenuators get useful as your system grows. For my Schulze modular there are 3x CP3A, 1x 995 and 1x CP35.

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