Controller Modules

Moog CV Busses

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

CV & Gate Control The original Moog modular has a set of internal busses that are pre-wired between the back of all the modules, to reduce the amount of patch cables needed. In a System 55 there are three sets of CV signal (CV1, CV2, CV3) and three sets of Gate or  S-Triggers (ST1, ST2, ST3), and they are typically hooked up to external keyboards, ribbon controller or a sequencer. The Moog Control panel modules (e.g. CP3) below the main modules are used to route the CV signals into the VCO’s , a 992 module next to the VCF’s does a similar job and a 993 module does the same for the ADSR’s and VCA’s.

A standard Euro Rack system does not have these busses, so Behringer have implemented a set of utility modules that provide the CV control but have to be patched into a VCO or VCF with cables. In my Schulze modular I wanted to implement these busses behind the modules as hard wiring  to the back of the sockets, in the end I decided to make my own modules and replace the Behringer CP3-AO and 992 – see below.

However many of the Behringer module inputs have normalised jack sockets to ensure the input is grounded with no jack inserted. This normalisation is a link between 2 pins of the socket, with the trace on the top of the PCB underneath the jack socket. This means desoldering the jack sockets, which is quite hard to achieve, and needs a wide nozzle desoldering station.

Behringer CP3A-0

Behringer CP3A-O This controller module is the left hand section of a Moog CP3A Mixer, and you should buy the other half , as this is also an important Mixer module. It is one of the cheapest modules as it contains very simple circuitry, and the user manual is here CP3A-O Quick Start Manual. The module has individual inputs and switches for the three CV signals, and a 4th switch and attenuator for an external signal.

There are three parallel outputs for connecting to VCO’s, either a (921A Driver or full 921 VCO). You can drive three 921B VCO’s directly but you loose the advantage of the 921A. In my Schulze system I have a bank of 921A/B’s on an upper row and the CP3A-O and CP3A-M below.

CP3A-O Op Amp

The key technical measurement is how accurate the transfer function of the CP3A-O is. Does 1.00V at an input get sent back out at exactly the same voltage? If it doesn’t it may mean going back and tuning the VCO’s or worse still adjusting the scaling if the offset is non-linear.

Measuring my CP3A-O shows a -24mV to -30mV offset on the output, across 0 to +6V, which is a surprising result given the module uses a SGM8273-2XS8G precision Op Amp, with only 1mV input offset specification. That is a 2-3 cent variation in tuning which can be nulled by the 921A frequency pot. The scaling error is below 1 cent and therefore acceptable, but means the 921A range and octave adjustments should be set with the CP3A-O in the pitch CV signal chain.

Behringer 992

Behringer 992 This controller module is a replica of the full height Moog 992 module, which is very similar to the CP3A-O but designed to drive filter CV inputs and therefore does not have to be as accurate. The original is a passive mixer but Behringer have used an Op Amp and the module draws a modest 10 mA, The 992 user manual is here 992 Quick Start Manual in case in didn’t come in the box. There is no setup and the module goes straight into the rack next to the 904 voltage controlled filters.

With no internal CV buses in the Euro Rack system the 992 will need to be manually patched to the CV sources (keyboard, sequencer), and from its 3 parallel outputs to your filters. You can also use the 992 to select and drive CV signals into the 902 VCA’s, using the same approach. A modulation from a 921 VCO could drive many VCA’s, or the 960 sequencer controlling volume over 3 VCA’s.

The 992 has normalised jack socket inputs, which means it cannot be hard wired into a CV bus behind the panel, but the outputs are NOT normalised and can be hard wired into the LPF and/or HPF CV input socket.

AM996 Osc Mixer

Summary A set of two nice and simple controller modules for CV signals. Do you need them? No you don’t, for simple systems. Save the money and buy another filter or VCO. They make a lot more sense when you have multiple VCO’s and VCF’s so you can select control from 4 different sources easily in a complex system. I have installed them in my Schulze modular for each of the VCO banks and the filter bank. The downside is that the internal Moog patching has not been replicated by Behringer, which means lots of external patch cables.

AM992 and AM996 In 2022 I developed AMSynths versions of the 992 and CP3-AO modules with more accurate Op Amps, improved potentiometers, and a 6-pin IDC connector for connecting to an internal bus (“behind the module”) that contains three pairs of CV signal and V-trigger. This replicates the Moog pitch CV and S-Trigger bus that was wired to the rear card connectors. I have also added a 2-pin Molex connector so that the AM901A VCO driver can be internally wired to the AM996, saving another external patch cable.

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