Moog Modular Routing

Inside a Moog CP4

Introduction The Moog Modular system has internal signal busses, which from 1968 onwards were controlled by utility modules, both full height like the 992 and 993, and half height like the CP3. These modules are very useful as they replaced external patch cables, provide switches for routing and potentiometers for mixing. The original control panel range of modules was reworked in the System 15, 35 and 55 as CPxA modules.

The Behringer System 55 includes some of these utility modules (992, CP3A-M and CPA-O) but there is no internal bus structure, and some Moog modules have not been replicated. The objective with  my “Big Moog” is to get as close to the Moog Modular workflow as possible, so I have designed a set of four AMSynths Eurorack modules, and an optional Bus Board, to fill these gaps and provide the much needed internal bus structure.

Moog Modular Wiring

CV & Gate Control The original Moog modular has a set of internal busses and external “trunks” that are pre-wired between the back of all the modules using edge connectors, 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 S-Triggers (ST1, ST2, ST3), They are typically connected to external keyboards, ribbon controller or a 960 sequencer.

I have replicated the Moog bus structure with a 6-pin cable that runs behind the modules, with IDC sockets on the new AMSynths utility modules. There are three sets of CV signal (CV1, CV2, CV3) and three sets of V-Triggers (VT1, VT2, VT3) which can externally patched into from Eurorack keyboards and sequencers (including the 960). The conversion of V to S-trigger for the Behringer 911 modules is done within the AMSynths AM-CP1 or AM-CP4 modules which are used to drive the bus from external signal sources. The receiving modules are the AMSynths AM-CP3 and AM992, which are used to connect VCO’s and VCF’s to the CV signals (CV1, CV2,CV3).

Trunking Explained

Trunk Lines A Moog Modular system has up to eight “trunk lines” which are simply wires from a front panel module (e.g. CP2, CP3, CP3A, CP35) to rear mounted 6.35mm mono jack sockets. They are not universally popular with owners but they enable connections to and from out board equipment (FX, Tape Recorders). They are usually marked as TRUNK LINES on the panels and are sometimes connected to multiples.

This feature was obviously left out of the Behringer CP3A-M (which is actually a replica of the earlier CP3 with transistors), as Eurorack has no rear jack sockets and customers are used to patching everything into the front of modular synths. The AMSynths AM-CP3 module does have “trunk line” capability, with two pairs of front panel jack sockets available via internal connectors to be wired to rear mounted 6.35mm jack sockets.

AM-990 This 4HP module is based on the Moog CP1 console panel, which provides front panel jack sockets connected into the Moog bus for three CV signals and three S-Triggers. The AM990 does the same job, with six jack sockets with LED indicators for V-trigger On. A 6-pin IDC socket on the back of the PCB enables connection with AMSynths utility modules, using a 6 way ribbon cable or via the AM900 BUS BOARD PCB.

Only one AM990 is needed in a Moog Modular Eurorack system, to bring the signals onto the 6-way IDC bus. If interconnection is needed with additional cabinets I recommend using a rear mounted 6-pin DIN socket and cable set, wired via a ribbon cable to the IDC bus on the AM900 BUS BOARD. The AMSynths utility modules take the three sets of CV and V-Trigger signals and apply them to VCO banks (AM996), VCF banks (AM992, AM-CP4) and VCA/Envelope Generator banks (AM993, AM-CP4).

AM-CP3 This 20HP module is a recreation of the mixer section of the Moog CP3 console panel, which is intended as a four channel mixer for VCO outputs. The oscillator CV input routing is provided by the AM992 module. The original CP3 was used in the Moog Modular I,II,III and then upgraded to the op amp based CP3A in the 15, 35 and 55 models.

The left hand side of the panel provides accurate CV selection and mixing for a bank of AM901A/B’s or Behringer 921A/B’s. The first three jack socket inputs are normalised to the internal IDC 6-pin bus whilst the fourth CV input goes via an attenuator potentiometer and is mixed into the other threes signals using a precision Op Amp. All four inputs have slide switches for selecting on/off with LED indication. The output of the mix is available as an internal connection for AM901A VCO Controllers and as a jack socket for Behringer 921A VCO Controllers.

The AM-CP3 has a faithful reproduction of the Moog transistor based four channel mixer, with input jack sockets at the top of the panel. The mixer has a master gain potentiometer and dual normal and inverted outputs. The click filter from the original has been retained, with an on/off slide switch and amber LED indication.

The front panel also has provision for Moog style “trunk lines” with two pairs of jack sockets, which can be connected from the back of the module to two rear facing 6.35mm jack sockets in the cabinet which the AM-CP3 is mounted in. The levels of the outgoing and incoming signals can be adjusted with potentiometers, which use the CP3A Op Amp circuit. There are also four jack sockets arranged as Multiples.

AM-CP4 This  16HP module is recreation of the Moog CP4 console panel with filter CV routing, and trigger & envelope control for two sets of adapted Behringer 911 and 902 modules. The CP4 was used in Moog 1C and IIP, and the nearly identical CP4A was used in the Moog 35. This handy module reduces external patching by providing CV and trigger routing via switches. with LED indicators. The AMSynths version contains dual V to S-Trigger converters, which are Eurorack compatible.

The left hand side of the panel provides filter CV routing with four independent inputs that can be switched on or off (red LED indication). The fourth input has an attenuator potentiometer and a green LED indicator. All inputs are mixed together at unity gain using a precision Op Amp and can be used to drive VCO’s as well as filters. The output CV can be manually patched into the CV input of a 904A or 904B filter module, to control frequency cutoff. The output is also available internally for patching to an AMSynths 904 or 904B filter module, which have internal inputs.

The right hand side of the panel provides trigger selection from two V-trigger inputs, which are converted to S-Triggers and internally patched to two 911 modules. At the top of the panel there are two toggle switches (red LED indication), to select the trigger signal inputs. The LED’s indicates the presence of a gate signal when in the switch is in the on position.

The left hand column of switches (orange LED indication) route the selected S-trigger signals to two modified Behringer 911 modules (LEFT and RIGHT). The right hand switches (with green LED’s) connect the DC control voltages from the 911’s to their respective 902 Voltage Controlled Amplifiers (LEFT and RIGHT).

AM992 This 8HP module replicates the Moog original with four front panel CV inputs, selected by slide switches with LED indicators. The first three inputs are connected to an internal IDC 6-pin bus and normalised to the front panel jack sockets. The fourth CV input goes via an attenuator potentiometer and is mixed into the other three signals using a precision Op Amp. The original Moog used a passive mix approach. The AM992 has replaced the Behringer 992 modules in my “Big Moog”.

AM993 Module

AM993 This 8HP module replicates the Moog original with eight slide switches and indicator LED’s. It is designed to work with a set of modified Behringer 911, 911A and 902 modules to provide signal selection and routing. An AMSynths AM997 provides the V-trigger to S-trigger conversion and is easily internally patched into the AM993. Daughter boards are provided to enable the supplied internal cable harness to be connected into the Behringer 911, 911A and 902 modules.

The Moog 993 was used in both the IIIP and System 55 to provide routing for a set of 3 VCA’s and Envelope Generators. In smaller system the Moog CP4 provides routing for two set of VCA’s and Envelope Generators and is replicated by the AM-CP4.

AM996 This 8HP module replicates the CV mixing section of the Moog CP3/CP3A with four front panel CV inputs, selected by slide switches with LED indicators. The first three inputs connected to an internal IDC 6-pin bus and normalised to the front panel jack sockets. The fourth CV input goes via an attenuator potentiometer and is mixed into the other threes signals using a precision Op Amp. The original Moog used a passive mix approach.

The AM996 has replaced the Behringer CP3A-O modules in my “Big Moog” and complements the Behringer CP3A-M modules.

Typical System Setups A smaller replica Moog modular will be based around one or two oscillator banks, a single filter and a pair of 902/911’s. The AM-CP4 is an ideal companion for this setup along with a AM-CP3. A larger system with 3 or more oscillator banks, more filters and three 902/911’s deserves multiple AM-CP3’s with both AM992 and AM993 modules. The AM-CP1 enables the bus to be expanded across multiple cabinets.

Availability This range of four new AMSynths utility modules (and internal AM900 BUS BOARD PCB, are available from our webstore, and the AMSynths 904A, 904B and 904C will be launched in 2022.