984 Four Channel Mixer


Moog 984

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 Moog IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years.

One of the modules missing from the Behringer System 55 range is the large Four Channel Mixer from the IIIP. I have recreated this module, as its an essential part and sound of the Klaus “Big Moog”, where it was used to mix and equalize the outputs from the rest of the IIIP. Moog originally created a two channel version, the 982 in 1965 but later dropped the 984 from the System 55.

AM984 PCB

Moog 984 The original Moog circuit dates back to the late 1960’s and is a very simple circuit that uses low noise transistors and active 12dB Bass and Treble boost controls. There are no Op Amps, as they were too expensive to use back then.

The circuit has a maximum gain of x2, but with the Bass and Treble boosted this can increase to x5. It is an audio signal mixer with any DC or CV signals blocked with capacitors. It is typically used as a final audio mixer, with a stereo output and a couple of effects looped in.

The four mixer stages (A to D) are arranged horizontally above each other. Each stage contains four level pots for the input lines (1 to 4) which can be mixed in each stage. Beside them there are two tone control pots (bass and treble) and an output volume pot. The output connectors are located at the bottom right of the front panel, the four inputs are located on the bottom left. This is a matrix mixer so that the four inputs can be combined and sent to any/all of the four outputs.

The column of pots above each input determines how much of that input is fed into each row of outputs. The mixer is great for EQ’ing audio signals but also for patching in external delay or reverb effects or an EMS 8 Octave Filter Bank.

AMSynths 984 This is a 38HP wide module, replicating the same Moog schematics but with modern low noise BC550C transistors and 1% metal film resistors. Matched Wima polypropylene capacitors are used in the 12dB filters, and high quality Nichicon DC blocking capacitors in the signal path.

I use the 984 as the final audio mixer in my Klaus Moog with outputs A and B used as stereo left and right, and outputs C and D used for the internal 905 Reverb module and an external Roland SDE1000 Digital Delay. There are 2 inputs from the final VCA’s and two from the FX. The front panel Outputs 1-4 are mirrored to four external balanced outputs (External Out 1 – 4) on 6.35mm jack sockets.

Outcomes The AMSynths prototype PCB was tested on 22 November 2020 and it works well, with the corrected resistor values. I was not happy with using the slim Lumberg jack sockets that I use in my 8HP modules (due to their small PCB footprint), so |I have reverted to standard euro rack jack sockets on my production module. Not black but more secure. I also added a 4 pin connector to drive the external jack sockets.

Copyright AMSynths 2019