904C Filter Coupler

Moog 904C

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.

The Behringer System 55 provides a great basis for developing a replica of the Klaus Schulze “Big Moog”, which was a IIIP manufactured in 1971. There are some modules missing in the Behringer catalog, such as the 904C Filter Coupler, so I decided to make one!

Moog 904c Filter Coupler This module dates back to the Moog IIIP of 1966, and provides passive routing and mixing of the 904A and 904B, so that you can switch them into Band Pass or Band Reject mode without re-patching. The original Moog circuit uses transistor based audio and CV mixers, and manual switching.

Klaus frequently used this module and you can see it patched into Band Reject in live shows.

AMSynths 904C PCB

AMSynths 904C I have used modern Op Amps for the signal and CV routing, and the massive 9 pole 3 way switch (!) has been replaced with a set of high quality analog switches and a 3 way 2 pole switch. I have designed a front panel and new PCB set, I have used THD components for an easier build.

I located some nice 14mm diameter control knobs which are not Moog style but close. It is also possible to reuse Behringer knobs by using a 995 module as a donor, but we need 28 knobs. The 904C module connects to both the CV and audio inputs and outputs of the 904A and 904B, and this can be achieved behind the panels.

The inputs and outputs of these two Behringer modules are NOT normalised (to GND), so we can connect signals to the jack sockets without having to cut any PCB traces. I have designed a daughter board for the 904A, which also holds a new set of polypropylene capacitors, and the PCB is used for the wiring into the 904B as well. These two daughter boards are wired using Molex connectors to the 904C.

Outcome The PCB’s were ordered in October 2020.

Copyright AMSynths 2019