AM904C Filter Coupler

One of the Moog modules that Klaus Schulze frequently used was the 904C Filter Coupler to create band pass and band reject sounds. You can see it patched into Band Reject mode in live shows from 1975 – 1979. AMSynths have recreated this module in Eurorack, so that you can build a Moog Modular III replica.

The Moog 904c Original This module dates back to the Moog IIIp and IIIc of 1966, and provides passive routing and transistor mixing of the 904A and 904B filter modules, so that you can switch them into series operation as a Band Pass filter or in parallel as a Band Reject (also called Notch) filter without re-patching. A 3-way switch selects between these two filter modes as well as having a central OFF position. The original Moog circuit uses transistor based CV mixing, and a massive 9-pole 3-way switch for audio switching.

There are rotary potentiometers for filter frequency and filter bandwidth, as well as 2x CV inputs for frequency and 1x CV input for bandwidth. There are signal input and outputs jack sockets with no level adjustment. The 904C requires delicate and specific use of the front panel controls to get the best out of it. The filter is unique as it has controls for both frequency and bandwidth, so that specific frequencies can be picked out (a typical 12dB BP filter only has frequency).

Early examples of the Moog 904C were constructed using plugboard and look like prototypes! They used large carbon and carbon composite resistors , as well as axial electrolytic capacitors.

The AM904C Back in 2020, the plan was to use modern Op Amps for the signal and CV routing, and analog switches to replace all of the massive 9-pole 3-way switch of the original. However the project was paused for a few years, whilst I located an excellent panel manufacturer (Meface) and designed a better module inter-connection approach than wired Molex connectors.

In late 2024 the AM904C design was revisited and ported across to SMD components, with a design that spans two PCB’s. The original transistor based “op amp” circuit (built from matched BC8X7 transistors) is matched with an easily available 2P3T Alps rotary switch. Analog switches replace the original 9P3T switch, which would be way too big and expensive for Eurorack!

An internal 6-way IDC ribbon cable connects the module to the Behringer 904A and 904B modules, with AMSynths Daughter Boards fitted to the rear. The new SMD assembly keeps the cost down and makes for a faster build.

The panel is a lovely black 2mm thick aluminum sheet, with graphics and side stripes etched into the surface (made by Meface). Very original looking!

Outcomes & Availability The new prototype was tested over the 2025 New Year holidays, and it works very well, making for some beautiful sounds. You can buy the AM904C module as two options;