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.

The initial design approach was to use the Behringer 904A and B but this requires kludging into the audio and CV inputs which is not viable for customers, wiring into the control node is really needed which means soldering wires to SMD components on the Behringer boards.

Redesign In late 2021 I decided to build 16HP AMSynths replicas of the 904A and 904B, and design a 904C that can plug into these module using a rear mounted IDC ribbon cable. This provides a customer friendly solution and enables me to design the filters with THD components and the original capacitor types and values. The modules are wider but also look more original.

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