AM904C Filter Coupler

8 HP Module | AM904C Filter Coupler | User Manual (PDF) | Modular Grid

OVERVIEW This is a recreation of the Moog 904C Filter Coupler module. Klaus Schulze frequently used this module to create band pass and band reject sounds and you can see it patched into Band Reject mode in live shows from 1975 – 1979. 

DESCRIPTION The Moog 904C module dates back to the Moog IIIp/c of 1966, and provides  routing and mixing of the 904A and 904B filter modules, so that they can be put into series as a Band Pass filter or in parallel as a Band Reject (also called Notch) filter without re-patching.

The AM904C closely follows the original design but it has been updated to use a 3-way rotary switch that selects signals via 9x analog switches, rather than the massive 9-pole 3-way switch Moog had to use. The AC coupled transistor based mixer for mixing the high and low pass filter outputs has been retained.

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 SEM like filter only has frequency).

The signal level of the 904C has been boosted in level by x2 to provider a hotter signal. This is because band pass and reject modes typically reduce the overall volume of the input signal. This is a simple Op Amp circuit which is totally transparent to the audio path.

The AM904C connects to the Behringer 904A, 904B modules using an internal 6-way ribbon cable. Small daughter boards attach to the rear of the Behringer modules, which the customer needs to install along with soldering 3 wires to the back of each module.

The AM992 Control module can be optionally attached to the same 6-way cable, this reduces the need for patching CV's into the 904A and 904B. 

CONNECTIONS There are five 3.5 mm jack sockets on the front panel: 

• Audio input
• Audio output
• 2x Frequency Cutoff CV Inputs (0 to +6V)
• 1x Bandwidth CV Input (0 to +6V)

The module package includes:

• AM904C 8HP Eurorack Module
• AM904A Daughter Board
• AM904B Daughter Board
• 6-way ribbon cable
• 6x M3 x 6mm Hex plastic standoffs
• 4x black M3 x 8mm mounting screws and washers

MODULE STATUS: PRODUCTION