Behringer 904B High Pass Filter

904B High Pass Filter

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.

Rather than rack up a set of Behringer modules and ride away making music, I thought it would be useful to explain how the modules work, how to calibrate them, and how I have set them up and augmented them. I will also check their specifications, to see if they replicate the originals accurately and so I can understand them better.

The 904B High Pass Filter The Moog 904B module dates back to 1966 and uses 4x transistor OTA’s to create a 24dB high pass filter, with two ranges. The schematics changes from the initial version to the later versions. The key changes were: simplified final audio amplifier design and core capacitor selection was moved from individual to parallel.

The Behringer module is a copy of the later design and uses good quality THD polyester capacitors in the filter stages, plus some SMD capacitors. The main filter capacitors are close in value to the Moog:

  • Low Range – Moog = 680nF,  Behringer = 680nF
  • High Range – Moog = 390nF,  Behringer = 400nF

The low range brings in the 680nF capacitors in parallel to the 390nF, and the 2nd and 4th pole also have another 22nF switched in. The 1966 version selected 1uF or 390nF individually for the low and high ranges.

904B New Caps

Setup The Behringer 904B has no user adjustable trimmers, but there are two trimmers mounted on the front of the PCB and are not accessible, unless you remove the front panel. There is a SCALE trimmer for adjusting the CV response to 1V/octave and an OUTPUT trimmer which adjusts the output offset, just like the original.

I have modified my 904B with matched 680nF polypropylene capacitors in the low range, and a pair of 180nF + 220nF polypropylene capacitors in parallel for the high range. Its a tight fit to get all the caps in, and the 180nF are mounted on the rear of the PCB.  There is a definite improvement in sound quality with the new caps.

AMSynths Version In 2021 I designed a 16HP Eurorack replica of the 904B that uses THD components, and the same Tropical Fish capacitors as the original. The design enables the 904C to be connected via a rear mounted IDC ribbon cable.

Copyright AMSynths 2019