Behringer 904A Low 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.

Behringer Caps

The 904A Low Pass Filter The Moog low pass filter module dates back to 1966 and is probably the reason you want a Moog modular! The Behringer module is a faithful copy and uses THD polyester capacitors in the filter stages, rather than ceramic SMD. The original Moog 904A used Tropical Fish and axial polypropylene capacitors.

This is how the Behringer module compares in capacitor values, notice the 2 octave (x4) interval in the Moog;

  • Low Band – Moog 1u2F – Behringer 1uF
  • Med Band – Moog 300nF – Behringer 270nF
  • High Band – Moog 75nF – Behringer 68nF

Moog Filter Caps

If you go back to the 1967 Moog 904A module it has larger capacitor values; 1u6F, 400nF and 100nf, and therefore lower frequency ranges. Does this matter? No not really, the Behringer caps are 10 – 20% lower in value then the Moog, which means the cutoff frequencies are higher. The difference is that the Moog is going to get down to a 1 Hz cutoff whilst the Behringer might not.

Self Oscillation Results Putting the Behringer stock 904A into self oscillation gives these ranges:

  • 45 Hz to 5.3 kHz
  • 65 Hz to 21 kHz
  • 140 Hz to 85 kHz

The lower limit of self oscillation is a bit disappointing, maybe because of the lower 1uF value chosen.  The Behringer uses Sancon capacitors for power regulation and higher grade Jamicon for the 2 reservoir caps in the final transistor buffer (100uF and 470uF).

1974 Caps

904A 1974 Version Behringer have chosen capacitor values that are easy to locate, they approximate the x4 interval range (2 octaves), but they are not the same values as in the Moog original. I have replaced the stock caps with Kemet 1u2F, 2x 150nF and 75nF polyester capacitors to implement the 1970’s 904A Moog spec. They are drop in replacements, with the 2nd 150nF soldered on the rear of the PCB.

The caps have been matched to 1% which helps the resonance quality. The frequency range of oscillation reduced down as expected but the low limit of self oscillation remained at 45 Hz.

Moog 904A Blueprint

904A 1967 Version To get to the 1967 Moog 904A is more difficult, as the capacitor values are much harder to locate and are large in sizey. These won’t drop into the existing Behringer PCB space but they do fit on a daughter PCB.

I have used 100nF polypropylene capacitors and replaced the original 68nF on the Behringer PCB. Axial leaded 1.6uF and 390nF polypropylene audio capacitors are mounted on the daughter PCB which plugs into where the old Behringer capacitors were on the main PCB using PCB pins.

Setup The Behringer 904A has two user adjustable trimmers, RANGE (5k) and SCALE (2k), which are not mentioned in the user manual which is here 904A Quick Start Manual in case you didn’t get one in the box. Out of the box the filter is good with the factory alignment. The SCALE trimmer adjusts the 1V/octave response, which can be done when in self oscillation. The RANGE trimmer is a frequency offset trimmer which I used to get exactly 5.0 kHz at the maximum cutoff in LOW range.

 

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