Behringer 904A Low Pass Filter


Moog 904A Blueprint

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 even 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 related to the resistance of the recovery amplifier at the top of the ladder filter, and there is not much that can be done to change this, and the results are in line with the original. Both Moog and ARP moved from bipolar transistor to JFET recovery amps to improve the low end oscillation. The Micro Moog achieving 10Hz and a more accurate CV control to improve self oscillation tracking.

The self oscillation in the 904A measurements show errors at low and high frequencies with a 2 octave sweet spot in the middle:

  • C3     136Hz     -35 cents
  • C4    262Hz     +4 cents
  • C5     521Hz     -3 cents
  • C6  1054Hz    +18 cents
  • C7  2132Hz    +32 cents

I have not adjusted the SCALE trimmer yet, to see if this non-linear response can be corrected.

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.

AM904A PCB

904A 1967 Version To get to the 1967 Moog 904A is more difficult, as the larger capacitor values are much harder to locate and are large in size, if polypropylenes are used. These won’t drop into the existing Behringer PCB space or fit easily on a daughter PCB. It is worth noting that the 1967 version of the filter did not self oscillate, and it was only later that the circuit was modified.

I have developed a daughter board (that is needed for the 904C) that uses a combination of 1.5uF, 390nF and 100nF matched polypropylene capacitors. It plugs into where the old Behringer capacitors were on the main PCB using PCB pins, and a R/A 10 pin power socket replaces the original.

Testing the new filter shows that the frequency cutoff is lower due to the larger filter ladder capacitors and is easily adjusted with the RANGE trimmer, there are holes in the new PCB so that both trimmers can be adjusted. The main benefit is that the filter resonance has improved, with resonance down to 40Hz and a 3rd order harmonic peak in addition to 1st and 2nd order which the matched polyester version delivers.

Subtle rather than dramatic improvements but certainly worth while, in terms of resonance and overall quality of sound.

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 to go 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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