Behringer 904A rewinds to 1967

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 VC LPF module is a faithful copy and uses THD polyester capacitors in the filter stages, with smaller values to ensure they fit onto the PCB.

It sounds very nice, but let’s make it sound fantastic and rewind back to 1967 and fit the same filter capacitors that Moog used in the IIIP.

Rewind to 1967 To get to the 1967 Moog 904A is tricky, as the larger capacitor values are much harder to locate, and are large in size – if the correct polypropylene types are used. These caps won’t drop into the existing Behringer PCB space, so the original capacitors need to be removed and a new daughter board fitted to the rear.

I developed the daughter board to use a combination of 1.5uF, 390nF and 100nF hand matched polypropylene capacitors. It plugs into where the old Behringer capacitors were on the main PCB, using PCB pins and has a cut out for the existing 10-pin power socket. The PCB also supports the AMSynths 904C, as it puts the jack socket inputs and outputs onto a rear mounted Molex connector.

Availability & Outcomes Testing the upgraded filter correctly shows that the frequency cutoff is lower due to the larger filter capacitors. The frequency cutoff is easily adjusted with the RANGE trimmer as there are holes in the new PCB so that both trimmers can be adjusted.

Listening to the audio output and measuring it with a frequency analyzer, reveals that the filter resonance quality has improved, with resonance down to a lower 40Hz and a 3rd order harmonic peak appearing in addition to the 1st and 2nd orders. This is as a direct result of using matched polypropylene caps.

These are subtle rather than dramatic improvements, but very worthwhile in terms of filter resonance and the overall quality of sound. It is now the sort of filter that you want to use all the time! 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 retained the self oscillation as its a key part of the Moog sound.

Outcomes The AMSynths PCB, capacitors and headers take a couple of hours to build, followed by some carefully desoldering of the Behringer polyester caps and the installation of the new PCB. This is an easy project for the competent DIYer. The production PCB’s are populated with matched capacitors and the PCB’s are on sale here.