Behringer 914 Fixed Filter

914 Fixed 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.

Moog 914 Inside

The 914 Fixed Filter Bank The Moog FFB is a classic and very well regarded filter module that dates back to 1966. It uses 28 hand wound coils to deliver twelve 12dB band pass filters, as well as a 24dB low pass filter at 100Hz and a 24dB high pass filter at 7.5kHz. The fixed nature of the filter is new to many users, who expect voltage control of the frequencies, but it’s an essential part of the Moog Modular sound and we know Klaus Schulze used it is his patches.

Accurate copies of this module are very expensive at £2000 and physically large, so Euro Rack has tended to use Op Amp replicas of the coil based filters with some possible loss of sound quality but much cheaper at £100 to £150. AJH Synths makes a hybrid version with inductors and Op Amps at a mid price of £475.

Behringer 914 PCB

The Behringer 914 goes down the full Op Amp route (TL074) and gets under the £100 price point. Six of the quad op amps are used in band pass channels, 2 Op Amps per channel. There are only 4 capacitors per channel (same as in the EMS 8 Octave Bank), so some compromise on capacitor values was necessary to fit the 914 circuit onto the PCB. Some designs use 8 capacitors per channel, so that the frequency band is more accurate for each channel.

The lower 6 channels use THD polyester capacitors in the lower frequency bands which will sound better than ceramic SMD capacitors, and are equivalent to the old Mullard PET capacitors used in the original. The 6 upper channels use SMD capacitors which could be high quality PPS.

Magic Sound JHaible, a great synth designer, was not convinced that the inductor version has a special magic and that it was more about the deep notches produced by adjacent filter bands. The inductor filter bands were designed so that the signal first passes thru a resonant LC-filter at a certain level, and then passes a second LC-filter at a lower level which filters out any harmonics created in the first filter.

Time to get the 914 in the rack and take a listen!

Setup The Behringer 914 has no user adjustable trimmers, which is the same as the original, so this module can go straight into your system. Here is a link to the user manual 914 Quick Start Manual, in case you didn’t get one in the box. The key question is how accurate the 12dB band pass filter points are and how good does it sound. Lets look at some measurements for the 12 band pass filters;

  • 125 Hz
  • 175 Hz
  • 250 Hz
  • 350 Hz
  • 500 Hz
  • 700 Hz
  • 1000 Hz
  • 1400 Hz
  • 2000 Hz
  • 2800 Hz
  • 4000 Hz
  • 5600 Hz

The 914 can be used to simply sculpt an audio signal with the filters set to specific frequencies, and its often used to get slightly resonant upper frequency “pips” which can be sequencer driven. But it can do far more:

  • Add a feedback loop using a 995 or CP35 attenuator to process the second Output back into the second Input.
  • Buy two and process different sides of a stereo pair with different band pass setups
  • Process echo delay tails via the FFB
  • Use a VCA and ADSR after the FFB to bring the processed sound in over time

Summary A cost effective way to add a Fixed Filter Bank into a modular setup with no setup required, albeit based on Op Amps without inductors. The active design does have the benefit of a louder output than the original. The Behringer 914 is very close in design to the Doepfer version, with the same approach and only 4 capacitors per channel, and at a similar price point as well. One potential modification is to improve the accuracy of the lower filter bands by adding capacitors on the rear of the PCB, but this is not an option for the upper frequency bands.

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