Behringer 921 VCO

Behringer 921

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 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 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 921 VCO This module was introduced by Moog in 1974 as a major update to the 901 VCO of the 1960’s which was rather unstable in terms of frequency. The 921 VCO it is an integrated design as apposed to the 921A and 921B configuration which has one driver and many oscillators. The 921 VCO has some extra features:

  • Individual frequency and pulse width controls
  • Sync via a clamping point
  • Two additional inverted waveforms
  • The additional of an Auxiliary Output which increases output levels by 5x
  • A dedicated LFO mode goes down to 0.01 Hz
  • An upper frequency above 20 kHz

The 921 can be used as a standard oscillator but it is even more useful as a LFO, with a lower frequency than the 921B, and the higher output signal making it more suitable to VCF/VCA modulation.

921 PCB

Setup My 921 has been aligned quite well by Behringer at their factory, but the user manual is usually missing from the box, and it can be downloaded here. Only two multi-turn (SCALE, HI-ADJ) of the 9 trimmers are accessible by the customer, There are 7 SMD trimmers on the PCB which can only be accessed by taking the front panel off, and invalidating the guarantee, they are;

  • CLAMP ADJ – sets the sync point
  • SINE SYMMETRY – sets sine wave symmetry
  • SINE SHAPE – set sine wave shape
  • TRI ADJ – adjusts triangle
  • RECT WIDTH ADJ – set the width of the rectangle wave at 10 and 90%
  • FREQ ADJ – adjust the semitone frequency range
  • RANGE ADJ – adjusts the switched octave range

My 921 was tuned close to F1 after being warmed up after power on, and with the Range switch set to 32′ and the Frequency potentiometer set to 0. But it was not exactly F1, and was sharp by 22 cents, however its easy enough to use the Frequency pot to get exactly F1. I would have liked to be able to¬† adjust the 921 to C0 (or C1) but it is not possible and its not part of the original Moog design.

Rear Trimmers

Tuning Accuracy Out of the factory the octave scaling was accurate and with a little bit of trimming I go these best results;

  • 1 cent down at F1
  • 0 cent at F2
  • +1 cent at F3
  • +1 cent at F4
  • +2 cents at F5

I made very slight changes to the SCALE and HI-ADJ multi turn trimmers on the rear of the PCB to get to this result which is excellent. However the HI-TRIM is right at the end of its travel and needed approaching up to max a few time to get this result. The instructions on setting SCALE and HI-TRIM in the user manual are fine, and they use 640Hz as a baseline as this is easy to divide and multiple in your head as the octave scaling is set.

Rectangular Wave

Frequency Range The Moog 921 has a stated range of 0.01 Hz to 400 Hz in Low Frequency Mode, and 1 Hz to 40 KHz in Normal Mode. The Behringer 921 has a higher frequency range and seemed to stall below 0.1Hz. The upper ranges are 810 Hz and 89 kHz respectively. It is nice to see the VCO can work at nearly 90 kHz but not really it is not useful above 20 kHz.

The wave shapes morph and collapse at low and high frequencies as you would expect, the rectangular wave survives the best. The 921 can certainly be used as a LFO, but its more like a low frequency clock when you go low and not smooth sine or triangle modulation.

Rect Width Trimmer

Rectangular Width Accuracy We know this as the more familiar Pulse Width. It is a more disappointing result with my 921 module producing only a 15% to 85% pulse width, when it should be 10% to 90%. Adjusting the SMD trimmer (RECT WIDTH AMT) enabled the pulse width amount to be symmetric, 15 and 85 rather than 18 and 82, but it does not shorten the width at the end points of the potentiometer.

So we are stuck with this result. The CV inputs control 8% of width per volt and by connecting a control voltage the rectangular width can be modified to 0% or 100%.

Waveform Levels The 921 has a set of individual waveform outputs, which to the new user seem too low in volume as they are below 2V p-t-p. These individual outs are designed to be summed together to form complex wave shapes, or they can be amplified by a factor of x2 by the CP35 Mixer. On the Behringer 921 the individual outputs measure as:

  • Sine 1.7V
  • Triangle 1.75V
  • Sawtooth 1.98V
  • Rectangle 1.36V

Sine wave

The Auxiliary Output enables any one of the six waveforms to be selected and then it is amplified by 5x by a simple Op Amp circuit. The sine, triangle and saw waveforms should be 7V p-t-p and symmetrical through zero. Whilst the rectangular waveform should be 0 to +5.5V. Here are the measurements, which are slightly higher than the Moog specification:

  • Sine +3.98V to -4.14V
  • Triangle +4.2V to -4.14V
  • Sawtooth +4.92V to -4.6V
  • Rectangle +4.7V to -0.94V

Summary The Behringer 921 VCO works well and needed very little calibration to be a stable and flexible oscillator. That’s good news as customers don’t want to fiddle with 7 SMD trimmers to get a great result. The higher Aux Output and wide frequency range makes this an attractive LFO in a System 55 setup, and I would be tempted to get more than one.

The sine and triangle waveforms are well shaped when played in the usual 5 octave keyboard range, but deteriorate,as I guess the Moog does, at very high or low settings. The big disappointment is in the limitation of the Rectangular Width range, which makes thin pulse shapes impossible to get with connecting a control voltage. If you do not need the 921 as a LFO I would suggest getting a set of 921A/B’s as the rectangular waveform is accurate and all the trimmers are on the rear making it possible to calibrate everything.

Copyright AMSynths 2019