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

921 PCB

Setup My 921 has been aligned quite well by Behringer at their factory, but the user manual is sometimes 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 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.

I subsequently set the HI-TRIM to mid travel and retuned the VCO, its more like E than an F at zero panel reading but I can dial in more high frequency trim.

Rectangular Wave

Frequency Range The 921 has two different frequency ranges selected by a front panel toggle switch; Normal and Low. On the Moog the switch manually puts 3 extra capacitors into being active when in Low Range. The Behringer uses a V411 analog switch to do this capacitor switching.

  • Moog Normal Range = 1 Hz to 40 kHz
  • Moog Low Range = 0.01 Hz to 400 Hz
  • Behringer Normal Range = 1Hz to 70kHz
  • Behringer Low Range = 0.01Hz to 500 Hz

Some of the wave shapes change at low and high frequency extremes 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. The normal mode gives a near smooth waveform at 1Hz whilst the low range has a glitched triangle and sine wave unless you trim these out – see below.

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. However PW modulation voltages will drive below/above 15/85%.

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

The 921 as a LFO The 921 strengths are as a standard oscillator but it is also useful as a LFO, with a lower frequency than the 921B, and a higher output signal making it more suitable to VCF/VCA modulation. However the triangle waveform deteriorates at frequencies below 20Hz, and becomes asymmetrical with an increasingly prominent glitch as the frequency goes lower. This also affects the sine waveform, but not the sawtooth or square waves. It is possible to trim the triangle wave to remove most of the glitch, using the trimmer on the front of the PCB,  but then frequencies above 20Hz will have the glitch instead and the sine wave is still asymmetric at low frequencies.

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, but the sine and triangle wave shapes in low range do have glitches. I use the 921 as a LFO but in normal range down to 1Hz.

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 Rectangular width is a limitation, which makes thin pulse shapes only possible when connecting a control voltage. The accurate scaling that can be set up, with low thermal drift, makes the 921 attractive as a primary VCO and I would be tempted to use an alternative module for LFO duties such as delayed sine or triangle modulations.

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