Behringer 921A/B VCO


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 921A Oscillator Driver The 921A pairs up with the 921B VCO and provides control voltages for the frequency and rectangular width, and that is all. It was introduced by Moog in 1974 as a major update to the 901A/B VCO of the 1960’s which was rather unstable in terms of frequency.  The 921A/B combination has some unique features:

  • Hard and soft sync
  • Up to nine 921B’s can be chained off a single 921A
  • One Frequency, Range and Width potentiometers controls all the 921B’s

Typically three 921B’s were cascaded from one 921A Oscillator Drive, and the oscillators are generally used as audio sources as they do not go down to a low enough frequency to be used as a LFO.

Setup My 921A/B has been aligned quite well by Behringer at their factory, but the user manuals are often missing from the box, so here are links to them;

There are three multi-turn (FREQ ADJ, SCALE, HI-ADJ) trimmers and another 7 trimmers are accessible by the customer, on the rear PCB. This is a big improvement over the 921 where the base frequency cannot be adjusted and the less used trimmers are all inaccessible.

  • SINE SYMMETRY – sets sine wave symmetry
  • SINE SHAPE – sets sine wave shape
  • SINE OFFSET
  • TRI ADJ – adjusts triangle wave shape
  • CARRIER OFFSET
  • PL OFFSET
  • RECT WIDTH ADJ – set the width of the rectangle wave at 10 and 90%
  • FREQ ADJ – adjust the base frequency

My 921B 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.

Copyright AMSynths 2019