Behringer 921A/B VCO

921B 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. The 921 design 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 set of 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.

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

921A trimmers

921A Trimmers There are two single turn (RANGE, OCTAVE) trimmers accessible by the customer from the rear of the PCB. The Moog original also has a SCALE trimmer, because the Moog 921A divides the 1V/octave inputs down to 500mV/octave. The Behringer design does not need to do this, as their 921B accepts 1V/octave, so there is no SCALE trimmer, but see below. Its worth remembering that 921A does not translate linear to exponential voltages for the VCO as in the 901, this is done by the 921B.

The RANGE and OCTAVE trimmers are best set using a DVM measuring the control output to the 921B and with the 921A mode set to whichever mode you use the most, semitone was my choice. Connect your pitch CV source to the CP3A-O and then onto the 921A. This is important as the 921B can correct the small offset and scale inaccuracies of the CP3A-O. The objective is to get an accurate 1.00V per octave when playing your CV source and measuring the output that goes into the 921B.

I use an Arturia Keystep as my CV source as its very accurate. It is NOT important whether the CCW and CW positions of the Frequency pot create exactly -6.00 and +6.00V, its the accuracy from the CV source that is important, as that is what you will be playing the VCO’s with.

The RANGE and OCTAVE trimmers are adjusted to give 1.00V/octave on the output to the 921B. I managed to get to with 2mV across 6 octaves, which is acceptable, but the single turn trimmers are hard work. Multi turn trimmers would be easier. Switching to Octave mode may create a change in pitch, mine dropped by 6mV. This is due to resistor tolerances and can be trimmed when RANGE and OCTAVE are correctly set.

The 921A uses two precision Op Amps (SGM8273-2XS8G) which should have 1mV offset. They are used for summing the Pitch and Width CV inputs. There is also an audio standard (4580) Op Amp for the Octave circuit which is an unusual choice for a precision voltage circuit. A possible upgrade is to replace the 4580 with a LT1013 precision op amp, and replace the 25K linear Frequency pot with an Alpha version and replace the single turn trimmers with multi turn Bourns 3296Y but on the rear of the PCB.

The jack sockets are all normalised (which is good practice) but the PCB trace is underneath the jack socket, so very hard to drill out and fit an internal bus.

New OCT ADJ trimmer

921B Trimmers There are three multi-turn (FREQ ADJ, SCALE, HI-ADJ) trimmers on the rear of the PCB, along with another 7 single turn trimmers. 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 shape symmetry
  • SINE SHAPE – sets sine wave shape
  • SINE OFFSET – adjusts the sine offset from 0V
  • TRI ADJ – adjusts triangle offset from 0V
  • RECT WIDTH ADJ – sets the width of the rectangle wave at 10 and 90%

There is also a single trimmer on the front PCB, with a small hole at the top of the rear PCB, enabling it to be adjusted. This trimmer is marked OCT ADJ and adjusts the accuracy of the RANGE switch positions and needs to provide 1.00V per octave. I have replaced the single turn trimmer with a multi turn Bourns 3266 trimmer so I can more accurately adjust the octaves.

921B Sync The original Moog sync circuit is considerably more complex than ARP or Roland synthesizers. The Moog sync implements a phase locked loop which measures the difference between the frequency of the “sync to” source (which Behringer call carrier) and it’s own frequency, it then generates an extra voltage that will drag its oscillator to the same frequency as the “sync” source. A sawtooth waveform is the best sync source. It is implemented with a balanced modulator chip (796/1496) to process the incoming waveform , a 3046 matched transistor array and an OTA.

The original Moog had three trimmers, the Behringer version has only the two:

  • CARRIER OFFSET – Phase Lock Filter Cutoff
  • PL OFFSET – Phase Lock Offset

The Phase Lock Offset trimmer (100k) is used to adjust the output of the Sync OTA to a zero voltage. This is an important trimmer, as any offset will cause the VCO to jump in frequency when the SYNC switch is engaged. It MUST be adjusted so that the center and right SYNC switch positions are the same frequency. Its a single turn trimmer in both the Moog and Behringer designs, and its very sensitive and tricky to adjust. I have replaced it with a multi turn Bourns trimmer.

I am not sure what the Phase Lock Filter Cutoff (10k) trimmer does, and its not mentioned in the Moog manuals. The Phase Lock Filter Balance trimmer (100R) has been replaced with fixed resistors, which makes sense.

Tempco and 3046

VCO Performance The pitch signal chain of the CP3A-O, 921A and 921B need to be carefully setup. The CP3A-O has a small offset which can be counterbalanced by the 921A setup. It is therefore important to check the pitch CV going into the 921B is 1.00V/octave (see above). It is also important to set the PL Offset trimmer before aligning the VCO.

The 921B, like all analog oscillators has some thermal frequency and scaling drift, which means it is important to align the VCO after 2 hours of warming up. I have upgraded the 3 multi-turn trimmers with Bourns 3296Y, which at £2 each are rather more expensive than the Behringer versions. I have also replaced the SMD tempco with a THD version that is mounted on the 3046 transistor array with thermal heat paste.

I have put in a Bourns multi trimmer for the OCT ADJ and replaced the PL OFFSET trimmer with a single turn cermet version. The 5k PITCH pot has also been replaced with an Alpha 9mm version to make setting the frequency easier.

My 921B alignment process is:

  • Let the VCO’s warm up for 2 hours.
  • Use an accurate CV source of 1.00V/octave.
  • Trim the 921A to give an accurate 1.00V/octave output with the CP3A-O in series.
  • Set the PL OFFSET trimmer on the 921B so that SYNC STRONG is the same VCO frequency as OFF in 2′ Range
  • Trim SCALE and FREQ to give 1.00V/octave with FREQUENCY pot at panel 0 frequency of C1.
  • This should be achievable to +/-2 cents over 6 octaves.
  • Trim the Range switch to give accurate octaves, within 2 cents, using OCT ADJ trimmer.
  • This will mean adjusting the FREQ trimmer to get to panel 0 = C1.

Technical Info The original Moog VCO (both 921 and 921B) have a 1K tempco resistor mounted underneath the 3046 chip that provides the NPN matched transistors for the exponential generator . The tempco has thermal paste between it and the 3046 chip to improve thermal stability. The Behringer VCO’s have a 1k SMD tempco resistor that is adjacent to the 3046 SMD chip.

I have replaced the SMD Tempco with a THD version, so that it could be placed on the 3046 with heat paste, this is not vital, but it is a small improvement. The Behringer 921B uses a 4580 Op Amp as the pitch CV buffer at IC4, and its a low noise audio Op Amp.




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