Behringer 902 VCA


902 VCA

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.

Rear of PCB

The 902 VCA This module is a simple transistor based voltage controlled amplifier with either linear or exponential response. The level of the VCA is controlled by a 0V to +6V voltage either from the front panel or via a control signal. The control signal can also vary down to -6V to give negative gain effects. It is one of the cheapest modules in the System 55 range but you may need to use a 995 or CP3A attentuator module with it, so you can adjust the level of the control signals going into the VCA.

It is important to realise that the VCA can amplify the input signal at higher control voltage levels, either via the Fixed Control Voltage potentiometer or a control signal. The Moog specification says in linear mode a +6V control signal will amplify by a factor of over x2, whilst in exponential mode the same control voltage will give +10dB (which is 6x). Take a look at the Moog 902 Brochure.

Measurements of the Behringer 911 show +6dB gain in both linear and exponential modes, and unity gain when the Fixed Control Voltage is set just above 5. At higher audio signal input levels above +2V you may need to be careful of the VCA distortion, which increases with hotter signals. Of course this is one of the charcteristics of the Moog Modular and you may want to use it!

Ring modulation like sounds can be created by using audio signals into both the SIG IN and a CONTROL INPUT.

Setup The VCA can be immediately installed into your system using the factory alignment. The 902 Quick Start Manual makes no reference to a trimmer but there is one on the rear and it is accessible by the customer. The trimmer is OUTPUT BIAS and it controls the offset of the final VCA stage and it should be set to 0.0V when the the VCA has no inputs and Fixed Control Voltage is set to maximum, or apply -6.0V to a control input and ensure the output is exactly the same. The trimmer should not need adjustment unless you notice clicks on the audio output. My 902 measured at

The original Moog 902 had three more trimmers but these have been replaced with fixed resistors and the use of modern components reduces the need for trimming.

Summary The Behringer 902 VCA is a well built replica of the original and it performs well with the correct gain at higher control voltages. It comes factory aligned and can be placed into your system immedately. You will probably want to use attenuators like the 995 or CP35, so that you can vary the control signals or balance the audio levels going into the VCA. I have added the AMSynths AM902 Daughter board on the rear of the Beringer PCB using spacers, this enable the CV input to be connected to a Molex connector and onto the AMSynths 993.

 

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