5U Classic VCA - Discrete core voltage controlled amplifier module

PCBs for this module are now discontinued

The suggested front panel design for the Classic VCA

This is a voltage controlled amplifier module inspired in part by a classic vintage design. The Oakley Classic VCA is based on the ARP4019 sub-module which was used on the wooden case ARP2600P semi-modular synthesiser. The design has a fully discrete core but uses op-amps for input summing and the output drivers.

Two signal inputs are provided, one DC coupled and the other AC coupled. The former allows audio and low frequency signals to be processed. While the latter has a high pass filter so that it can be only used to process alternating frequencies. Both inputs can be used simultaneously and each one features an input level control.

Two control voltage (CV) inputs are provided. CV1 is linear and is provided with a standard attenuator pot that can control the sensitivity of the input. CV2 can be operated in either exponential or linear mode.

CV2 features a reversible attenuator. When the pot is turned right of its central position, the pot acts as a normal attenuator; increasing the sensitivity of the CV input. Left of centre, the pot will act in inverting mode and the CV will be tipped upside down so that increasing positive voltages will decrease the gain of the VCA.

When no input jack is inserted into the CV1 input, the CV1 pot becomes a gain control. This allows the pot to be used as a simple volume control but when set high it also allows CV2 to turn the VCA off.

The 5U version of the Classic VCA has two outputs. OUT A is the standard output of the VCA. OUT B, gives you the difference signal between the sum of the two inputs and the final output of the VCA. This is a sort of anti-output; when the VCA output signal goes up in level this output goes lower. You can use it as a ring modulator (with gain pot set to max) or in conjunction with OUT A as a voltage controlled pan.

Power (+/-15V) is provided to the board either by our standard Oakley 4-way header or Synthesizers.com header. Current consumption is approximately 35mA per rail.

The main PCB is 89mm (deep) x 126mm (height). As with all Oakley projects both PCBs are double sided with through plated holes, have tough solder mask both sides, and have bold component legending for ease of construction.

The prototype Classic VCA module using a natural finish Scheaffer panel. Note the use of the optional Sock6 socket board to help keep the wiring to the sockets neat and tidy .

Project Downloads

5U Classic VCA User Manual

5U Classic VCA Builder's Guide

Construction Guide Our handy guide to building Oakley DIY projects

Parts Guide Our handy guide to buying parts for Oakley DIY projects.

Schematics are only available to purchasers of the PCB and will be sent via e-mail when the board(s) are shipped.

Front Panel database

A Schaeffer front panel can be made for this module. The databases can be found by downloading the following links:

5U format 1U wide Classic VCA module in traditional black

5U format 1U wide Classic VCA module in natural finish

To read, edit and print these files you will need a copy of 'Frontplatten Designer' from Schaeffer. Panels can be ordered via the program using its secure online ordering system.

Schaeffer are based in Berlin, Germany and can send panels to anywhere in the world. Delivery to the UK normally takes around ten days. For North American users you can also order your Schaeffer panels from Front Panel Express.

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Copyright: Tony Allgood.
Last revised: February 11, 2024.