Classic VCA - Discrete core voltage controlled amplifier module


The 6HP wide Oakley Classic VCA is a new version of one of my first Eurorack modules. This is a voltage controlled amplifier module inspired in part by a classic vintage design, 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 inbuilt high pass filter so that it can be only used to process audio frequencies. Both inputs can be used simultaneously and each one features an input level control.

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

CV1 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 either CV input socket, the CV pot of that input becomes a gain control. This allows that pot to be used as a simple volume control. With no jack inserted into the CV2 input and the CV2 pot is set high it allows the CV1 input to turn the VCA off.

This newer 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 audio inputs and the final output of the VCA. This is a sort of anti-output; when the Out A output signal goes up in level this output goes lower. You can use it as a ring modulator (with either CV pot set to max) or in conjunction with Out A as a simple voltage controlled pan.

A single +5V CV input with its associating pot turned up full with produce unity gain (0dB) through the VCA.

Current draw is approximately +/-30mA at +/-12V.

The PCB set

The Classic VCA module comprises of two printed circuit boards (PCBs) connected together with three 0.1” (2.54mm) single in line (SIL) headers and sockets. The main board on the rear of the module houses the power input and conditioning, and the VCA core. The pot board has the pots, switch, and sockets that are attached to the front panel, as well as the CV and audio input circuitry. The main board is a four layer design, while the pot board is a two layer design. Each board is the same size; 29 mm x 107 mm.

The majority of the components are surface mount parts. The ICs and THAT340 array are SOIC, while the resistors and surface mount capacitors are 0805. The PNP transistor array is SOT457. All components are standard parts.

Project Downloads

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 PCBs and will be sent via e-mail when the boards are shipped.

Front Panel database

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

6HP Eurorack format in natural silver

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.

Back to Eurorack projects page

Back Home: Oakley Sound

Copyright: Tony Allgood.
Last revised: January 02, 2024.