Classic VCA for Eurorack - Discrete core voltage controlled amplifier module

The PCB for this project is no longer available.

The popular ready made euro rack version of the Oakley Classic VCA as made by Krisp1 using the narrower 8HP format.

This is a high quality single channel voltage controlled amplifier designed around an all discrete transistor core. It is based on the 4019 sub-module which is found in the classic 2600 semi-modular synthesiser.

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 provides a DC block to process only alternating frequencies. Both inputs can be used simultaneously and each one features an input level control.

The unit features both linear and exponential control voltage (CV) inputs. Both can be used together or individually.

A gain control is also available to provide a fixed bias to set the initial gain of the VCA. This way a negatively going CV can reduce the overall gain of the VCA. The gain pot can also be used a volume control.

Current draw with a +/-12V supply is around +/-20mA with no output signal. A standard eurorack power header is provided on the board.

This is the original prototype module using the original wider 10HP panel design.

The issue 2 PCB

The PCB is double sided with through plated holes. The topside of the board is clearly legended for easy construction. The board size is 69mm (deep) x 102mm (height).

The control pots are Alpha/ALPS 16mm types and for extra rigidity can be optionally mounted with matching brackets. Because the board is designed to mounted to the panel with the pots no other mounting holes are provided on the PCB.

Project Downloads

User Manual

Builder's Guide for issue 2 Classic VCA

Construction Guide Our handy guide to building Oakley DIY projects

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

Front Panel database

A Schaeffer front panel can be made in either the 10HP or 8HP versions:

vca-10hp.fpd 10HP module in standard natural finish. The socket hole sizes are for Cliff 3.5mm sockets.

vca-8hp.fpd 8HP module in standard natural finish. The socket hole sizes are for Thonk PJ301 sockets.

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: April 25, 2022.