The Envelope Follower and Gate Extractor (EFG)

PCBs for this project have been discontinued.

The double width EFG-Deluxe (left) and the single width EFG (right) - both shown in the suggested standard finish MOTM format.

An envelope follower takes an audio input and converts it into a control voltage. This control voltage will rise and fall with the overall volume of the input signal. The louder the input, the bigger the output of envelope follower. The Oakley issue 6 EFG has a fully variable speed CV output thanks to its internal four stage slew generator. Set to fast the CV output will respond very quickly to incoming audio and is therefore suitable for tracking fast moving cymbal and hi-hat patterns. Set it more slowly and it is more useful in processing overall audio volume. By using the CV output to control VCAs and VCFs it is very easy to create synchronised patterns that keep in perfect time with a drum machine or other musical input.

Another feature of the Oakley EFG, is the provision of a gate extractor. This part analyses the peaks in the music and creates a gate type signal in time with these peaks. This can be used to trigger envelope generators in time with an external drum loop or click track. Two rotary controls, threshold and response, enable you to get clean gate signals from all sorts of input material.

The Oakley EFG now comes with a wide range three stage input pre-amplifier. This allows guitars and other unbalanced external instruments to be processed simply. You can use this output for driving other modules.

The double width deluxe version of the module utilises a built in lag or slew module. Unlike earlier versions, issue 6 of the EFG main board contains all the circuitry required. Builders of the EFG-Deluxe do not now need a separate, and now defunct, Little-Lag PCB. If making the single width standard EFG the parts pertaining to the Little-Lag circuit are simply omitted.

And finally the EFG, in either guise, has three LEDs. One showing input overload, one for gate status, and one for a visual indication of the slewed CV output.

For DIY builders of this module it is possible to purchase the Sock4 circuit board which is available separately. This little board speeds up the wiring of the four 1/4" sockets and reduces the chances of any wiring errors. Two Sock4 boards would be required to make an EFG-Deluxe.

Power (+/-15V) is provided to the board either by our standard Oakley 4-way header or header. The board size is 143 mm (high) x 107mm (deep). Current draw is around +50mA and -40mA.

This is the prototype of the standard single width EFG behind a natural finish Scheaffer panel. The unpopulated section in the lower left hand corner of the board belongs to the lag circuitry used on the double width EFG-Deluxe module. Note the use of the optional Sock4 board to facilitate wiring to the four sockets. The peak LED is not fitted to this particular module but it would normally sit to the right of the green 'gate' LED.

Project Downloads

Builder's Guide for EFG issue 6

User Manual for EFG

Construction Guide Our handy guide to building Oakley DIY projects

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

Links to schematics are available to purchasers of the PCB or module 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 (EFG issue 6) module in traditional black

5U format 2U wide (EFG-Deluxe issue 6) module in traditional black

To read these files you will need a copy of 'Frontplatten designer' from Schaeffer. The program also features on-line ordering, so its now even easier to buy your panels from Scheaffer.

The company are based in Berlin in Germany and will send out 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: January 29, 2024.