Demeter - Compulator

[This post also contains info on the Bajaman Optical Limiter]

The Demeter Compulator has been a popular compressor for a while now. It's got a great level of transparent compression available, is built like and tank and uses decent quality components.

Here's the description from the Demeter website:

"The sound of classic studio optical compression of the 60's and 70's at your feet! The first compressor pedal not to suck the life out of your tone.

Operation is fairly straightforward. There are two inputs on the front side of the unit: a 1/4" input jack on the right and a 1/4" output jack on the left with a battery-ground switch on the input jack.

There are two controls on the unit: compress and volume, plus a foot switch to bypass the effect, and an LED to indicate effect operation. On the side there is a trim pot to set the overall gain of the unit.

Compress affects the amount of gain reduction (compression) of the input signal. Turning this clock wise will give you up to 30dB of gain reduction (depending on input gain). Please note that the Compulators max gain is 26dB (see Trim Pot) so in some circumstances if your instrument is very hot you could achieve less than unity gain if you turn up the compress knob too much.

Volume increases and decreases the output volume of the Compulator. Use this for level matching between the effected and unaffected signal.

Trim pot sets the gain of the compressor's pre-amplifier. If distortion occurs turn this down until the signal is clean. The unit is set at the factory at 20dB of gain which is perfect for most instruments. If you have weaker pickups or want to push the envelope turn it up to its max gain 26dB."

And here's a demo video of the Compulator in action:

Works well doesn't it. Well, I've built a couple of these and I love the tone they provide. I can fully recommend the circuit for anyone who needs a quick and easy compressor. The DIY project has been available for a few years now and was provided by the great Bajaman over at freestompboxes.org, here is the schematic and PCB layout files:

PCB Layout:

PCB Transfer:

So there you go - one of the best compressor pedals out there. I will say many people have asked about the VTL5C10; you've got to use it or at least construct your own optocoupler that matches it's specs.

Bajaman liked the circuit so much he based his own design around it with the Bajaman Optical Limiter, here's the schematic - LA2A style optical limiter:

Here's what he says regarding the optocoupler: "The LEDs used are both standard yellow 5 mm types. the LDR has a resistance of about 90 ohms when exposed to the light - sorry I cannot give you the exact number for this part but i found by purchasing from a number of suppliers that there are two common types in this size (5mm circular with top and bottom sliced off). You can test with a multimeter across the terminals - expose it to sunlight and observe the reading - if it is 350 ohms or higher, then you have the wrong type for this project - it should read around 90 ohms."

And a PCB layout:

And a PCB Transfer:

Limiters and compressor, although similar, do not function in the same manner. A limiter will only limit the peak of the wave and won't increase sustain as a compressor will. This is due to the way the envelope follower is configured. Notice that with the compressor the envelope is taken from the end of the circuit (A feedback config) whereas with the limiter the signal is taken from in front of the attenuation circuit (A feed-forward config). This difference may seem trivial but you need to take into account that the feedback config is effectively reacting to it's own changes. I.e - the envelope follower detects a strong sound wave, it decreases gain in the circuit, as the wave strength falls over time the envelope follower detects this and therefore forces the gain in the circuit to rise, the envelope follower then detects this rise and forces the gain back down and so on and so forth. It is this "feedback" action that gives the illusion of increased natural sustain. In a limiter there is no reaction or feedback effect, the circuit simply sees the strong first peak of the wave and attenuates, once the signal drops below the trigger voltage for attenuation to occur the envelope follower is essentially off. Therefore with the limiter you don't get the pseudo increase in sustain.

For reference here's the freestompboxes.org forum topic: http://freestompboxes.org/viewtopic.php?f=7&t=317