A few years ago I bought a Rocktron Big Crush compressor pedal. It’s a nice transparent compressor and it sounded great right away. As long as it was active. When in bypass mode, it was the worst pedal ever. It had buffered bypass, which isn’t necessarily a bad thing, but in this case it was terrible. There was a big volume drop and a major loss of bass frequencies. So I’ve thought about turning it into true bypass for a while, and now i finally had the time (and courage) to do it.
Why true bypass? True bypass means that the circuit is mechanically removed from the input and output when the pedal isn’t active. This is usually done with a DPDT-swich (two on-on swiches in one package), but because most pedals have a led that lights up when pedal is activated, there’s need for either a third switch (which calls for a 3PDT-swich) or one of the existing switches has to be used for this. Another solution is the millennium bypass, of which I already wrote in the fuzz-post. When the signal path is completely removed from the circuit in bypass mode, there’s no extra load on the signal, and therefore no loss of any frequencies, which might be the case when using (bad) buffered bypass.
So the pedal already had a DPDT-swich, which isn’t always the case in buffered bypass pedals (most of them use FET-switching). What I had to do was to disconnect the inputs and outputs from the rest of the circuit, and because I intended to use millennium bypass, I also disconnected the led. This was done by simply cutting off the copper trace on the circuit board in the right places (red lines).
I then soldered wires to the right places to the underside of the board, green wires for bypass and red for current to the led.
Because of the way the cicuit board is held in place inside the pedal, I had to cut a small “bay” in the side of the board so the wires could pass through.
The led switching circuit was placed right next to the switch using double sided mounting tape. The circuit is roughly the same as in the fuzz, it uses a different diode and a smaller resistor, but the principle of operation is exactly the same.
I found the schematics online (bad resolution, but it’s readable), here’s what it looks like after the modification:
(Note that R24 needs to be connected to C25. The easiest way to do this is to solder pins 5 and 6 on the ribbon cable base togehter). The parts used in the bypass circuit are: Q = 2n5457 FET, R = 560 ohm resistor, D = 1N1418 silicon diode.
One thing that I noticed is that when there is no plug in the input of the pedal, the led stays on all the time (if the power cord is connected). First I thought I had done something wrong, but this i simply because the ‘ground’ of the whole cicuit isn’t really grounded until there’s a plug in the input, a design meant to save batteries. This is not a big deal, because I never run my pedals on batteries. Apart from this, it works totally fine. Now as the pedals major flaw has been corrected, it’s a really good compressor, and will find its way back to my pedalboard!
EDIT: By request, I added three more pics with more detailed information on which wire goes where (see numbers) and a stripboard layout for the fet switching circuit. I hope this will be helpful!
N.B. the pin order on the fet might differ depending on the fet used. Check the datasheet.
What's wrong with this circuit? 
