In the previous post, I described the circuit that I’m using to emulate a potentiometer using a couple of opto-isolator (or opto-couplers?). There is one small problem with the circuit: the input impedance has to be quite low.
If you just plug a guitar in the circuit, which generally has a high impedance output, it will have a hard time keeping its signal straight. Mostly, you will loose some of the high frequencies present in the guitar signal. Even more so if the cable between the guitar and the circuit is long. For a real and very detailed explanation, take a few minutes to read http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp
The best way that I know of to correct the impedance mismatch is to use a line buffer. So I decided to include one right into my VC-1. This way, the user (sorry… guitar player…) can plug his/hers instrument into the PU-2 and the signal will be brought down to the proper impedance. There is another benefit to this: the impedance buffer/VC combination will keep the impedance fairly low all the way to the output. This way, the rest of the guitar signal run will receive a lower impedance signal, which is always a good thing.
The circuit I’m using:
The TL072P Op Amp is just used for pin placement. I’m actually using a NE5332A Op Amp because it has a very low noise circuit and is only marginally more expensive than the TL072. The 2M resistor are Metal Film for even lower noise. The input impedance is 1M, which is plenty for any guitar. The ouput impdance is only a few ohms. The gain, which is controlled by the connection between pin 1 and 2 (and 6 and 7 for the other Op Amp) is set at 1, so the signal at the output should be at the same volume as the input.