If you need the schematic, just tell me and I'll draw it for you as soon as I'm home again.
Anyway, it consists of 3 transistor stages with the first one having the boost control by regulating emitter resistance. Each stage is a simple common emitter design,and the last one has a diode as feedback between collector and base to further clip the waveform.
So, when I test it using Waveforms it does work just fine, then I tested it with the guitar as an input and the output to an oscilloscope: found out it makes much noise,the same noise I can hear in the speaker connected to my computer (so I'd say it is something in my house and the pickups sense it).
To try and avoid it, I used the same oscilloscope to understand what frequencies this noise has, and since they range between 8kHz and 12kHz, I simply put a low-pass filter (simple passive one) with cut-off frequency at 4kHz so that the signal to the amplifier can remain almost unaltered.
At the oscilloscope now everything seems fine (just guitar signal from strings and no more noise superimposed). But when I connect the circuit output to the amplifier the noise is there again and massive too (it's so much that its volume is greater than the string's).
How could I get around this?
Keep in mind that I'm using a breadboard since I would like to be sure that the circuit is working correctly before soldering it, but other times using the same breadboard worked out just fine to test other circuits.
Could it be faulty design? But then even if it's so, why does everything seem fine at the oscilloscope?