Fuzz Circuit Questions

[Cloudless]

Hello,

I am trying my hand at a homemade fuzz.  So far I am just building the circuit on a breadboard, with no switching.  I am using this diagram: http://fuzzcentral.ssguitar.com/fuzzface/fuzzfacepnpschematic.gif but with a 330OHM instead of 470OHM, as the silicon diagrams I have seen use 330.  I am using 2 2N2222A transistors.

Unfortunately I don't have pots of the correct values off hand (I had the rest of the parts in a misc parts box I had!), could I just hook up a pair of permanent resistors for each pot for testing purposes?  I am not sure which value I would use in that case, probably something in between 0 and 100K for the 100K pot?  So far, with pots that were a bit off, all I get is a buzzing sound.

Edit: I tried this layout also which has a negative ground: http://fuzzcentral.ssguitar.com/fuzzface.php (scroll about halfway down to NPN silicon diagram).  Having no appropriate 1K linear pot, I tried putting a 1k resistor from ground and to the other two sides of the "pot" circuit.  My intention was to simulate the fuzz pot turned all the way up.  This actually yielded some results!  However, it is quite noisy, with some high pitch sound in the background when not playing, and a bit more when playing also.  I'm not sure if this is what is referred to as "oscillation".  This circuit just will not work with a positive ground (battery switched, caps reversed) either.  Or will NPN transistors only work with the negative ground??

[hannibal827]

but with a 330K instead of 470K, as the silicon diagrams I have seen use 330.  

The diagram you're linking to shows a 470-ohm resistor, but not a 470k.  If you're substituting a 330k resistor in that spot, that's probably part of the problem you're having.

I am using 2 2N2222A transistors.

These are NPN transistors and won't work in the circuit you're trying to build.  Just as an experiment, though, you could try reversing the +/- on your power supply, as well as the electrolytic capacitors, and then you would have a circuit suitable for NPNs.

As long as you have about half the supply voltage on the collector of Q2, you should be in the ballpark.

Good luck!

[Cloudless]

Hannibal, thank you for your reply!  It is a 330 ohm, sorry for the typo.  Yes, I did a negative ground circuit (diagram half down this page http://fuzzcentral.ssguitar.com/fuzzface.php) with about half volume and "full" fuzz and it functioned.  It does make a fuzz sound!  As I edited in my original post though, it is quite noisy and has some high pitch sound coming through.  Any thoughts as to the cause of this?  Maybe I need a proper fuzz pot, and I'm sure it doesn't help that this is a breadboard setup and probably vunerable to outside noise.

[hannibal827]

If you hear a really high-pitched noise, that oscillation might be mitigated by placing a small capacitor [try 100pf or so] across the collector and base of Q2.  This will reduce the treble a bit as well as diminishing the oscillation, but most reports I've read indicate this is an acceptable trade-off.

Even "low" or "medium" gain NPN silicon transistors are likely to produce this effect.  In my fuzz face build I think I wound up going with 2N2369As or something around 100hFE, and didn't need the collector-base cap.

Some hiss can be mitigated by placing an electrolytic cap across the power supply--try 100uF.

Again, check your voltage on the collector of Q2, and try to get it to around 4.5 volts.  Adjust that 8.2k resistor to get it there.

Hope this helps.

[hannibal827]

Sorry, didn't notice this earlier:

Or will NPN transistors only work with the negative ground??

Yeah, that's pretty much the case.  If you're building this circuit for negative ground, use NPN.  Positive ground, use PNP.

[Cloudless]

Great info, thanks!  I still can't believe I basically built what can amount to a $200 pedal for free with parts I already had!  I figure a basic enclosure, some switches, etc can't cost much over $30.