Silicon fuzz face hiss?

[Phend]

Try different cables.

[Rob Strand]

No, I rechecked everything again today, still can't find anything wrong, transisitors are ok, no shortages anywhere, cold soldering, everything seem fine, except it isn't!!
I will keep looking for an error, but this damn seems haunted!

When things are it crazy like that it's a good plan to step back and check parts of the circuit in isolation.

For example  disconnect the base of Q2 from the rest of the circuit.  Connect the base to +V with
a large resistor, or a large resistor in series with a 1M pot.   See if you can get Q2'c collector to bias.
The base resistor probably going to end up around the 3M ohm.  At the end of the day all you are
doing is testing the transistor gain in-circuit.   It seems pointless *but* doing it in circuit will
expose whatever is going wrong.  If a single transistor stage isn't working there is something
really wrong.   You can measure the base, collect, and emitter voltages to help identify the problem.

Repeat for Q1.

[iainpunk]

I'm starting to think... maybe I should try with a different guitar cable?

i'm building an amp, and it had the same problem untill i swapped guitar cables, hiss was nearly gone.
i'm not saying its the guitar cable for sure, but you might as well try another one if you have one.

cheers

[Silvio55]

Thanks guys!! I became very frustrated with this project so I'm taking a break for some days, I built another pedal, a silicon treble booster based on the rangemaster and the naga viper, works fine and I'm tweaking some details, testing different transistors but I'm very happy with it.
I will come back to the FF and report results in a few days, I'm not giving up on this!

[pinkjimiphoton]

Sorry for double posting, but after some experimentation I discovered that adding a cap between colector and base at Q2 helps, but it also limits the highs, like putting a tone control, not a great thing for me, and adding resistance in series, around 5 or 6k at the input eliminates the hiss but changes the sound and I don't like it. I think I have an oscillation problem, but these solutions don't convince me, maybe biasing Q1 differently would help??

sounds to me like q1 is a noisy transistor. try changing it for another.

you can snub most of the noise out by adding a cap, but as you found, its a dodge. it will change the tone (in fact if ya put a "fake variable cap" here, you can control the overall tone of the fuzz)

5 or 6 k resistor in series with the input is a lo pass filter. that's why the noise diminishes. but that's a LOT of resistance, and will mess with the tone and response of the fuzz itself.

my money is still on a noisy transistor. it happens. try changing it out and seeing if the issue goes away.

[pinkjimiphoton]

Well, I tried, no luck!
I'm intrigued... why the hiss stops when I add resistance in series at the input? It's oscillation, some interference or something else?

resistance in series at the input is for all intents the same as turning your guitar down slightly.

i've built somewhere around 1000 si fuzzface variants at this point... almost every single time i've encountered the issue you described, it was ultimately a noisy transistor. try changing it out. betting that will fix your problem.

also remember, you can change the GAIN of the transistors by adding small resistances between e and ground.

anywhere between about 22 and 220 ohms will work. the commercial ones i build use q's with an average hfe of 200. i use 100-220r between e and ground on q1, and 47-100r between e and the rc network on q2. gets the gain in the ballpark, so that a quick bias adjustment and i get pretty fair consistency.

i generally add a 47p snubber between b and c on both as well. that helps keep the noise and hiss down.

metal film resistors will help, as well. its a brilliant circuit, and toneful, but can be noisy.

proper gain is crucial imho to making a si fuzzface that really sings.