Silicon fuzz face hiss?

[Silvio55]

Hi, Just finished a silicon fuzz face, I used two 2n3904 hfe around 210 for both. Biased q2c lowering the colector resistor at 5.2 volts, q1c is at 1.45, with stock 33k resistor, the pedal works fine, but unlike my Ge FF, the more I turn down the guitar volume the more hiss I have, especially when I turn down the volume down to zero, it's not very bad but I was wondering why it happens. My Ge FF is more hissy in general but the hiss doesn't vary with my guitar volume,  and when turn down to zero the hiss stops.
Any ideas? Thanks!

[antonis]

Does the hiss diminish (or even vanish) when you touch your guitar strings..??

[Silvio55]

Does the hiss diminish (or even vanish) when you touch your guitar strings..??

No, it only gets affected by the guitar volume, the more I turn down, the more the hiss, worst case is with the guitar volume at zero.

[Silvio55]

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??

[Big Monk]

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??

Try the cap on Q1.

[Silvio55]

I tried bit it did nothing 

[nocentelli]

Very small resistor (like 10 ohms) between ground and the negative leg of the fuzz pot capacitor?

[antonis]

I think I have an oscillation problem,

It's possible you have RF oscillation so try to put a 10μF cap in parallel with 100nF ceramic disk one also in parallel with a 1nF ceramic disk one across the power supply of FF particular, in case of PS daisy chain, circuit..
(in any case, place those caps as close as physically posible to the junction of 33k and 330R resistors..)

[Silvio55]

Very small resistor (like 10 ohms) between ground and the negative leg of the fuzz pot capacitor?

I dont have this problem with the fuzz control maxed, I have it at any setting.

I think I have an oscillation problem,

It's possible you have RF oscillation so try to put a 10μF cap in parallel with 100nF ceramic disk one also in parallel with a 1nF ceramic disk one across the power supply of FF particular, in case of PS daisy chain, circuit..
(in any case, place those caps as close as physically posible to the junction of 33k and 330R resistors..)

I'm using battery only and already added a 330uf filter cap in the power supply rail. But maybe adding smaller caps in parallel would help?

[antonis]

maybe adding smaller caps in parallel would help?

IMHO, yes..
(just be sure to be ceramic disk type..)

[Silvio55]

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?

[antonis]

why the hiss stops when I add resistance in series at the input?

Could you plz be more specific..??
Are you talking about lowering guitar volume..??

[Rob Strand]

I was thinking oscillation until you got to the part where the hiss keeps going up as you roll the volume back.

Are you 100% sure you have 10uF input cap orientated with correct polarity?  When you the guitar volume can you measure any change in the DC bias on Q2?   That could imply the 10uF has a problem, either incorrect polarity or leaky.

Beyond that it's probably wise to replace Q1.

[Silvio55]

why the hiss stops when I add resistance in series at the input?

Could you plz be more specific..??
Are you talking about lowering guitar volume..??

No, I put a pot wired as a variable resistor at the input,  around 3k of resistance the hiss starts decreasing and around 6k it stops. The problem is that it changes the sound of the fuzz and I don't like it.

I was thinking oscillation until you got to the part where the hiss keeps going up as you roll the volume back.

Are you 100% sure you have 10uF input cap orientated with correct polarity?  When you the guitar volume can you measure any change in the DC bias on Q2?   That could imply the 10uF has a problem, either incorrect polarity or leaky.

Beyond that it's probably wise to replace Q1.

Yes, I'm pretty sure the input cap is oriented correctly, negative to the input jack, positive to Q1b. The pedal is NPN negative ground.
I will.meassure the bias at Q2 as I roll the guitar.volume back and see if it changes.

[antonis]

I put a pot wired as a variable resistor at the input,  around 3k of resistance the hiss starts decreasing and around 6k it stops. The problem is that it changes the sound of the fuzz and I don't like it.

Put a 5k6, or so, resistor and lower input cap value according to your sound taste..

[Silvio55]

I put a pot wired as a variable resistor at the input,  around 3k of resistance the hiss starts decreasing and around 6k it stops. The problem is that it changes the sound of the fuzz and I don't like it.

Put a 5k6, or so, resistor and lower input cap value according to your sound taste..

It's not that I don't like the tone with a resistor at the input, it's the way it changes how the circuit interacts with the guitar, and that's is what I love about the fuzz face, so a resistor at the input it's not an option for me.
I checked the input cap and it's perfect, no leakage and the bias doesn't change when I roll down the guitar volume knob.
I'm starting to think... maybe I should try with a different guitar cable?

[Fancy Lime]

Hi,

sounds like an RF resonance problem to me. The inductance and resistance of the pickup, resistance of the guitar volume pot, capacitance of the cable, input impedance and capacitance of the FF and a bunch of other parasitic capacitances and resistances form a rather complex filter with many unknown values. It may very well have a resonant peak in the RF range that shifts around depending on the series resistance of the volume pot. What you hear as noise is then not the RF, obviously, but the result of this RF interfering with some other RF oscillation in the circuit. I suspect that that is the reason why the noise goes away when you put that cap across Q2, because that is where the second RF oscillation lives. What value cap did you use there? What happens if you use a smaller one?

More things to try:

Put a 50-100pF cap from base of Q1 to ground.

Put a film cap (10-100nF, value is not important) in parallel with the input cap. Depending on what the actual source of the problem is, this may make it better or worse.

Put a 100 Ohm resistor between emitter of Q1 and ground.

Put a small cap in parallel with the resistor from emitter of Q2 to base of Q1. Try 1nF. If it does nothing, try 10nF or even 100nF. If 1nF changes the sound in a bad way, try 100pF.

Hope any of that helps,
Andy

[Silvio55]

Hi,

sounds like an RF resonance problem to me. The inductance and resistance of the pickup, resistance of the guitar volume pot, capacitance of the cable, input impedance and capacitance of the FF and a bunch of other parasitic capacitances and resistances form a rather complex filter with many unknown values. It may very well have a resonant peak in the RF range that shifts around depending on the series resistance of the volume pot. What you hear as noise is then not the RF, obviously, but the result of this RF interfering with some other RF oscillation in the circuit. I suspect that that is the reason why the noise goes away when you put that cap across Q2, because that is where the second RF oscillation lives. What value cap did you use there? What happens if you use a smaller one?

More things to try:

Put a 50-100pF cap from base of Q1 to ground.

Put a film cap (10-100nF, value is not important) in parallel with the input cap. Depending on what the actual source of the problem is, this may make it better or worse.

Put a 100 Ohm resistor between emitter of Q1 and ground.

Put a small cap in parallel with the resistor from emitter of Q2 to base of Q1. Try 1nF. If it does nothing, try 10nF or even 100nF. If 1nF changes the sound in a bad way, try 100pF.

Hope any of that helps,
Andy

I will try!! But I'm having some problems now, the pedal doesn't work, I have 1.6v at Q2c!! 8.5v before Q2c resistor. And I still can't find the problem!! One hour trying to fix but nothing for now, I don't see anything wrong, I even changed the transistors.
This is so frustrating!

[Rob Strand]

I have 1.6v at Q2c!! 8.5v before Q2c resistor. And I still can't find the

That means Q2 is turning on too hard, which can happen when Q1 isn't turning on enough.
Are you sure you didn't swap c and e of Q1?

[Silvio55]

I have 1.6v at Q2c!! 8.5v before Q2c resistor. And I still can't find the

That means Q2 is turning on too hard, which can happen when Q1 isn't turning on enough.
Are you sure you didn't swap c and e of Q1?

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!