mosfet and hiss?

[rpjones]

I am trying to track down the source of some excessive hiss in my AMZ mosfet boost build. I understand that the bs170 mosfet is delicate. The pedal sounds great, but is very hissy when not playing. If the mosfet is functioning well, is it possible that the hiss could be coming from it? I built the AMZ mini-booster and it is very quiet. Thanks.

[Processaurus]

The mosfet boost I built wasn't particularly quiet, the hiss was definitely in there.

[DeeBug]

I have an AMZ Mosfet Boost, but it has no discernible hiss.  I mean, it's dead quiet.

Having said that, mine was built by a boutique builder (with permission from AMZ, I understand).  It's an SCP.

I know that doesn't help much, but I'm curious to know why both of yours have hiss (I was thinking of building one myself).

Hopefully more folks will chime in.

[grapefruit]

I've built a circuit similar to the AMZ Mosfet Boost and it didn't have any discernible hiss.

Stew.

[petemoore]

  Had hiss once, but no hiss is possible, shouldn't really hiss.
  Tried a different transistor?

[rpjones]

I think that will be next, I socketed it so maybe I'll just throw another bs170 in there. Other than the hiss I love the sound of the circuit.

[PerroGrande]

Any time there is noise, hiss, etc, noticeable from a circuit that has gain, one has to isolate whether the hiss is being produced by that circuit or is being amplified by that circuit (or a combination of both!). 

One easy thing to try is to short the *input* to the booster to ground.  This will isolate the gain circuit and make sure the input isn't floating and picking up room noise, etc.  If this doesn't change the hiss, then you're certain that the booster circuit is the source.  Check at various boost levels, all the way up to maximum. 

Secondly, you want to make sure you don't "help" hiss by using components of low quality.   Basic carbon resistors are a notorious source of noise.  This is especially true when you're dealing with higher resistance values.  Wirewound resistors are the quietest, then metal film, metal oxide, carbon film, and lastly, carbon composition.

Also, you might try replacing the 10Meg resistor in the bias circuit with a lower value (1-2 Meg).  This will lower the effective input impedance of the circuit, but shouldn't cause roll-off in the frequency response.  A really high input impedance does allow more "stuff" (including hiss and background-induced noise) to enter a circuit.   Just something to try.

Lastly, check to make sure your socket is making good contact, etc. 

Curious -- what are your voltages (quiescent?) for this circuit?

[PerroGrande]

Oh -- and one other thing...

Are you running this from a battery or from a power supply circuit?

[petemoore]

  Another way is shown in updates to Mosfet Boosters, GEO.
 Noiseless biasing.
 [though you can get 'noiseless' from a 'Mosfet Boost'].
 

[markm]

Any time there is noise, hiss, etc, noticeable from a circuit that has gain, one has to isolate whether the hiss is being produced by that circuit or is being amplified by that circuit (or a combination of both!). 

One easy thing to try is to short the *input* to the booster to ground.  This will isolate the gain circuit and make sure the input isn't floating and picking up room noise, etc.  If this doesn't change the hiss, then you're certain that the booster circuit is the source.  Check at various boost levels, all the way up to maximum. 

Secondly, you want to make sure you don't "help" hiss by using components of low quality.   Basic carbon resistors are a notorious source of noise.  This is especially true when you're dealing with higher resistance values.  Wirewound resistors are the quietest, then metal film, metal oxide, carbon film, and lastly, carbon composition.

Also, you might try replacing the 10Meg resistor in the bias circuit with a lower value (1-2 Meg).  This will lower the effective input impedance of the circuit, but shouldn't cause roll-off in the frequency response.  A really high input impedance does allow more "stuff" (including hiss and background-induced noise) to enter a circuit.   Just something to try.

Lastly, check to make sure your socket is making good contact, etc. 

Curious -- what are your voltages (quiescent?) for this circuit?

Truly very good advice however, after having built more than several MosFET circuits IMO, it's overkill.
My guess is there's a bad solder joint or the device is "junk".

[rpjones]

I shorted out input to ground and hiss was still there, no change in level. It is there whether using battery or dc jack power. The quiescent voltages are:
D=4.5V
S=2.1V
G-2.63V

I have no idea if these voltages are proper or not, I am just learning. I think I will try replacing the gate resistor with a metal film and check out all the solder joints. Thanks for the input!

ps: when the level of the circuit is maxed the hiss sounds like Niagara Falls. I am sure this is not proper!

[PerroGrande]

No -- niagra falls is not proper...  The circuit should be extremely quiet.

A quick observation based on voltages:

Your gate voltage is only about 0.5v above your source.  It is very likely that you are operating this MOSFET too close to its threshold voltage.  Noise *IS* one of the side effects of being on the borderline with your Gate-to-Source voltage.

So -- check your wiring carefully.  I'd look carefully to make sure that your drain resistor and source resistor are the same value and properly connected.  The source is bypassed with a capacitor and the gain pot.  Make sure the cap is good/correctly installed and that the pot is also properly installed.    Oh -- and make sure you don't have the zener diode installed in the wrong direction.

It *is* possible that you have a bad MOSFET, too. 

[Gus]

When the input resistance is above a few meg leakage currents can cause hiss.

Leakage current can be caused by flux on the board.  Try cleaning the flux by the gate connection.

[dmk]

i've built two and even on full they are both remarkably quiet