Why do Clipping Diodes add Lots of Hum and Noise!!!

[Minion]

Hi folks, I have built several OD/distortion circuits that use clipping diodes but every one of them causes a very undesireable amount of Hum and Hiss when the Diodes are in curcuit, the Circuits are dead quiet with the Diodes disabled (I usually use a switch to put the diodes in or out of Circuit) ...Why is this?? is there a way to use clippers without them causeing a lot of Noise?? I think Fet baced OD circuits are probably quieter but I have never been able to get a fet to work no matter what biasing arrangement i use ,I guess maybe all the fets I have bought were either bad or way out of spec and therefore un-biasable??

Thanx

[ayayay!]

Well, wouldn't the hum and hiss be just the microphonic nature of increased gain?  I know what you're asking, but the physics involved would dictate more noise.  I mean, when you look at clipping graphically on a scope, you "see" noise.  The waves get squared or "chopped" off the more it clips, so I think you're just stuck with it. 

[Minion]

well I think that there is Less gain whith the Clipping diodes enganged because the output signal is determined by the Voltage drop of the diode (0.7v?) and I didn"t think clipping would introduce more Noise because when tube amps clip they just distort but don"t add any extra noise (my Guitarists Mesa Dual rec is dead quiet with the gain cranked and nothing being played) and even many Many SS amps don"t add a lot of noise with the Gain cranked.... Is it maybe that many SS amps don"t use Clipping diodes but maybe use saturated Transistors??

[johngreene]

the Circuits are dead quiet with the Diodes disabled (I usually use a switch to put the diodes in or out of Circuit) ...Why is this??Thanx

I think there may be a clue in this statement. Perhaps there is a problem with how you are wiring them to the switch? Long, unterminated leads maybe?

--john

[Papa_lazerous]

I think someone just hit that one straight on the head 

high gain circuit with long wires going to the switch, lots of noise there, maybe screeened wires ? or shorter run of cable

[Minion]

Yes I also thought of that before but when I hard wire the Diodes in Place there is no differance in the noise ,I also use a 3 conductor shielded cable (2 signal wires and shield) for my switches so any interfernace should be shunted to ground....also if I drive an opamp into clipping I don"t get the noise but opamp clipping isn"t a very pleasant OD sound...

I guess maybe I"ll try a couple transistor based OD/Distortion circuits without any diodes and see if I can get a pleasant clipping sound out of them.....

Anyone know if a very heavilly distorted OD that uses transistors (Not fets) that has a low noise (hum) level?? I"m thinking of a sort of Slayer or Dimebag type of Distortion....

Thanx

[Paul Perry (Frostwave)]

Another possibility is that the nonlinearity of the diodes, is rectifying high frequency electrical noise. If you take an AM transistor radio & tune it across the band, ther eis a huge buzz of hum and noise. the diodes can rectify this, like a crystal set, and it gets amped up.
See whether running on battery instead of wallwart makes a difference, or (better) if you have a battery amp or a headphone battery amp, try it out in a park. (I'm serious, when you are chasing noise, you have to do these things!)