Fuzz picks up radio stations...

[earthtonesaudio]

This comes up all the time, people hearing a radio station broadcast through their fuzz.  Often the suggestion is to add a capacitor to ground on the input, but that can actually make it worse.

This picture might help explain why:

http://www.zen22142.zen.co.uk/Theory/diode_pump/Image188.jpg

Notice the similarity to a Fuzz Face circuit.  Notice also the capacitor to ground at the input!  That's to help tune in the station!  Note also the absence of any resistor in series with the input.  This is important.

(An RC or LC lowpass filter at the input is a good way of keeping the radio out of your circuit, but only when implemented properly.  A resistor or inductor in series with the input is the bare minimum, relying on the transistor's inherent capacitance to form the "C" part of the filter.  Better still would be the series R or L, plus a cap, either to ground, or between the transistor's base and collector.)

More info here: http://www.geofex.com/circuits/what_are_all_those_parts_for.htm

[petemoore]

  I was recording along one day, and the Fuzzface picked up anti-communist propaganda radio from the 60's, sounded like Wallace or McCarthy, fades in perfect time with the tune, transforming the message into a much more interesting one.
   My Fuzzface radio works great, tunes in 'haunting' voices from the past, right on time...amazing !

[frequencycentral]

The best fix seems to be a low value cap between the collector and base, like the One Knob Fuzz, but I like Pete's solution, just make it part of the music.

http://fuzzcentral.ssguitar.com/schematics/oneknobfuzz.gif

[petemoore]

  Just a 10k resistor might to it.
  Depends on the gain and the shielding and...
  Little cap to ground at the input should take out every last bit of RF, yet leaving the relevant HF's mostly intact.
  Usually shouldn't take much to squelch the RF input.

[Electric Warrior]

Notice the similarity to a Fuzz Face circuit.  Notice also the capacitor to ground at the input!  That's to help tune in the station!  Note also the absence of any resistor in series with the input.  This is important.

but the fuzz face part in this circuit is only used for amplification, right?

[Blue_Toad]

but the fuzz face part in this circuit is only used for amplification, right?

The fuzz face part of the circuit is only used for amplification, the forward diode resistor to ground and capacitor make what is known as a peak detector circuit which in AM receivers is a simple way to demodulate the signal into AC which would then get amplified by the fuzz face stage.

The reason the fuzz face picks up radio is that it has no RF filtering at all (I've never built one, I've just checked out the schematics, also remember that the transistors will act like a diode getting us back to the peak detector circuit in some cases) for the most part a resistor and capacitor low pass filter will do the job or a cap between base and collector, but remember that this will only give you a 20db / decade rolloff, so if you have your RF filter at about 40kHz, 1 decade lower (4kHz) the sound will be basically unchanged, but at 1 decade higher (400kHz) the sound will be 20dB less. Keep in mind that Fuzz faces will only pick up AM radio stations, which are around 1600kHz so if you have any gain in the circuit after the simple RC filter, you can actually end up cancelling out its effects and bringing the radio back in, (sounds like a cool effect on the recording though).

For the record if you want to figure out the cutoff frequency of the RC filter the formula is f=1/(2*pi*R*C)     
R is in ohms and C is in farads, 1uF = 0.000001F and f is in Hz.

I really hope I didn't make this more complicated than it should be. 

[earthtonesaudio]

I think that's as simple as possible, but no simpler. 


Basically the FF circuit is a wide-band amplifier that can also act as a radio demodulator under many common circumstances.  The point of my first post was that the common advice of placing a capacitor to ground at the input-and nothing else-is not a good approach because it makes the circuit even more like the AM/FM demodulator shown in the first link of the first post.

A better approach is a genuine low-pass filter, but as Blue_Toad pointed out, the basic FF circuit has so much gain, one pole of filtering might not be enough.

[slacker]

It's a good point about needing a series resistance in front of the cap to ground to make an RC filter. However, in my limited experience of Fuzz Faces the ones I've built only pick up radio when the guitar's volume pot is rolled down at all, when on full they don't. In this case the portion of the guitar's volume control between the hot lug and the wiper forms a series resistance, which is possibly why the cap to ground trick works.

[punkin]

I'd be looking at PN junctions where the AM signal can be rectified. Using a small capacitor (at rf freq ranges) across the junctions should clean the mess up by bypassing the rectification opportunity.

[Quackzed]

...i know that the input is acting as the antenna, so if you used shielded wire/made the input connections very short you could probably (marginally) reduce this effect. you can also make the fuzz a BETTER radio by hooking a long unshielded piece of wire to the input of the circuit. its kinda cool tho... and i can relate to having weird coincidental radio interference while playing, cool!
i'd try a short shielded input wire from the jack to the switch, as well as the switch to the input of the circuit plus a small cap to ground at the input!

[petemoore]

  Yeap, I forgot [~intentionally] about typing or trying out 2-pole LP filter.
  Because I get enough HF's that taming them later on in the FF's is the case.
  But that said, I generally do like "some high frequency hash let in.. and tamed later" sound and approach [output>HF cut control].

[Electric Warrior]

...i know that the input is acting as the antenna, so if you used shielded wire/made the input connections very short you could probably (marginally) reduce this effect. you can also make the fuzz a BETTER radio by hooking a long unshielded piece of wire to the input of the circuit. its kinda cool tho... and i can relate to having weird coincidental radio interference while playing, cool!
i'd try a short shielded input wire from the jack to the switch, as well as the switch to the input of the circuit plus a small cap to ground at the input!

cool. and how can I tune in a different station? 

[kelmark]

Picking up Fm radio is one of the my personal benchmarks in knowing I've build a good fuzz circuit. If when, I plug in, turn up the gain on the fuzz up with the guitar volume turned down I heard public radio, I know life is good.

[biggy boy]

Picking up Fm radio is one of the my personal benchmarks in knowing I've build a good fuzz circuit. If when, I plug in, turn up the gain on the fuzz up with the guitar volume turned down I heard public radio, I know life is good.

I guess mine is no good, it only picks up AM radio.

[petemoore]

  Re-orient the box, move it away from the wall or floor.
  Clip to the collector or somewhere, a 6'' or longer wire..a guitar string fits in with the C lead into the socket.
  Turn the guitars controls and the gain knob, adjust bias.
  Change location.
  Wait until there is a solid cloud layer, you may be able to tune in radio waves with a delay signal added.

[spaceace76]

Picking up Fm radio is one of the my personal benchmarks in knowing I've build a good fuzz circuit. If when, I plug in, turn up the gain on the fuzz up with the guitar volume turned down I heard public radio, I know life is good.

I guess mine is no good, it only picks up AM radio.

occasionally I pick up my neighbor's cordless phone... They are Korean and when messing with the fuzz, I hear Korean chatter and some Asian TV channel in the background. I can tell because of the laugh track. It's bizarre.