Broadcasts picked up from Guitar Amp

Dewd! Play Stairway!! You're being channeled by Bonham!!

No seriousry. High resistance and high impedance inputs seem to allow radio waves to be picked up. Those two things are built into your amplifier, and there is little you can do about it.

Are you using shielded cables? I have a 1.5 foot unsheilded cable that cna pick up a Mexican mariachi station no matter where I am.

If you unplug the guitar, do the voices go away?

Are you using any effects when you hear the radio signals?

You might be able to shield the guitar (try borrowing a different guitar and see what you get), or it just may be the amp.

What kind of guitar and amp are these, btw?

-phaeton

The first stage of the amplifier is acting like a rectifier, ie a crystal radio, and converting radio waves into audio that the rest of the amplifier is amplifying. The cable from the guitar acts as an antenna. Of course, this works because there is a strong signal nearby, strong enough to overload that first stage.

It is a common thing, and is a side effect of the active devices. Happens with both tube and solid state devices. The only variable is whether or not there is a local signal strong enough to do the overloading.

It is not the fault of the transmitter in this case.

The solution is to keep the radio signals out of that first stage. The traditional method is a small value capacitor from the base (if it's a bipolar transistor) or grid (if it's a tube) of the first active device in the amplifier to ground, using as short leads as possible. This bypasses the incoming radio signal to ground, so the first stage can't be affected by it. The trick is to use a value of capacitor large enough to "short out" the radio signal, but not so large that it affects the audio response of the stage.

Note that some equipment will have such capacitors in place, because they know it can happen.

If you live in a really strong RF field, you may need to get more complicated, maybe putting an RF choke in series with that element of the first stage of the amplifier. The stronger the radio signal, the harder it will be to keep it out.

Michael

yeah, but radio waves? These are not audible?

I didn't try.

I'm using a overdrive pedal.

Its a Line6 H147.

I'm not worried about the stuff because it doesn't interfere with the guitar sound as its much much much softer. I was just curious as to where the sounds were coming from and maybe how they were being picked up.

How does that work? Do you mean there is an sorta of AM demodulation equivilent circuit in the amp?

Yeah, maybe this happens but since I'm amplifying so much that I can still get them. like I said, they are barely audible and don't effect normal playing.

Its not a big deal. Its not loud enough to cause any problems. (Atleast I think. Maybe it could be the reason why my guitar sound sucks ;)

I was just curious as to where the sounds were comming from and how they were being picked up.

I was thinking that they could be from AM radio(kinda sounds like an AM radio) but I don't understand how one could demodulate the signal with digital equipment(I assume that the amp's first stage is an ADC).

Maybe the signal is being picked up and demodulated in the overdrive pedal I have... it should be the only analog thing circuitry in the chain.

I didn't try to take it out and see if it made a difference I but next time when I feel like messing with it I will.

Thanks, Jon

Try this trick. Put a 1K resistor in series with the signal lead directly at the input jack and put a ferrite bead on one or both ends of the resistor lead. My choice woud be on the lead NOT connected to the jack. That lowpass network will have nearly no effect on the audio but kill the RF. I've done this on hi end audio receivers, mixing boards and guitar amps with good results.

GG

most likely you're picking up CB radio (or some other AM broadcast) possibly a christian AM radio station.

Probably AM radio and/or CB.

Guitar amplifiers are notorious for picking up radio transmissions.

are you using a properly designed receiver?

Bye. Jasen

No, it is a guitar amplifier. Didn't you read what you replied to?

The input stage is overloaded and forced into a non-linear mode which will then detect the modulation. The fading is caused by changes in the RF signal strength being fed to the input, just like any other RF detector.

When you're listening to variable noise, you can hear pretty much anything you want to. My office door opens into a machine shop, and lots of times when they're turning a large part, the sound of the cutter sounds like it's saying stuff. Usually something like "Gotcha,gotcha,gotcha,..." ;-)

Or, yes, it could be radio, TV, cell phones, hams, taxicabs, police, fire, ambulance, aircraft, etc, etc, etc...

Good Luck! Rich

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It's caused by the detection of some sort of radio signal, most likely a local, close by AM station. It could be the Ham down the street as well. The obvious fixes are well shielded, pick-ups, inputs and cables as has been mentioned .Also, adding small bypass caps, ferrite beads or even series resistors or inductors may help as well. However, the biggest culprit is not a lousy front end stage but is rectifying junctions caused by lousy connectors, jacks, plugs and screw connections. Slight corrosion or oxide on connections can cause enough non-linearity to detect strong radio signals in sensitve high gain circuits. The solution: Change out all phone jacks, plugs and connectors in the low level signal path with GOLD plated ones. That's why gold is used for high quality contacts. It does not oxidize and will not cause a rectifying junction. Bad solder joints can do the same thing so check those as well. Bob

They're not audible, but they (especially basic AM radio) aren't all that difficult to detect (demodulate), either. Remember, this is something that can be done by a hunk of galena crystal and a pair of cheap headphones.

Bob M.

Yup. It's called a semiconductor junction (followed by some capacitance). http://66.102.9.104/images?q=crystal-set+demodulator

Bob Eld talks about series inductance (a ferrite) as well. Trap out the unwanted band before it gets demodulated. . .

Yup. It's pathetic that the guy who designed the gear wasn't aware of this phenomenon and didn't include an appropriate trap.

yeah ;)

But sometimes it might exactly be saying something ;)

cool. I didn't know it was that simple to do AM and FM.

Cool. Thanks for explaining a lot of the issues. I had ideas that it could possibily be from many of the things you mentioned but wasn't sure if I was just making stuff up.

Right now its not an issue since it does not effect the overall sound AFAIK. But atleast if I ever get into the situation where it does then I know whats going on.

thanks agian, Jon

I might end up playing around with it later. Right not its not an issue though. I might just try some of these ideas to see how well they work and try also to isolate the where the problem is comming from.

Thanks, Jon

I knew that, I was reminding him. He asked why it faded in and out...

Bye. Jasen

Which you didn't explain to him.

That's because nobody really knows. It could be anything from reflections from traffic, or varying atmospheric conditions, or something in his setup that's picking up a couple of freq's that are beating with each other, or it could be the spooks fading in and out, to give their arcane messages. They can't do it Full-Time, because "God" is watching them, and disapproves of anything that's not within established "scientific" dogma.

Good Luck! Rich