amp picking up mobile phone noises

[HTH]

Hi all,

I'm having trouble with my amp picking up mobile phone noises (that noise of the phone searching for a network signal) and its driving me mad.

The amp is a build of my own based around a 2203 Marshall circuit and I'm happy that its not the build - I've used this amp for a couple of years now.

Anyone else had this problem or have ideas on what it might be?

I'm at the stage of thinking about adding a Faraday cage round the input jack to see if that helps.

[jrfrond]

Not much you can do except to kill any cell phones around the amp area. FWIW- it seems to happen mostly with A T&T phones, and I don't know why.

Unless you build a Faraday cage around the entire rig (and I'm not even sure that would help), you are probably chasing ghosts that you will never catch up to. It's mainly an issue with high-impedance, unbalanced audio systems, just exactly what any instrument amp is.

[defaced]

FWIW- it seems to happen mostly with A T&T phones, and I don't know why.

It's because they use a GSM network. AT&T, T-Mobile and when it was around, Cingular, use(d) the same type of network. The other guys, Verzion/Sprint, use a CDMA network, which you can't hear through audio equipment. The best thing to do is to move the phone a few feet away from the amp. I seem to remember 3' being the magic number with my computer speakers when I had a Cingular phone.

[Jazz P Bass]

Maybe try a small cap (47P) across the input resistor (1M).

[jrfrond]

Maybe try a small cap (47P) across the input resistor (1M).

Won't help. Thanks to the network type info supplied by "defaced", I went and did some rudimentary research and found that the interference is ELECTROMAGNETIC not ELECTROSTATIC in nature, so traditional methods of choking RF like shunt caps and ferrite beads won't work. Here's a little rudimentary info: GSM Interference

[Ronsonic]

Don't throw your gig bag with the phone in it behind the amp. Or, better, turn off all phones during a gig.

There really is no practical fix for this one.

Just keep the phones away from the amp.

Welcome to the 21st Century.

We spent decades getting the CBs and Country music stations out of our Marshalls, but the cell phones have defeated us.

[km6xz]

The problem is no different than any other RF interference problem before cell phone technology except there is a new twist. Equipment that is hardened against RF pickup adn rectification should not be impacted by the low power transmitters in GSM phones. They use a time domain multiplex modulation scheme that has a baseband pulsed envelope occurring every 1/217 second, There is a great deal of energy in the harmonic series that can extend quite far up the spectrum, with harmonics of 217hz beating against the high rate data signal being burst on each pulse.
The same methods of disinfecting an amplifier of RF signals help guitar amps but the baseband signaling starting so low, within the passband of an audio amp makes it a little trickier. The RF signal frequency is quite high, 900 or 1800Mhz which is very easy to get rid of, is not the source of the noise, that comes from the modulation itself that is inducing pickup as a 217 hz, high harmonic content pulse that is being picked up and detected.

[dai h.]

Do none of the typically suggested countermeasures work? I.e. chassis connecting the input jack right where it comes in to the chassis, small cap (10nF-ish) from ground lug to chassis. FWIW, (something I remember seeing a web profile/interview piece on Matchless amps) setting the phone down on a CD would (apparently) abate noise. (What that does I don't know--aluminum acting as some kind of ground plane capacitively coupling and doing ?something.)

[jrfrond]

.....a 217 hz, high harmonic content pulse that is being picked up and detected.

Which is smack in between the 3rd and 4th octaves of the audio spectrum, and right around the A string on a guitar (220Hz), so basically, any attempts at removing this sideband interference also kills the guitar signal.

[bkahuna]

When working on an amp, I used to be able to tell I had an incoming call before the phone would ring! The amp would start chirping about a second or so before the phone would ring. I'm on ATT also, but since getting a new Android phone, I haven't noticed this at all, so maybe the answer is to update your phone?

[Jazz P Bass]

I have read of people hearing the GSM buzz through amps that are turned off.
How the heck can that happen.

[HTH]

thanks for the responses guys.

I make sure all mobile phones are switched OFF within the band at our gigs, but it really makes no difference to when they're on. I think its a wider issue of general interference from mobile phones since they are everywhere these days.

The small cap from the input to ground is one I've heard of before (Aiken) and will try for sure.

[Enzo]

Mobile phones are indeed everywhere, but if the band turns theirs off, the remainder in the house are probably not going to be on stage.