A few months back, we spent the day in Avatar Studios in New York with Neil Jason, John McCurry, Ivan “Funkboy” Boding letting them play with WiC for Guitar wireless guitar systems. Each took the opportunity to play the guitar or bass of his choice, through a DI, and into a WiC for Guitar plugged into the Marshall amp. We suddenly noticed one of the radio stations playing through the amp! My first reaction was, “What the hell is that?” I was a bit surprised that the Avatar sound engineer just took it in stride, like it happens all the time.
Our engineers live to hunt noise problems like this in RF systems. For them, this is like a cage match – no one eats or sleeps until it is fixed. While we were in the studio we were able to isolate it to the amp by cabling straight from a guitar to the amp, swapping out different models of guitar and bass and trying different cables. So what would cause the interference, and what should you do if it happens to you?
What would cause the interference? There are a couple possibilities. Amplifiers, particularly high gain amps are susceptible to interference because those radio signals are always there; the signal level is just too low to be heard. The high gain amplifies that noise to a level high enough for you to hear it. In NYC there are lots of radio stations and they transmit at high wattage, which makes it worse than most places. Some amps are simply not designed for noise immunity. Having saying all that, if you’re hearing radio stations it’s like your amp has other problems; for instance, weak or cracked solder joints, corrosion on tubes, long unshielded wires, or bad shielding and grounding.
What do you do about it? Remove corrosion on the preamp tubes. Take out the tube and clean the pins with Scotch-Brite or steel wool. Then put them back in and out a few times. Check the jack leads for cracks, and re-solder the connections on the jacks and other connection points from the input jack. Doing these things should get rid of the radio interference, but if not your problem comes from the amp design. If you find yourself in that situation, feel free post a question.