“In my studio certain instruments and amps will pick up an AM radio station which is about 1/2 mile away. We tried different mic cables hooked to an amp, different 1/4 inch cables, lifting the ground, going direct through a DI, and moved the electrical cord to another outlet. Nothing has worked. Most recently, with a bass player, he just went home and got another bass, which did help. The building is located beside a huge electrical tower and it had been raining all day. I have several guitar amps, and some days certain ones will play the radio station. Usually it’s not a hassle, but today the station was loud and clear.”
Once you realize how easy it is to make an AM radio receiver you begin to realize how many things can cause this type of problem. A working AM receiver can be made with one diode and a resistor. “Yeah, so what?” you say. One of the attributes of ‘cold’ or poorly made solder connections is they can take on some of the properties of a diode and/or resistor. Guitars and basses, with their coil pickups can also make excellent radio receivers. So any connection in your studio (or even inside your guitar) can potentially pick up radio and inject it into your signal. Further complicating matters is that AM signals are dramatically effected by weather conditions, cloud density in particular.
This is one of those unfortunate situations where there are so many things that could be contributing to your problem that it can be hard to nail any of them down. As always, we recommend a rigorous troubleshooting scheme. Strip the components down to the bare minimum. Does that bass pick up the radio when it’s just the bass and the amp connected with no other equipment? What about a different amp, or the bass directly into the recording mixer? You said a different bass was better, so that tells you right away that there may be a fault with the first bass that exacerbates the problem. Does the direction the bass is facing make any difference? These are just a few of many possible questions that can begin to lead you down a path towards at least making some improvement. Here are a few other things to check for and possibly fix:
Is all of the equipment properly grounded? Don’t just say yes. Really investigate this. It makes a difference. If it is, try ungrounding different pieces just to see what happens.
Is your facility properly grounded? Some electrical grounds (especially in older buildings) are very poor and cause all kinds of problems with electronic equipment.
Is the shielding inside the bass or guitar thorough? Many instruments have very minimal internal shielding and are prone to pick up all sorts of EMI.
Try grounding the instrument to a known good earth ground. Run a wire from the bridge to this ground.
Make sure you use balanced lines everywhere possible.
If you try all of this stuff and still can’t get anywhere, you might try looking around for a local electronic technician who can come observe the phenomenon and make additional suggestions. You could always build a metal cage around your studio and ground it. That would almost certainly cure things, though the expense would be extreme. A more workable solution is sometimes just to move your studio location.