RF Interference?

Hi all,

Not an EE but I have a truly perplexing problem my EE friends have not been able to solve. At a particular venue in the Boston area, there is a strange interference between guitar amplifiers during live performances. Bass guitar plays through their amp and cab, and a "crunching" noise will come out through the guitar amps and cabs, plugged into different outlets but on the same circuit. During the last test, the bass amplifier was solid state, guitar amp 1 was solid state, guitar amp 2 was valve. These "ghost notes" came out of both guitar amps. Tried all sorts of different outlets with no success.

Aside from a remedy, I am curious as to how the hell this can happen. How is it possible that the bass frequency is running back through the lines and over to another outlet? I'm fairly certain that there is a grounding issue somewhere in the system, but I don't want to rule anything else out. Any insight would be greatly appreciated.

Best,

-BM

My next step would be to check the supply voltage when the amps are blasting away - could be the line voltage is sagging when the bass is cranking away. Just a thought

or use a isolation power transformer and make sure that all neutrals and phase connections ar connected proper.
Also explain how this all are connected together mixing panel and and amp feed back recording etc.

I note that you say, "At a particular venue". If the problem occurs _only_ in this particular building, it suggests the fault is not necessarily in the band's equipment, but in an interaction with some electronic environmental factor, perhaps an internal communications system in the venue.

Does the same equipment work perfectly elsewhere ?

Do the power lines in the building have good earth continuity and proper grounding ?

As for your suggestion that a signal is running through the power lines, this can be tested for and traced with an oscilloscope. (Tiny digital oscilloscopes are now very cheap.)
In which case, the solution is to de-couple the signal from the power lines.
An audio systems electrical engineer would probably give you an instant solution.

Have you checked all the cables for continuity of ground and screening ?

Are all the amps and mixers etc (particulary valve) fully enclosed and screened and grounded ?
(Faraday cage effect should fully eliminate RFI.)

Hi there. Is it a possibility that you are getting accoustic feedback via a guitar string vibrating from another guitar? This could also possibly be venu specific (echoes etc).

Same equipment has worked fine elsewhere, grounding is iffy. Two of the three amps are fully enclosed, all are certainly grounded within their enclosures.

Hi Rodney,

We did actually try the experiment once with the guitar unplugged, and the bass' "crunch" still squeaked through the guitar cabs.

"grounding is iffy" ! This is simple to fix. Make sure there is good continuity (very low or zero resistance) from the metal casing of every equipment to the ground connection in the power line.

As I suggested before, make certain the building's power system itself is properly grounded, otherwise it could be acting as an antenna for RF signals and noise !

Radio transmitters have a great fondness for causing interference. This is not surprising, since their primary job is to pump 100 watts or more of radio energy into the sky. Ideally, all of this energy would be sent off towards the distant receiver, but this is not the case. Antennas, particularly small ones, radiate in all directions, and worse yet, any imbalance in the antenna system causes the coax cable, power wires, and every other interconnection to become part of the antenna system and radiate also. In the days before digital communications this was a nuisance at worst, but when modems and computers get interconnected with transmitters and radios the potential for chaos is great. This is especially problematic for small installations such as boats and RV's where the antenna and ground system literally wrap around the radio and other components.


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