RF Interference?

[BigMess]

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

 

[Skogsgurra]

Can't see this happen unless the amplifiers are a few kW. Are they?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[BigMess]

No, each is 450w at most.

 

[Skogsgurra]

Hard to believe. RF? Don't think so.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[BigMess]

I know, very strange. The wiring on this particular circuit is a jungle, so anything is possible I suppose. Any other possibilities?

 

[MikeHalloran]

I'd guess a { loose connection | too small wire }, i.e. a shared impedance somewhere upstream of the AC connections for the various amps, such that the peak current draw for the bass amp is basically 'dimming the lights' for the other amps on the same circuit.

Scan for anomalous heat in the facility wiring, e.g. with a IR thermometer, or just 'looking' with your forehead or hand, with the bass amp plugged in and making some noise.

Do it soon.



Mike Halloran
Pembroke Pines, FL, USA

 

[VEBill]

"...each is 450w at most..."

Each is 450 watts audio OUTPUT max (RMS), or AC power max INPUT (as per the name plate)?

If they're rated 450 watts RMS audio output then the input AC supply power will be up to about twice that. If so, then you're probably overloading the circuit when the bass hits the peaks. It's worth noting that many circuit breakers will be very slow to open as per their design.

Probable solution: buy a heavy duty (12-ga) extension cord, and find a separate outlet for the bass amp.

 

[VEBill]

To clarify, overloading the circuit to the point where you're modulating the 115 VAC voltage (voltage drop) with the bass (effectively a rectified version, thus horribly distorted).

 

[SceneryDriver]

Some guitar/bass amps are designed terribly (read as cheaply). They will inject all sorts of crap onto the ground wire, and expect it to be shunted to earth. It's no problem if the circuit has good grounding, but you said the wiring was a "jungle." It's a common cause of all sorts of hums, buzzes, and other audio "ghost in the machine" issues.

Don't discount the above theory from VE1BLL though. If the bass amp is solid state and the input rectifier has a poor power factor (only drawing power on the peaks of the AC waveform) it may be causing distortion that the other amps' power supplies can't cope with. Bass amps typically have LARGE filter capacitors in their power supplies to cope with the sudden power demands of a "plucked string." This only makes their power factor worse, and intensifies the distortion effects on the AC line.

The answer V1BLL is the right one - get a heavy duty extension cord and run the bass amp off another circuit. Alternatively, fix the wiring at the venue.


SceneryDriver