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Strange interference every 30 seconds
I was helping out a friend with an issue they have where they get a regular, maybe every 30 seconds - 2 minutes buzz that lasts for 1 second through the speakers.
We isolated the device causing the issue and it’s on a different circuit (different fuse/break at least). Not sure what it is at this stage, there is a rack (although everything in it was powered down), computers, photocopier and other things. We'll find out what it is soon via further elimination within that area (unless someone can have a guess at what it might be?)
The thing I'm interested in is why the issue is occurring.
- We isolated the amp, with just it powered on and no inputs (no cables at all). No issue.
- Plugged in the input but not having it connected at the other end. No Issue.
-We powered on the desk with no inputs or outputs at all. No issue.
- Plugged in just the desk output to the amp, and with nothing turned up. Issue appears. The desk was plugged into the same circuit as the amplifier (power point next to the one amplifier was plugged into). We tried a different desk and the issue remained.
Basically it only occurs when the amp and desk are connected via the signal cable (it’s a balanced cable which was tested to confirm no issues; we also tested with other cables in any case).
We couldn't test a different amplifier as the only other one available didn't have enough power so if it’s part of the problem we wouldn't have been able to hear it.
If it was a grounding problem, shouldn't that have been eliminated by going into the same circuit? Should we test by lifting ground on one end of the balanced lead? If so, which end?
Regarding the device causing the problem and the fact it’s going through the fuses. I had a guess that it might be a plugin pest control device, those ones that emit high frequency and modify the electromagnetic field in the wall wiring.
If it were a ground loop it would be continuous, or at most change in amplitude over time, but not cut in and out at regular intervals.
Most likely it is a device that turns on and off that is injecting noise back in to your power. Things that do that, refrigerators, HVAC, etc should all be investigated. You probably need an electrician to come in and tell you what all is on the phase of power you are using for your sound system. Ideally your sound system will be on it's own phase.
I came to the same conclusion as bladeaudio in that you appear to have a piece of equipment somewhere that is injecting noise back into the power wiring. My initial guess would be to look for something with a motor in it. When a motor starts it will draw a high current until it has run up to speed. Your one second (ish) amount of noise could fit into this category. The only way you will identify the culprit is by a systematic search. Be patient - you will find it...
At least you have identified which breaker the offending equipment is hung off. Identify all the equipment on this breaker (should be relatively straightforward) and then look at each piece of equipment in turn for high voltages or electro/mechanical components.
The next thing is to isolate each piece of equipment in turn (if this is possible) and see what effect it has on your audio. You want to be in the situation where there is no interference with the item switched off - but that the interference appears when the item is switched on (i.e. the method is scientific and repeatable).
The next thing is how to fix it...
I read your post and came to the conclusion that it may be the mixer desk that is susceptible to the interference. You say that the amplifier is OK on its own - and that the amplifier plus the signal cable is OK when not plugged into the mixer desk - but the amplifier with the signal cable plugged into the mixer desk is susceptible. Can you try the mixer desk on its own with a pair of headphones? If the mixer desk was susceptible to electrical interference then you would need an amplifier to hear the interference (which is what you appear to be saying if I have read your post correctly).
Time it carefully. If it is every 30 sec, or every one minute lasting 1 to 2 sec, it may be a signal from a master clock or a repeater for a master clock system to a bunch of impulse clocks. These were sent either hard wired or by a transmitter that sent a signal over the power lines. The buzz could be from a power transformer only when it is loaded.