How to fix loud thunderous noise from Peavey PA system?

Question

I am having a severe problem with my Peavey PA system.

After connecting all the system components together, the PA system is left ON: A loud continuous noise of distortion starts coming out of the speakers I connected.

It is so loud people actually run away. It does not happen all the time. Happens like 2 out of ten events.

I asked some local experts, they even didn’t know which part of the whole PA system is responsible for that. Either it’s the cables, the amplifier or the speaker which is faulty ???

Does anyone know what causing this, and how to eradicate this problem?

Someone told me that it might be a loose cable.

Gear

• Amplifier PVi 8b, 150 watts / 4 ohms http://peavey.com/products/index.cfm/item/700/116558/PVi%26nbsp%3B8B

• Speakers Peavy PV 115 (8 ohms) http://peavey.com/products/index.cfm/item/683/114376/PV%26nbsp%3B115

• Wireless microphones sennheiser ew 100 G3

• Cables Prolink. performer 500. Monster cable

Is the combination of speaker and amplifier technically correct? The amplifier is 150 watts and the speaker is 400 watts.

Does any one know what causing this, and how to eradicate this problem ???

Answer 1

This is most likely an issue related to microphone cable, connectors or wireless receiver. Try the following to determine the source of the error:

Test amplifier

1. Connect amp and speakers only - NO input cables connected at all, and turn it on with the volume a bit up.
2. Try gently shaking/pushing the power cable.
3. Try gently pushing the main volume fader and the various channel gains/volumes.
4. Try gently moving the amp (shake it a little, turn it around)

If you can't provoke the error this way, then move on, otherwise deal with it (fix the cable, clean the faders etc).

Test receiver / input cable

5. Make sure amp is up running (see first bullet above).
6. Now connect the the wireless receiver (turned OFF)
7. Turn on receiver (but not mic)
8. Move around the cable between receiver and amp, gently push connectors

Test microphone

9. Turn on the microphone and make sure you hear it clearly through the speakers
10. Bump it gently into your palm etc.

If none of these steps reveal the problem, it is most likely internal defects in the amplifier. The speaker cables are very unlikely to cause such sounds, since failures here typically causes drop outs and scratchy sound.

Personally I'd suspect the cable between the receiver and the amp to be the problem: a shield wire is shorting with the signal once in a while.

Answer 2

Without any recording of the "loud continuous noise of distortion" it's hard to tell, but obvious things that come to mind:

• audio feedback through the mic. It's enough for some [audio] noise to get picked by mic to initiate this
• amplifier heats up, gets unstable and starts oscillating
• much less likely: some machine nearby starts and you get electrical noise via capacitive coupling or the mains, which gets picked up and amplified.
• And since this is a wireless mic: any sort of RF interference in that band could cause the mic's receiver to act up

You need to debug this in stages: start measuring/monitoring the signal at the mic, its receiver, move to the amp etc. The speakers are the least likely culprit. Also, just physically shaking stuff up might trigger it, or it may very well not; you may need to hook up measurement equipment and wait.