Microphone squeel - ideas

paulmapp8306

Hi - not looked myself yet as Im not at my mums, but shew runs a dance thing, and uses a PA/music system.  She has had no problems for ages, but now both her micrphones just cause squeel when switched on (have on/off switches).  They both have dedicated cables so not likely to be that - and there both Low Z.

They have a third mic which is duel impedance - again with its own cable.  When on Low Z she gets the same squeel.  When switched to High Z all is fine.

So - what has happened that now causes all low Z micrphones to just produce squeel but high Z micrphones work OK.  

bossarb

High Z microphones will typically have a lower output level, so they are less likely to feed back at the same preamp gain setting on the PA as the equivalent Low Z mic. What's the PA system and can she not just reduce the gain a little on the PA? Someone may have inadvertently changed the gain or deactivated a pad etc. on the channel.

That's assuming that none of the other obvious factors have changed like speaker position.

paulmapp8306

Its an all in music system - basically a music player with built in speakers and a mic input.  That feeds an external power amp that then goes to proper PA speakers.

The level hasnt really been changed - and the mic input level (if there is one - need to look) would always be low - and the whole volume of the system isnt large by my standards.  Its old couples ballroom dancing - not a rock concert.

John_P

Definitely sounds like a gain issue -  I suspect somewhere there is a switch - impedance, pad or input type  that has been knocked so it's now getting a huge signal compared to how it was set before.

paulmapp8306

hum - might be an input gain.  Spoke to my  mum and there is a little white knob (I think its the recessed type with screwdriver slot) next to the mic input.

The system has recently been PAT tested (or just PAT - I have that), and the issue has arisen since.  Could be the tester adjusted that knob for some unknown reason, so the input gain is now too hot for a Low Z mic.

maltingsaudio

Certainly sounds like somethings been "tweeked" and more than likely during the PAT process, without knowing almost exactly what kit we're dealing with very difficult to give an answer or a way to problem sort

paulmapp8306

Problem solved. Was indeed the input gain turned up full.

Also had something wrong with a speaker cable, intermittent loss. I'd already made up new cables. Mum was shocked, I'd used good quality summer 2.5mm core stuff. Weighs a tonne compared to the crappy string she was using (supplied with pa), but sound is so much better.

PolarityMan

Stupid question but has the system been physically moved so that now the speakers arent in the mic's rejection zone?

ICBM

paulmapp8306 said:

Problem solved. Was indeed the input gain turned up full.

Bad practice to "service" equipment and adjust a preset like that, unless there is a very good reason for doing so, or without informing the customer.