100v line

[mikelx]

Some help please friends

 

I am running a 100v line amp to power 4 speakers FOH and I have a "hum" on the speakers

I know its something obvious but some help with fault finding would be helpful

Should tthe amp be "grounded" if so how?

Thanks

[MarkPAman]

Hi

 

If you can give some more details like the model of the amp, what you've got plugged into it and the type of cables you are using, then somebody may be able to have a stab at identifying the problem.

 

As it is there's far too little information to go on I'm afraid.

[Jivemaster]

Hum is NOT picked up by 100v lines! It will likely be picked up by the input circuits though, so look at whatever you feed your amp with, Mics with, unbalanced mics, phono connectors etc.

 

Take out all the amp inputs and see whether the hum has gone, then put back one lead at a time till you find the source of the hum.

[mikelx]

The amp is Eagle PA4120E

The input microphone is on the balanced line input and is a Shure microphone and on a short xlr lead in SM corner to amp under SM desk

 

3 speakers are eagle with a separate volume control on each formerly used in dressing rooms on a show relay system and now redeployed to Foyer positions

One speaker is of unknown origin with no volume control-its part of an "inherited" system in the foyer which we have extended with the 3 eagle speakers

We tested the system some weeks ago with just putting recorded music from a minidisc player through the amp and I can't remember if there was a hum then

Its only when we have got it set up for FOH announcements when the hum is noticeable so I will check the microphone side of things

as suggested but this won't be until next week

Thanks for the help so far

[Shez]

Does the volume of the hum change when you change the mic channel's level control? That will quickly tell you whether the hum is coming from the mic / its cable or somewhere else.

[sam.spoons]

Have you checked the amp's safety earth? Tried the amp with no mic connected?

[djmatthill]

It's worth checking to see if the Shure Mic is actually true balanced , the lower end Shure kit like C606 from memory is not balanced but still has a xlr connection.

 

Of course if it's an SM 58 it will be , but check the internal wiring or better still try another Mic that you know is a true Balanced Mic..... check Mic cable for breaks etc too .

 

 

 

Matt

[Jivemaster]

Remember that the best mic, if used with an unbalanced cable becomes unbalanced, the presence of three pin connectors is no guarantee that screened cable has been correctly used.

[mikelx]

Thank you everyone- problem solved - it was the microphone!