low frequency hum on a show relay mic run

[CARWYN]

Hi Guys and Gals

 

a bit of a weird one for you I have a bit of a low frequency hum on a mic line that we have running for our show relay and before anyone asks I've tested the cable run and the mic mic has been plugged directly into my sound desk with a short lead works all fine and with a cable tester the cable tests all fine. where does this hum materialise from ??????

 

Thanks in advance

Carwyn

[d_korman]

Exactly what type of cable was used for the run to the stage?. Are the correct cores used for the three pins on the xlr? A cable tester will not tell you about that, it will only tell you if correct pins are linked. In is critical that the screen is not used as one of the signal conductors. you have to open up a connector and look.

[Jivemaster]

50Hz hum would indicate mains being picked up somewhere and an unbalanced lead could be one culprit, but also and earth loop could cause issues over a substantial distance.

[CARWYN]

im convinced as I re soldered the control room end thisafternoon and it was a proper mic cable

 

thanks carwyn

[mackerr]

im convinced as I re soldered the control room end thisafternoon and it was a proper mic cable

 

thanks carwyn

 

Even with the right mic cable, a cable tester will only tell you that there is continuity on the 3 lines. It will not tell you if the shield has been compromised, nor if the cable runs near power. If the channel is the relay mic directly to the mixer a ground loop seems unlikely as there should be no ground connection at the mic, but if the mic is mounted to the building steel or anything that is connected to building steel it may be worth isolating it from that possible alternate ground.

 

Show relay mics tend to be run a fairly high gain because of the weak input signal from distance mic'ing so any noise induced in the line will be greatly amplified relative to the same level of noise on a close mic'ed channel.

 

Mac

[Jivemaster]

Is there any chance that you could put a pre-amp near the mic? This way the signal to noise over the mic line may be improved. Is there a phantom pre-amp available?

[Whiskers]

Is the mic mounted on metal, thinking of earth loops or gradients

[sandall]

Also, are the XLR shells connected to pin 1 (a nasty habit of cheap made-up cables), in which case you may well have an earth loop ?

[dbuckley]

Even with the right mic cable, a cable tester will only tell you that there is continuity on the 3 lines. It will not tell you ....

 

... if some friggin' idiot has got the pins wired the wrong way and wired the cable shield to pin 2. Not once but on dozens of tie lines.

 

You can guess how I know this......