Main Buzz on Dressing room monitor

[Ian_P]

HI all,

I have a dressing room monitor that comprises a SM58 in the stage pros arch, a 25m mic cable running through the attic to the dressing room, plugged in a powered speaker pair - a Behringer SM16 if I recall correctly.

We use this for the cast to monitor the audio from the stage - usually plays so spoken word.

All works fine until I turn on the stage betapack3 dimmers, when a mains buzz is heard - which persists until they are turned off.

The stage PA doesn't have this issue, and I recently installed a battery powered DI box to the monitor to try lifting the earth - but this makes no difference. The building is single phase so everything is on the same dist board for power.

Any ideas what to try next?

[themadhippy]

Is the mic input balanced? and if it is are you running it balanced.

[Bazz339]

Following on, even if the cable is balanced mic (Not DMX etc.) The input to the speaker needs to be balanced as well. 

If it is balanced at both ends, is there an earth lift switch on the speaker? Have you tried that in both positions?

Does the cable run adjacent to lighting cable, if it does move it well away. 

If that doesn't do anything you  could try replacing it with quad cable.

 

P.S. The mic, being dynamic might be picking interference directly if it is adjacent to dimmer cables or lights. If everything is on the same phase it may be mains carried. 

Edited March 9, 2023 by Bazz339

[KevinE]

I'm happy to be corrected but isnt the MS16 a computer speaker (pair) ? The manual does not say if it's balanced or not, but the mini 3.5mm TRS jack would suggest to me a standard PC microphone pinout, ie screen, signal and possibly DC bias. It just doesn#t say. 

 

I'd be tempted to use a local pre-amp at the mic end and then run the signal at line level into one of the phono inputs. I suspect the cable's just getting induced hum.

[Simon Lewis]

The MS16 is indeed a computer monitor type speaker (I've used them for this purpose for many years) although it's marketed as a studio monitor.
The mic input is designed for dynamic mics.

 

[sunray]

One of the guys I work with has some behringer speaker pairs , look like about 4 inch plus a couple of 1inch holes above.

Whenever we use sm58 it's with a transformer, XLR to jack.

IIRC the jack I/p is unbalanced and fairly high impedance so presumably the transformer is balanced to unbalanced and with some impedance matching.

 

I've had a dynamic pickup AFILS and the cure was condenser mic but that will require phantom power or a battery.

[sunray]

13 hours ago, sunray said:

One of the guys I work with has some behringer speaker pairs , look like about 4 inch plus a couple of 1inch holes above.

Whenever we use sm58 it's with a transformer, XLR to jack.

IIRC the jack I/p is unbalanced and fairly high impedance so presumably the transformer is balanced to unbalanced and with some impedance matching.

 

I've had a dynamic pickup AFILS and the cure was condenser mic but that will require phantom power or a battery.

Confirmed they are MS16s and transformers are 1 to 3.3