feedbackhi

[joshuashu]

hello, i know the answer to this question, but my worship pastor does not get it so i need some help convincing him. When the folks before me ran the dmx cable they ran mic cable, it runs down the wall parallel to some other network cables and such. It also runs in a tray around the room with florescent lighting. When our guitarist is playing, and we have our par cans on, we get feedback. as soon as i kill the pars the feed back goes away. Im pretty sure if we were to replace the cable it would help with the noise, some suggestions or feedback would be great. Thanks
josh

 

[DuckJordan]

Can you describe the noise, feedback is generally a screaching noise. Feed back comes from microphones picking up their own noise out of the speakers and reproducing that noise in a loop

 

[joshuashu]

yes it is a hum. kinda like when you get a bad cable it hums. its an electrical interference, not like sound feedback where its a really loud noise, this is a low hum.

 

[Les]

yes it is a hum. kinda like when you get a bad cable it hums. its an electrical interference, not like sound feedback where its a really loud noise, this is a low hum.

Ah, the 60Hz hum. I would recommend shielded DMX cable, and rerouting it if possible. I've never had a problem with DMX near instrument or audio cables, but it really shouldn't run parallel to any AC cables (crossing cables perpendicularly is usually permitted if kept to a minimum). That said, DMX isn't a strong enough signal to induce feedback - at least in my experience. Especially not a 60Hz hum.

I assume these par cans are on dimmer packs. Sounds like a ground loop issue to me, meaning the dimmers and the amp aren't electrically isolated from one another.

 

[JD]

Sounds more dimmer pack related. Three ways it broadcasts:
1) Ground loop
2) Close induction (Dimmer power and output cables to close to audio cables.)
3) Harmonic broadcast (Especially true on guitars without humbucking pickups.)

 

[gafftaper]

Do you hear the hum coming from the sound system or the PARs themselves?

 

[Footer]

Its not the DMX. DMX is a low voltage signal that won't effect anything like this. I'll put a hundred bucks on it that your dimmers are on the same power as your stage power that powers the guitar amp. Light should be on its own power AND the audio power should be on its own as well... and preferably on an isolated transformer. Also, ensure that everything in the PA, including backline gear, is grounded and has properly shielded cable. We run AC, dimmed AC, and all of that type of stuff right next to audio stuff all the time without issue. We have clean isolated power onstage and cable that is in good shape.

Finally, it could be lamp sing that you are hearing from the pars. Cheap dimmers tend to have undersized chokes that can lead to really bad lamp sing.

 

[FMEng]

This is a sound problem, not a lighting problem. The responses might be better if the question was posed on the sound forum. We need some details in order to help:

1. Is the guitar acoustic or electric?

2. Is it picked up with a mic or a direct box?

3. If a direct box, what kind?

4. Does the guitar feed connect to the mixer using an XLR mic input?

 

[Footer]

This is a sound problem, not a lighting problem.

Ha, wish it was always that easy! Its both. Odds are a solution from both sides will fix the problem. Getting the lighting on separate power on better dimmers is a start. Ensuring that everything in the PA is isolated and properly grounded is the other side of this. I'm pretty sure if I told an engineer "that hum you hear when the lights turn on is your problem, not my LD's", I would get punched in the face. Both need to work together to come to a solution.

 

[Ric]

This could be also as easy to fix as changing a guitar lead or getting the guitarist to change the pick up setting.
Some guitar pickups are notorious for picking up stray electrical noise.
Try those first !

 

[techietim]

We've had issues like this before. Could be one of the two things mentioned above but in my experience will be one of these.

Lack of a D.I. box - so the cable can pick up interference from various sources, especially over a long cable run.
Signal/Power cables too close - as you say the cables are all close to eachother in that 'box' which could cause the problems.

I would recommend trying a D.I. box if you are not already then try spreading the cabling out. You could also try shielded cables (which should help), as mentioned above.
Flourecent lighting can be a pain too!

Good luck

 

[StNic54]

If your pastor needs convincing, then it looks like he can have his music in the dark. If you are having any problems that is specific to your dmx run, your lights would be doing something funky, with ghost data or no data. Are you running portable dimmer packs?