Mic receiver rack causing electrical hum

J

jpcantalino

Member
Recently I was placed in charge of our school auditorium's lighting design and electrical crew. We have a show coming up in about a week. Currently we are running two floor to ceiling racks of ETC SR48 dimmers and one smaller rack of a few of the same dimmers for the architectural lighting. On friday, I set up our road case of Shure mic receivers used to, well, receive the actors beltpack mics. As soon as I plugged in the power cord to the backstage socket, a loud, low frequency hum filled the room. Upon unplugging the case, it stopped. We have had this problem in the past, however we simply ignored it, and the hum eventually stopped. We're not entirely sure why it only occurs when the mic case is plugged in, but we are sure that the problem may stem from the fact that every electrical socket in the auditorium derives its power from constants on the SR48 racks.

Any ideas on how to stop the hum?
 
I would guess that this is a classic ground loop situation as dimmers are great tools to find these. In all audio systems, there must be one ground. We run into problems all the time from churches calling to say that their dimmers are making noise in the sound system. After a brief chat, we find that they have added a new guitar or keyboard to the service and it is grounded on stage with the audio line running back to the mixer. Big ground loop.

As an experiment, you can run a large extension cord from the power at the mixer to the mic rack. Or, depending on where your mixer is, you can run setup the mic rack at the mixer.

Another option is to employ a ground lift on the audio send, not the power cord!!!

David
 
DavidNorth said:
I would guess that this is a classic ground loop situation as dimmers are great tools to find these. In all audio systems, there must be one ground. We run into problems all the time from churches calling to say that their dimmers are making noise in the sound system. After a brief chat, we find that they have added a new guitar or keyboard to the service and it is grounded on stage with the audio line running back to the mixer. Big ground loop.

As an experiment, you can run a large extension cord from the power at the mixer to the mic rack. Or, depending on where your mixer is, you can run setup the mic rack at the mixer.

Another option is to employ a ground lift on the audio send, not the power cord!!!

David
Click to expand...
I would second that. Ground loop. Not sure if you have an audio return for each receiver, but my bet would be you could unplug all the audio sends except one, and you would still have the buzz to some extent.
 
I think after we finish our production next weekend, I'm going to take a look at the amplifier rack for the house speakers. It happens to be a few feet from the SR48s. I just want to narrow the problem down to the dimmers themselves. Again, every single outlet in the auditorium, including the ones powering the amps, run from constants in the SR48s.

Unfortunately, my school's maintenance department doesn't have enough electricians to cover all the work being done, so this is really up to me to at least diagnose.
 
This appears to be a ground loop which is not caused by the lighting system. Is there a way that you can plug your receivers into the same circuit as the audio rack for power or the mixer?

Moving this thread to the audio forum as audio guys deal with this all the time and can offer better advice.
 
Depending on the quality of the transmitters/receivers and building construction you probably can run the receivers from the loft or from next to the mixer.
 
The problem with that is there are 14 SLX receivers inside a road case that we wheel out for our drama productions. We generally leave it plugged into power and an XLR snake off stage right. That's the closest (hidden) position we can get it to the actors. Our control booth is at the back of a 1,200 seat auditorium where the signal would be terrible. Running that extension up into the loft is my best option for getting the rack and the amps on the same lines.
 
The primary way to solve a ground loop on a balanced line is to break the connection to the audio cable shield at one end of the line. In this case, I would do it at the back of each the receivers. You can do this by cutting the connection to pin 1 inside the female XLR connector that plugs into the back of the receivers. Take care to trim and insulate the shield so that no whiskers of it can come in contact with the shell. Clearly mark the cable as "ground lift" so that it does not get used where it could cause problems, such as with a microphone. For this to work, all of the audio lines in and out of the rack need to be lifted.

Another way to accomplish the same thing, without modifying the cables, is to use a ground lift adapter.
http://www.markertek.com/Audio-Equi...Accessories/Hosa-Technology-Inc/GLT-255.xhtml

There are circumstances where lifting the shield on one end will not solve a ground loop, due to the design of the output and input stages, or where one or both sides are un-balanced. In those cases, inserting a good quality audio transformer provides ground isolation and common mode rejection that cannot be achieved any other way. Transformers can be audio life savers.
 
jpcantalino said:
The problem with that is there are 14 SLX receivers inside a road case that we wheel out for our drama productions. We generally leave it plugged into power and an XLR snake off stage right. That's the closest (hidden) position we can get it to the actors. Our control booth is at the back of a 1,200 seat auditorium where the signal would be terrible. Running that extension up into the loft is my best option for getting the rack and the amps on the same lines.
Click to expand...

I would try it from the back of the house. It'll only take a few minutes. The SLX is a very good quality system and I've seen longer throw distances than that out of cheaper gear. It all depends on the shape of the room and the type of construction. You may be very surprised. I had a Shure wireless unit about 15 years ago that could reach 400'-500' down the hall of the school through multiple classroom walls and sound crisp and clear.
 
This may also perhaps be a very classical case of what happens when lighting and audio power intermix. There is a very specific whitepaper I ran across some time ago detailing similar situations and the idea that lighting throws out dirty power. Will see if I can dig it up somewhere.
 
Lighting dimmers can create electrical noise due the extremely fast rise time of the load currents due to the way the dimmers switch. However, my experience is that audio systems are oblivious to it unless there are shortcomings in the way the audio equipment is interfaced or designed. Even the best equipment in the best environment will have hums and buzzes if the basics of proper interconnection are not followed. First, any un-balanced line longer than a few feet must be avoided. Secondly, care must be taken to prevent ground loops. If those two are neglected, it's just luck that a system is clean. This is just scratching the surface of a very complex topic.
 
Generally, if it's a 60 Hz hum, it's a ground loop problem.
If it's a buzz that changes as lighting levels are changed, it's dimmer noise.
The better dimmers have a choke coil inside that slows down the high frequencies.
Cheaper dimmers have small coils (or none at all).
Better dimmers have choke coils the size of a donut.
 
You must log in or register to reply here.

Similar threads

P
Pls Help! ETC Dimmer Rack going out!
Replies
12
Views
1K
John Palmer
J
W
Installs IT guy tasked with replacing middle school auditorium sound system (long post)
Replies
4
Views
3K
Jay Ashworth
Jay Ashworth
Crisp image
Control/Dimming Pathport, ETC ION and Ovation 910FC
Replies
1
Views
888
jtweigandt
jtweigandt
kiersplat
Dimmer pack flicker
Replies
5
Views
3K
almorton
A
StephenParticipatesInTech
Control/Dimming ETC Sensor & Unison control issues
Replies
8
Views
5K
brucek
B

Users who are viewing this thread

Total: 1 (members: 0, guests: 1)
Back