High-Freq "Hum"

[Anonymous067]

I have been working on a show this week, and the house system is giving me a rather annoying high frequency (15k +) humming noise.

It was not anything I have hooked up, because I powered the system up, and it just started humming. It is intermitent for sure however.

The ONLY thing I can think of would be the receiver I removed from the rack of fixed devices, and it is fed directly into a DSP Automixer....could this be the culprit? I put the receiver back, and the noise continued however...

I'm at a loss for where it could be coming from???

 

[g15]

have you tried connecting an audio source? if memory serves, our system used to hum if just the amps were powered on...

 

[epimetheus]

I'd pick a spot in the system and start disconnecting cables to see which piece of equipment the signal is coming from. For example, you could start at your amps and unplug their input; if you still have the hum, then it's from amps downstream. If that stops the sound, then start working upstream, one device at a time. You should be able to determine at least which piece of equipment is the culprit. The key thing is to only change one variable at a time - unplug a cable, if that doesn't work, plug it back in and go to the next.

 

[FMEng]

Take Epimetheus' advice and carefully and methodically go through the system piece by piece. Do not let this go because it may get worse to the point of destroying speaker drivers.

The noise is probably circuitry in one of the pieces of gear going into oscillation. This often happens when the power supply filter capacitors dry out from heat and age. Not to pick on Rane, because they build fine equipment, but I have had this happen on their graphic EQ's.

If you can find the offending unit, and are handy with component level repair, the easiest cure is often to replace ALL of the electrolytic capacitors, especially the ones place near heat sinks.

This is why is always desirable to leave air space above and below equipment. Cool equipment lasts much longer.

 

[Anonymous067]

Got IT!!! (well..kinda)

I ended up putting the wireless mic receiver back in (that I had taken out). When I hooked it up (antenna's and all) it worked fine, no weird noises.

Just for curiosity sake, I started pulling cables from the receiver one by one to find the problem.

Power cable out-no change.
Mic cable out-no change.
Antenna A cable out-high pitch noise. Unplug Antenna A cable from active splitter, noise disappears!
Antenna B cable out-high pitch noise AND what sounded like 60 cycle hum. Unplug cable B from active splitter, just the humming noise, no high stuff.
WTF???

I ended up just pulling a different receiver out of the rack, and unplugging those cables with no problems??
It worked...but I'm still curious...

 

[epimetheus]

My first guess would be a bad splitter, also is the splitter grounded correctly? Maybe you have ground lift switches on the wireless receiver outputs? 60Hz hums are almost always ground loop problems.

 

[Anonymous067]

But why does this high pitch noise come into the system when the ANTENNA CABLE is disconnected...and how would an antenna cable cause a ground problem?

The weirdest part is if I disconnect the cable not only from the receiver but the splitter, the noise remains, (but only on this one receiver). I didn't try switching ports on the splitter (which is probably the problem...) but I'm just too tired of the problem to do that...or so I say at this hour.

Any other thoughts?

 

[epimetheus]

When the antenna cable is unplugged at only one end, it can essentially act like another antenna. Maybe it's picking up noise for a nearby switching power supply that is filtered out when the cable is plugged on both ends. At this point I'm just throwing out some suggestions, it's kind of hard to trouble shoot via forums.

As for the possible ground problem (if that's really what it is, again that was just a suggestion), you're antenna splitter is active, right? So it's got a power supply, and your receivers have power supplies, and your receivers are plugged into the sound board, which has it's own power supply. The BNC shell of the antenna cable can cause a ground loop just like the shell of an XLR can.

 

[Anonymous067]

Allow me to clarify.

The receiver is not IN the rack when the hum is created. It is REMOVED.
The splitters for the antennas are..well...I don't know.

One would think they're active, because we have three receivers, and remote antennas....
But I can't find ANYTHING that looks like a psu for the antennas. For the record, the DC on the antenna's on the ULXP Receivers have been disabled per post-factory modifications.

 

[crewcap]

the same thing happend to me i use my own pa for my theater and the first show we did it was giving off a huge hum then we tryed another amp and it went away