Transient EtherSound Buzz

[Chris Chapman]

Ohhh the headache I have right now.

Rig Description: Yamaha M7CL, running 2 ES Stageboxes outboard. Box 1 for this show has 8 condenser mics with phantom power.

Setup was in for a week with no problems for rehearsals, 2 different shows, one being a road show. Fall Choral Concert completes rehearsals with no problems and clean sound.

After dinner break on show night, a 60 cycle hum was in all condenser lines on SB1. No additional lines had been patched, configuration was completely untouched. Reboot SB1, hum persists. Reboot M7 & SB1, hum persists. Take Audio System power down, reboot all again, hum persists. Swap SB1 w/ SB2, do a softpatch change addressing, hum persists. Recycle power on all systems, hum persists. Ended up going up late, and ran the sends at a lower level so hum would not be as present in house mix. Not clean as I'd like, but OK.

This morning I'm troubleshooting what happened. Power up all sound systems, and no hum. Bring facility up to show configuration, running light and sound cues, no hum.

What did I miss? Hum appeared BEFORE we opened house too.

Oh, and you should NEVER have 3 people troubleshooting an audio problem like this simultaneously. We ended up totally "gophering" the record patch. "Gophering" is when multiple hands are rooting around cable and signal path at the same time, making changes that should "help" and end up making the problem worse. Rushed incomplete patch and a lack of understanding the signal path made my record sends into total trash.

 

[JD]

New technology, but your problem is not digital, it is old world. Look at the analog end of things. Although ground loops will not affect a digital signal, they will affect an analog one. Things to suspect:
1) Any direct boxes being used?
2) Defective cables or microphones that allow the phantom voltage to drain. Moisture?
3) Possible problem in Phantom supply.
Ground/ground lifts on any equipment. Could very well be a ground loop appears only when some facility equipment is turned on. Label the stage lines at the box and start disconnecting things to see what equipment is involved in the loop.

Often, the only way to trace down a loop is the divide and conquer strategy.

 

[Chris Chapman]

New technology, but your problem is not digital, it is old world. Look at the analog end of things. Although ground loops will not affect a digital signal, they will affect an analog one. Things to suspect:
1) Any direct boxes being used?
2) Defective cables or microphones that allow the phantom voltage to drain. Moisture?
3) Possible problem in Phantom supply.
Ground/ground lifts on any equipment. Could very well be a ground loop appears only when some facility equipment is turned on. Label the stage lines at the box and start disconnecting things to see what equipment is involved in the loop.

Often, the only way to trace down a loop is the divide and conquer strategy.

That's my problem on this troubleshooting end, JD. Where the SB and the breakouts for it are were totally dry, with nothing else plugged into it with no changes. Tracking this ground loop buzz is awful because today, with again no facility change, it's totally absent.

 

[FMEng]

When you swapped stage boxes, did the hum follow the box or stay with the same sources? Consider everything plugged into that stage box. Was it only mics or did you have any DI feeds from something like a bass guitar or keyboard? Anything connected to the outputs? That is more likely where a ground loop would occur, but it could be more audible in the inputs.

 

[Chris Chapman]

It followed when we swapped boxes. Condenser mics sent from different areas, different snakes, and different hard patches. Only mics were used on this show. Mix of Shure Drop, AKG Floors. The outputs were sent to our video rig though...

 

[FMEng]

It followed when we swapped boxes. Condenser mics sent from different areas, different snakes, and different hard patches. Only mics were used on this show. Mix of Shure Drop, AKG Floors. The outputs were sent to our video rig though...

If, after the box swap, all inputs and outputs were connected the same, then my guess would be a faulty power supply for that stage box. If it happens again, turn off the phantom on a noisy channel, plug in a dynamic mic, and see if it's clean. That would tell us whether the problem is just on the 48 V rail. Since the problem showed up after dinner break and didn't show the next monrning, it might be a thermal intermittent. Leave the system powered up for many hours and then check it.

 

[Oldman]

One other suggestion is that the source of the offending signal may be some other piece of equipment that only operates some of the time. It could be totally unrelated to your audio system but simply creating a ground loop or coupling the 60 cycle signal into your system.