An elusive hum at the mains

[One_Dude]

I did a sound job today where the band, in order to save some money and my back muscles, wanted to use their sound system with me doing the engineering. After initially agreeing to this arrangement, I decided I was more comfortable using my mixer and amp. The band provided the FOH speakers, the monitors, and their own mics and cables. I provided my passive mixer plus an additional powered mixer.

 

They provided powered speakers for the FOH, and passive speakers for the monitors. I got the monitors working first and had a nice clean signal with no hum. When I hooked up the powered speakers we got a good signal, but also got a hum. Through process of elimination I proved the signal path good all the way to the powered speakers. The hum followed the powered speakers. I reversed polarity at the mixer which made no difference; there was no polarity switch at the powered speaker. This is the first time I have used powered speakers and have never had this problem with my passive speakers. I was finally able to reduce the hum by lowering the sensitivity adjustment at the powered speaker. Although lower in volume, the hum was still there. The hum was only marginally noticeable when there was program content coming through the powered speaker.

 

Is this an issue with the powered speaker, or with the mixing of powered speakers for FOH with passive speakers for monitors? Or is it a matter of neither I nor the owner of the powered speakers knowing how to adjust them properly?

 

Thanks in advance for your comments.

 

One_Dude

 

 

[BillESC]

Were your mixer and the powered speakers on the same electrical phase?

 

[onelife]

Were you using balanced lines from the mixer to the powered speakers?

[One_Dude]

Both good questions. Thanks.

 

The AC outlets were on different walls at the venue (a large multi-purpose room at a church) and about 80 feet apart, but I don't know if the circuits were in phase with each other.

 

I was using unbalanced speaker cable (1/4 inch ends) for both the powered speakers and the monitors. The powered speakers had speak-on jacks that accept either XRL or 1/4 inch. As I write this I realize however, that when I daisy-chained the powered speakers I did it with a balanced output jack on powered speaker number one to the speak-on jack at powered speaker number two.using an XRL cable. Maybe that was the problem.

 

One_Dude

 

 

[RoadRanger]

I did it with a balanced output jack on powered speaker number one to the speak-on jack at powered speaker number two.using an XRL cable.

Oy Vey, I hope not .

 

[onelife]

I was using unbalanced speaker cable (1/4 inch ends) for both the powered speakers and the monitors.

 

That was your problem. Because the speakers are powered, they will amplify any noise that is picked up by the cable that feeds signal to them. Speaker wires are unshielded and very susceptible to EMI (Electromagnetic Interference) which is everywhere.

 

Cables feeding any amplifier must be shielded to cut down on the EMI. Shielded, balanced cables are the best choice because they are also designed to cancel any noise that does get picked up by the cable.

 

If your mixer does not have balanced outputs (usually XLR) then you could use an unbalanced 1/4 inch cable but it would have to be shielded.

 

Since you were feeding the passive monitors with an already amplified signal, the unshielded speaker wire was actually your best choice because it can handle the larger amount of current from the amplifier. Shielding is not necessary in that case because any noise that would get picked up by the cable would not be amplified and therefore insignificant.

 

Under the circumstances, your choice to turn down the volume of the powered speakers and turn up the signal going to them was a good one because in gave you a better signal to noise ratio but it didn't solve your problem.

 

 

We learn from our mistakes and we all make them. I remember when I first learned about ground loops. It was a humbling experience but it has served me well ever since.

 

 

 

 

 

[onelife]

I think the OP confused the TRS/XLR combo jack with a speakON connector - so it wasn't as bad as it seems

 

I had to read the post a couple of times to fully understand it

[Craig Vecchione]

That might have been the problem, but the hum could also be from using two different AC receptacles, possibly on different circuits, and possibly on different phase legs. Either that or the unbalanced, unshielded speaker wire being used for line level signals are possible causes for hum.

[dboomer]

If you are trying to marry unknown equipment up to yours you need to carry some 1:1 transformer boxes that can also provide ground lift. They will almost always solve problems like this, no matter what the cause.

[Shaster]

Agree with others that it was either a circuit problem and/or using speaker cable for the powered speakers.

 

When possible, my mixer and amps or powered speakers live on the same circuit. At the very least, I check for ground and other electrical problems in advance - hums, shocks... and rectify to the best of my ability. There are also some cheap ground fault detectors that may or may not come in handy to you. http://www.homedepot.com/p/Gardner-B...3501/202867890

 

You should not use speaker cable for powered speakers, and BTW you should not use line cable, balanced or unbalanced for passive speakers.

 

Consider learning the difference between a Speakon connector and a XLR/TRS combi input, and while you're at it, even though it's probably rare in your circles, you should learn about PowerCon jacks. I just ran across PowerCon connectors in a hotel lounge, so they are out there, even at the bottom rungs of the sound ladder; which is wear I like to hang out

 

https://en.wikipedia.org/wiki/XLR_connector scroll down for a picture of a combi jack

 

https://en.wikipedia.org/wiki/Speakon_connector

 

https://en.wikipedia.org/wiki/PowerCon

 

One last thing; way back when, some speaker manufacturer's were using XLR connectors on their passive speakers (Yorkville comes to mind) this confused the issue even more, and this practice stopped AFAIK. Just thought I would mention it to muddy the waters even further!

[One_Dude]

Thanks for the excellent analysis; you are correct in thinking that I confused the combination XLR/TRS jack with a speakOn connector. The powered speakers did not have speakOn connectors. And in reading the user manual for the powered speakers, the only output jack listed is an "XRL out". So if daisy chaining is desired, using that output jack is the only option.

 

With regard to the unshielded cables; right again. My speaker cables are definitely not shielded, and I suspect that my 100 ft snake is also not shielded. It makes sense that, as explained, lowering the powered speaker sensitivity and increasing the signal sent to them will improve the signal to noise ratio and decrease the audible level of the hum. As mentioned, the hum was still there, but at a lower volume that didn't interfere with the program content.

 

I'll also check into the 1:1 transformer boxes that were recommended. This group was happy with the sound, so they may ask me back in the future with the same shared equipment arrangement. Next time I have a similar problem I will also remember to try using the same A/C outlet to power both my gear and the powered speakers.

 

Thanks for the suggestions and the great detective work; I'm sure your are right on the money as to the cause or multiple causes for the trouble. This was a good experience for me since no equipment was damaged, and a lesson was learned.

 

One_Dude

 

[onelife]

Just a quick comment about your snake.

 

Snakes are usually made up of balanced cables with the direction of the signal flow determined by the connectors on the end of the cables.

 

I would not run signal from an amplifier to a passive speaker through a snake. 100 Watts into 4 Ohms is 5 Amps and that is a lot of current.

 

If you are using a powered mixer, you would be better off getting some long lengths of lamp cord and soldering your own connectors on the and running them along with your snake. If you are using passive speakers and a passive mixer, you will need an amplifier and it's best to put the amplifier on stage near the speakers. You can use the balanced lines in the snake to connect the output of the passive mixer to the input of the amplifier(s).

[One_Dude]

Thanks again for the advice on running the output from my powered mixer through the snake to the speakers. I can see that the wire gauge in the snake is too small to handle the powered signal. I have done this a few times and have probably been lucky that there have been no problems. I typically run sound for small acoustic groups rather than high powered rock groups, which might explain why I have not run into trouble with this arrangement. But the repeated use of this practice could certainly end up damaging equipment somewhere in the signal path.

 

I also have some heavy duty speaker cables that I use sometimes. I see now that I should either use the heavy duty speaker cables all the time, or locate the amp at the stage end of the signal path.

 

One_Dude

 

[Shaster]

100 ' snake! So were you using line level outs on your mixer and then using speaker wires from the snake box.? I can't imagine you were sending amplified signal down a TRS cable, at least I hope not.

 

IIRC StratGuy22 had a snake with speaker cables and XLR cables that Yorkville made or perhaps still makes for their PM series mixers. Must be heavy though, and maybe not something you want. Perhaps SG22 will chime in.

 

Found the link. You have to scroll down the side bar to see them - 14 gauge wire 100' and 12 or 16 XLR's. Might just be cheaper or better, to buy a mid sized passive mixer and use the powered mixer for monitors. I dunno....

 

http://accessories.yorkville.com/cables/snakes/pa/

[Craig Vecchione]

I'm totally confused after reading this thread. The original post described active FOH speakers, passive monitors, a passive mixer and an powered mixer. The complaint was that the active speakers reproduced a hum. There is no description of what mixer is driving the signal used as input to the active speakers. There's a later description of a 100' snake with unshielded speaker cables.

 

Is this a "drive snake" which has some balanced cables used as mic and instrument inputs TO a mixer, and then has large gauge speaker cables typically with 1/4" TS plugs that are used from a power amplifier to the speakers, assuming the amplifier is located near the mixer, or a powered mixer?

 

If you could describe the entire signal path from an example microphone to the speakers, citing what mixer is connected to what by what cables, it might be clear whether there's a mistake in connectivity at play as well as any other signal issues. Brand and model of gear helps, as we can see what connectors are available.

[cappttenron]

I had a similar problem bought the transformer and poof problem went away. It stays in my bag now.

[One_Dude]

Sorry Its taken so long to respond; I was traveling all day yesterday.

 

Here's the gear lineup;

 

Passive Peavey mixer receiving all mic and instrument signals through the 100 foot snake.

 

Mixed signal sent via return 1/4 inch snake line to powered EV speakers.

 

Passive mixer monitor out sent to powered Peavey XRD mixer.

 

Amped signal from powered mixer sent to monitors via 1/4 inch snake return line.

 

Except for sending the amped signal to the monitors through the snake, I think the rest of the setup was OK.

 

Thanks One_Dude

 

 

 

v

[One_Dude]

Whoops forgot to include that nothing was shielded to my knowledge.

 

One_Dude

 

[agedhorse]

Almost certainly you had or created ground loops at several points throughout your system. Your snake is shielded, and the shields all leak together so you could have easily had a ground loop between the various inputs and the monitor power amp output.

 

ANYTIME you think outside the box, you really ought to know how all of the parts of the box really work. This could have turned out really bad if the monitor amp was class D and internally bridged. Then, you would have had voltage (and high current) on the shields leaking through to ground (mixer ground bus for one path) causing all kinds of costly problems.

[onelife]

Since you are only using the powered mixer as an amplifier, you could put it up on the stage.

 

My only concern is what might happen because it's unsupervised.

[Shaster]

Since you are only using the powered mixer as an amplifier, you could put it up on the stage.

 

My only concern is what might happen because it's unsupervised.

 

I would suggest putting the mixer at the stage as well, keeping in mind that ground problems could arise from having the mixer and the FOH and monitors on different circuits - just depends. If the powered mixer was a rack of power amps, that's where it would live anyway.

 

As agedhorse (and others) have stated, the OP is lucky that a hum was the worst of his problems. But don't push it!

 

[Craig Vecchione]

Whoops forgot to include that nothing was shielded to my knowledge.

 

One_Dude

 

Drive snakes are the only type that have some speaker cables, which may or may not have shielded jackets. All of the input channel cabling would be shielded. These types of snakes are very uncommon, so it's more likely the 1/4" cabling is simply returns that are shielded but, if they are TS and not TRS, are not balanced. TS would also be very unlikely if the ends are original....there's little point in using balanced lines for inputs but not outputs...it's still a hundred feet of cable.

[agedhorse]

Snakes with speaker cable are becoming extremely rare now that powered speakers have taken over. They never were very common, and can suffer from cross talk issues when gain is high on a mic channel and signal level is high on the speaker cable, there isn't enough CMMR in the board to cancel this out at higher frequencies.