# Persistant hum



## Soxred93 (Sep 2, 2009)

Hi, I was setting up a stage monitor today, and I noticed that there was a loud hum coming out of it. Our current setup consists of 1 TRS monitor socket on both sides of the stage. I hooked up a mic to the board, and turned on the monitor out. The MONITOR OUT then goes to a feedback destroyer, and then to the monitors. 

To try to solve this problem, I first tried a different amp. Still humming. I then tried that amp on the other side of the stage, with the other plug. Still hum. I tried a different cable to the amp. Still hum. I tried going straight from mixer to monitor, bypassing the FD. Still hum. I even put a 50 foot TRS cable from the booth to the amp, and it still was humming.

Is there anything else I should try?


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## rwhealey (Sep 2, 2009)

Did everything work fine before?

If so, try to track down what changed. 

With a hum, I would suspect that there might be a ground problem. Did someone plug something into an outlet it wasn't plugged into before?


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## Soxred93 (Sep 2, 2009)

I set up a monitor about 4 months ago, which was the last time the stage was used seriously. I plugged the amp into the same outlet as before, using the same cable, on the same amp. This amp has worked normally earlier today when it was used for a guitar.

I even "reversed" the signal for a mic plug. Normally, there's a XLR plug that goes to the board, and a TRS back. I took the XLR plug at the board and put the output from the FD into it. I then plugged the amp in through the mic plug using a XLRM to TRSM cable. Basically, I tried a whole different circuit.


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## SHARYNF (Sep 2, 2009)

Make sure you plug the source and the amp all into the same outlet. This way they share the same ground connection. Check to make sure the ground/shield on the xlr cable is still secure, the it is not unusual for the ground connection inside the xlr to break. 
Sharyn


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## Soxred93 (Sep 2, 2009)

SHARYNF said:


> Make sure you plug the source and the amp all into the same outlet. This way they share the same ground connection. Check to make sure the ground/shield on the xlr cable is still secure, the it is not unusual for the ground connection inside the xlr to break.
> Sharyn



Ok, I'll try those two options. Currently, the amp is running through a rackmount power distribution center in the booth, so that will serve as a troubleshooter only. I'll also try another XLR cable. 

(unrelated to quote) Maybe it has something to do with the fact that the output from the wall is unbalanced mono, and the amp takes unabalanced mono, and I'm using a mono balanced TRS cable. I wouldn't think so, but it's worth a shot.


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## rwhealey (Sep 2, 2009)

A long unbalanced run could introduce noise into a system- I've seen it when a bad solder connection breaks the shield on one end. As soon as it's soldered back on, noise goes away.


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## SHARYNF (Sep 3, 2009)

If your cable connection is in fact an unbalanced run that could certainly be the problem. 
Sharyn


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## howlingwolf487 (Sep 3, 2009)

It sounds to me like you are using the wrong type of cable for the speaker. Not only is the wrong connector on the end(s), the guage of the cable might not be correct for the application, either (which could at least lead to a fried cable or worse, a fire).

You will need to use appropriately-rated/-gauged speaker cable with 1/4" TS connectors on each end. The standard is switching over to Speakon connectors, which lock and can carry multiple sets of connectors; if your speakers have them, I would look into using them.


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## museav (Sep 3, 2009)

Maybe I'm just slow this morning, but I'm a bit confused. You go from a console output to a feedback destroyer and then to what? Is the amp on stage or in the booth? What is the amp? What is the monitor? Is the run from the booth to the stage balanced line level out of the feedback destroyer to a TRS connector?

You mentioned that the amp worked fine with a guitar earlier in the day. Are you by any chance using an instrument amp as a monitor and plugging into the instrument input?


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## Soxred93 (Sep 3, 2009)

Ok, in order. Console -> FD -> stage. The amp is the monitor, I'm using a regular guitar amp as the monitor (It's all we have, and it's what I used before). I would think that the booth2stage connection is balanced, but I'm not sure if it is. 

I am using an instrument amp as a monitor, and plugging it into the only input it has. I would like to have an actual stage monitor, but that's just not in the budget right now.


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## SHARYNF (Sep 3, 2009)

The instrument amp will be in almost all cases an UNBALANCED input connection. It really is not set up to have a long run on the input. You would have to see if the cable that is running from the FB to the stage is a balanced 2 wire plus shield cable, if so you could look at getting a transformer based conversion from balanced to unbalanced, keeping signal balanced all the way to the instrument amp and then converting it. 

Sharyn


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## Soxred93 (Sep 3, 2009)

I set it up so that it was going through an XLR cable to the stage, changing to TS at the very last minute, and it still hummed.


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## mstaylor (Sep 3, 2009)

How did you make the conversion between XLR and TS?


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## museav (Sep 3, 2009)

What you essentially have is a long unbalanced run (it is going into an unbalanced input so even if the cabling to the wall is balanced, the circuit is unbalanced) going into a very high impedance termination. That makes for a nice antenna. In addition, you have an input with its own preamp intended to receive a very low level signal being fed line level. If the amp has an input that is after the preamp stage then that might be an option to try. Otherwise you might want to look at something like this, Radial ProRMP reamping device.


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## Soxred93 (Sep 3, 2009)

mstaylor said:


> How did you make the conversion between XLR and TS?



I'm using a passive DI box.


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## rwhealey (Sep 3, 2009)

The ground lift on that Radial box would probably also solve any grounding issues you're having that would contribute to the hum.


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## 2mojo2 (Sep 6, 2009)

Have you got something like an EBTECH Hum Eliminator you can run the signal through at the stage end to remove the possibility of a ground loop?
That is the most common cause of a persistent hum.


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## teqniqal (Sep 13, 2009)

Even if the output of the Feedback Destroyer is balanced, it is likely that the line level input to the musical instrument amp/speaker is un-balanced (Tip-Sleeve, not Tip-Ring-Sleeve), so somewhere the signal is getting unbalanced anyway. You did not say if the instrument amplifier has a two-prong or three-prong (grounded Edison or NEMA 5-15) plug on it. Speculation: If the power plug is two-prong, and the unit is an older device, it may be that the "difference" from "before" to "now" is that the plug is reversed in the wall outlet -- try flipping it over. If the Feedback Destroyer and/or the Amplifier/Speaker DOES have a grounded three-prong plug, then DO NOT use a 'ground lift adapter' -- THAT IS VERY DANGEROUS.

It is difficult transport an audio signal very far if it is unbalanced, so keeping the signal balanced as far along the way as you can is always a good idea. Adding an XLR-to-1/4" TS adapter that has an isolation transformer in it at the amp/speaker end of the balanced cable might help clean-up the signal. Adding a XLR-XLR isolation transformer at the output of the Feedback Destroyer might help, too, as you may be getting a ground-loop there, but it is unlikely, and this would leave you with a long unbalanced line that would still be susceptable to picking-up noise.

Another thing that may have changed is that the ground connection for the AC receptacle or the device plug at either end of the system may have failed over summer break. Yes, this can happen, and its VERY DANGEROUS. Check each outlet for proper grounding with one of those three lamp plug-in testers. Unplug and check each piece of equipment that has a three-prong power plug to see if the ground pin has continuity to the device chassis / and pin one of the XLR connector(s) and/or the "ring" of the TRS connectors.

Another 'difference' might be the introduction of another noise source that wasn't there before. Is the long cable between the Feedback Destroyer and the Amplifier/Speaker running parallel to any power lines along it's path? This can couple a 60 Hz powerline "hum" (it's humming because it doesn't know the words!) or a higher harmonic of it if it is a dimmed lighting circuit (bzzztt!). Separating the two lines will reduce this effect.

One more possibility is that the cable you are using today wasn't the same one you used before, and it is wired differently. Maybe the ground (shield) is not connected? Maybe one or the other of the signal lines is shorted the shield or each-other, or open circuit. Solder joits can fracture if they are 'cold' (i.e. - the wire is moved ever so slightly right before the solder cools from liquid to solid).

Finally, most modern devices have solid-state balancing on inputs and outputs, only the older and/or more expensive new stuf uses actual transformers. Chips fail. Maybe the output stage has failed on the source device, or the input stage on the destination device. Something a simple as a capacitor 'drying-up' can do this. This scenario isn't too likely as you said the problem remained when you omitted FD from the system.

Great resource for knowlege: JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS


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## Chris15 (Sep 16, 2009)

teqniqal said:


> Unplug and check each piece of equipment that has a three-prong power plug to see if the ground pin has continuity to the device chassis / and pin one of the XLR connector(s) and/or the "ring" of the TRS connectors.



If you are checking the ground connection on a TRS, you ought to be checking the sleeve not the ring...


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## teqniqal (Sep 17, 2009)

Chris15 said:


> If you are checking the ground connection on a TRS, you ought to be checking the sleeve not the ring...


My BAD. You are absolutely correct. Sleeve = shield = screen = ground = shell = pin 1 = chassis.


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