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I finished wiring 4 stations around my music room to my mixer, and tested everything and I have continuity through all the cables to the mixer. I tested the guitar jacks, and everything is sounding great. When I tested the mic connections, two of them are great, they are both on the same wall that is an inside wall.

However the other two XLR's are behind the baseboard on an outside wall. There are electrical outlets on that wall, they are about 12 inches above the baseboard. One of these two XLRs is presenting a buzzing sound in the cable.

I have checked continuity with a meter, and I have no cross connects, I used a cable tester, and all the lights are fine, so the cable appears to be connected properly. If I plug in a microphone at the workstation, and look at the board everything appears fine, except the level light for the gain is slightly lit. If I remove the microphone I get a loud buzzing in the speakers, the gain light stays slightly lit, and if I reattach the mic, the buzzing disappears.

If I press the PFL button to see how strong the signal is, I am getting a very high signal in that cable with out a microphone attached, and if I reconnect the microphone, the buzzing goes to the background. And the microphone work pretty good.

None of the other XLR channels/cables are doing this.

I have pics but could not find the place to add them. They show the entire mixer, all the setting are the same on each channel, then the next two pics show the PFL activated, and the steady on level light, and the huge noise that is induced by the cable to the mixer... any idea what this might be?

Thanks

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2 Answers 2

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Balanced line is supposed to be good at rejecting induced signal such as mains hum - but you shouldn't tempt fate by running parallel to mains at all.

If you need to cross mains, cross it at 90°. If you absolutely can't avoid it try to stay at least 2ft away at absolute worst.

I'm assuming it's mains hum - quick test would be to switch off that ring & see if it disappears.

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  • When you say mains, do you mean the speaker cable runs? My speakers are on speakon connectors, and the speakon cable runs across the ceiling and the mic cable is behind the baseboard, so they are far enough away from each other. And I am not sure what you mean by" switch off that ring"????
    – Dusty0b9
    Commented Jan 24, 2021 at 13:28
  • Mains voltage, depending on country - 240v 50Hz or 120v 60Hz. If you put a tuner near the audio interference, 50Hz comes out a bit sharp of G, 60Hz is slightly flat of B. You kind of get to recognise the pitch after a few years (or decades) of chasing it down at a myriad gigs & cheap studios ;)
    – Tetsujin
    Commented Jan 24, 2021 at 13:34
  • I just tried something, I put an XLR connector on a 20 foot piece of XLR cable, and ran it across the floor in the center of the room away from all the power outlets, and that new cable has the same noise. I started turning things on and off, and noticed that the led lighting in the ceiling changes the amount of interference when switched on and off, but did not eliminate it entirely. is is possible that the sum of all the wire runs, 200 feet over 8 connections could be the problem?
    – Dusty0b9
    Commented Jan 24, 2021 at 13:57
  • It's certainly a possibility. Just to eliminate your preamp, though, swap to another one. Make sure it's not the unbalancer that's not doing it's job properly - as I say, balanced line ought to be pretty good at rejecting induced hum
    – Tetsujin
    Commented Jan 24, 2021 at 13:58
  • is there any kind of filter or ground I can install that will fix this one run??? Or do you have any other ideas for me to try, I am at a loss to figure this out!
    – Dusty0b9
    Commented Jan 24, 2021 at 14:00
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In the comments you write "200 feet over 8 connections". That's just asking for trouble. Balanced cable is very good against interference (assuming a balanced source and a balanced differential input: if you use an adapter for using an unbalanced source or recipient, a significant part of the advantage goes out the window). But what is good against interference is the balanced cable. The connections are quite weaker. While in a pinch you may connect several XLR cables, 8 connections are just nonsensical. If you say "wiring my music room", the idea would be to run an uninterrupted piece of shielded balanced cable between your target panels. This will also make it less likely that some connection has bad wiring. In your case, an interrupted shield would be one possibility: the microphone would still work, but the shield following the interruption would stop doing its part for minimising interference. Another possibility is that the shield has contact somewhere with ground or other potential, creating a ground loop or worse.

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