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Hum In Monitors

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This one is really odd and I can't seem to solve it so far. I started to get whats sounds like a ground loop hum that comes and goes but mostly stays  coming from my studio monitors. I have a background in basic electronics, so I knew all of the things to check...or thought I did. Any additional ideas or insight much appreciated. 

Things I have checked:

- Focusrite  Scarlett I8i8 2nd generation interface, changed sample rates, tried various software I/O configurations. Tried re routing wires, removed all input cables. Plugged power supply into multiple outlets. Turned down all inputs.

- Tried different monitor cables. Moved monitors in relation to where they were. Moved electrical items in close proximity to different locations.

-Removed florescent/LED light bulbs from space. Powered down all routers and audio recorders.

-Checked the basement below studio for any appliances running. Checked elsewhere in the house to make sure nothing was running.

The hum seems to come and go. FWIW my monitors are plugged into the same  power strip as my computer. I have a well grounded electric service, modern 200 amp. I put it in. The only thing I can determine is maybe recently something is somehow feeding interference into my main power. This shouldn't be happening. 

I ordered a WattBox that has EMI/RFI interference protection. I hope it helps. I haven't changed my setup though and it's been like this for a long time with no worries.

Suggestions appreciated.

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Could it be the monitors themselves failing? That seems to be the one variable you haven't been able to test.

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First I would check you've not got any loose connections in your audio cables. The fact that it's intermittent points to something can could maybe be moved and unsettled somehow. Things like dry joints in balanced cable connections are prime candidates.

Also, have you any rack cases?

The reason I ask, is that I once had a ground loop I could just not explain in my live rig. I had two ABS cases of gear I used live: one for my vocal fx & mixer, and the other my PA amp. I could never reproduce the ground loop at home, but live it happened every time.

In the end it ended up being a ground loop through the edge of the rack case when they were sitting on top of each other. Sealing the edge of the case sorted it.

A lot of rack gear has it's case grounded. So check that it's not touching something else occasionally and causing the loop.

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Just a thought: have you or any of your neighbours recently bought a robotic lawn mower and in doing so installed perimeter wires? If so, make sure the wire is turned off when the mower isn’t running.

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Posted (edited)

When you say hum, is it actually 60 Hz?  If it is 60 Hz, no plug-in EMI/RFI filter is going to help you.  Is the hum coming from both monitors simultaneously?  If the hum is intermittent, I would suspect that maybe the electrolytic filter caps maybe going bad, but this would likely not affect both monitors at the same time.  I don't know what monitors you're running, but some of the M-Audio models, like the BX series suffer from poor magnetic shielding of the main power transformer and hum continuously.

I would try:

  • Powering up only one monitor at a time - is the hum only in one monitor?
  • Disconnecting all audio connections - do you still get hum?  If so it's power supply related - if not, it's likely an earth loop.
Edited by Bill Ruys
Updated

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There are some subtle house power wiring mistakes that can cause large 60 Hz EMFs intermittently.  If the hum goes off and on with nearby lights or appliances, there could be a split neutral.

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forgive me in advance for this, but.....the reason the monitors are humming is 'cause they don't know the words!

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