Trying to track source of transformer hum

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Old 12-07-17, 10:41 AM
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Trying to track source of transformer hum

The torodial transformer in my stereo power amp will emit an occasional, slightly loud hum, louder than the normal, near-silent hum. I have somewhat narrowed down my hunt to the 50A sub panel breaker inside my main panel. May not be the root cause, but when it's humming slightly loud, I can open the breaker and the hum silences to a barely-audible hum. Even if I close the breaker right away the louder hum does not come back right away.

However, if I leave the 50 amp breaker closed and go to my sub panel and turn off all the circuits it won't change.

The circuits on a sub panel consist of, washer, dryer, fluorescent lights, refrigerator and microwave ...pretty much everything to work against me.

I have replaced the 50A breaker in the main panel to no avail.

Something I noticed this morning that when the microwave is on there is a louder home from the amp and when the microwave stops the loud hum diminishes.

I have also opened all breakers in the main panel except for the dedicated 20A feed for the amp, hum is still there at times.

House was built in 78, 200A service, any chance it could be a grounding issue outside? Perhaps oxidized or tarnished contact areas?

I do have dimmer switches, ceiling fan remote control dimmers, as well as some illuminated light switches in the house, but not on sub panel circuits.

I took the amp to my mom's house, different county, and it only emitted a normal, very quiet hum all day.
 

Last edited by i6pwr; 12-07-17 at 10:53 AM. Reason: Content
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Old 12-07-17, 10:58 AM
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A small amount of DC on the AC power line can cause a lower quality toroidal transformer to hum. I wouldn't be particularly worried about it.

Yes.... the microwave can introduce DC on the AC line.
 
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Old 12-07-17, 11:59 AM
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Would it introduce DC when not in operation, aside from the clock? I agree about lower quality transformers but this one is a good one and it doesn't hum any louder than having to press your ear against the case to hear it when operating normally.

When it's loud I can easily hear it from 4 feet away.
 
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Old 12-07-17, 12:42 PM
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Maybe the transformer mounting needs to be tightened.
 
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Old 12-07-17, 01:25 PM
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I did check that out....seems to be secure.
 
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Old 12-08-17, 06:28 AM
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I would remove any mounting hardware from the toroid and retest. If the toroid continues to emit noise unmounted, then replace it. You could try to encapsulate it in epoxy, but even that may not eliminate the vibration/hum.
 
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Old 12-10-17, 02:28 PM
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I have illuminated light switches in the hallway, the switch on the left works the foyer light in conjunction with the foyer switch.

All appear to be 3-way switches, but the foyer light only goes from 24V to 35V when switched on one leg.

Here's a sketch of the 3 switches, the top number is when the switch is on, bottom number is when off.

This is measuring to ground.
 
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Old 12-10-17, 02:32 PM
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If you are reading the traveler voltage with a digital meter..... it's very likely you'll see odd, ghost, voltages caused by inductive pickup from a dead wire picking up induced voltage from the live wire in the same cable. That's a fairly normal occurrence. Usually those voltages are checked with an actual voltage checker that has a load in it and it ignores the induced voltages thru loading.
 
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Old 12-10-17, 10:02 PM
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I guess I didn't fully investigate the circuit. The left switch is joined to the 3-way foyer switch. So the 24/ 35V wire is the traveler from the foyer and becomes 122V when the foyer switch is flipped.

I also noticed the 3 hall switches show .08V when the breaker is open along with my living room outlet breaker that's open. But when I close the living room breaker and leave the hall/foyer breaker open, the .08V on black at the hall switches jumps to 2.-2.1V.

Is that normal? Doesn't seem like much, but always curious about ground loops or stray voltages while chasing this possible gremlin.
 
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Old 12-10-17, 10:20 PM
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Yes.... perfectly normal.

Finding grounds loops is next to impossible. A ground loop is a circuit that connects to multiple grounds or grounds at different locations. Since everything is at ground potential.... there is no easy way to determine what the actual points are.
 
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Old 12-12-17, 08:32 PM
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What baffles me is it will hum constantly, but as soon as I turn the tv on, it goes dead silent, like you shut it off, except for shortly after it will hum in waves....about 8-10 seconds apart and gradually rise and fall. On the down slope of the so called wave, it goes silent, but will hum louder on the peak then 10 seconds later it repeats.
 
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Old 12-16-17, 12:58 PM
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Without opening all the receptacles on my sub panel to check for wiring issues, I did notice something and wanted to ask about it.

My sub-panel is near the HVAC, but the HVAC is fed through my main panel. There happens to be an outlet mounted next to my air handler and that outlet is fed from the sub. My whole house humidifier is powered from that outlet but the humidifier is also wired to the air handler for the 24V to automatically turn on the humidifier.

Could this possibly cause any unwanted loops?
 
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Old 12-16-17, 02:25 PM
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A hum in waves is the transformer resonating.

The humidifier uses a very small bit of AC power. Possibly a small fan inside and small transformer. I wouldn't think that would cause any noise to your circuit.
 
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Old 12-17-17, 10:12 PM
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The humming in waves is only with the plasma tv on. I agree the humidifier is likely not the culprit, but was curious about a ground loop between the two panels.
 
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Old 01-05-18, 09:27 PM
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Ok,.let's set aside the plasma tv, as they have been known to cause this type of hum. This is occasional and is not an issue.

What's more interesting is my sub panel and how it affects the amp. Instead of shutting down the entire sub panel, I shut down each circuit individually. The amp was humming today and I shut off the microwave breaker and the humming stopped.

The microwave was not in use except for the clock and maybe the surface lights...which are incandescent.

Could the microwave leak DC without being in use?
 
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Old 01-05-18, 09:40 PM
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Have you tried unplugging the microwave but leaving the breaker on? That would narrow it down. There's no guarantee that someone didn't tap into that circuit.
 
 

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