(How) Can an earth loop be intermittent?

[Sashoir]

One of my subwoofers intermittently develops a hum which I *think* is an earth loop (if I put a 1:1 trafo between the source and the sub, the hum disappears, and the hum is at the right pitch for 50hz AC). So I can "solve" the problem.

But if I turn the sub's plate amplifier off and then on again, often the hum goes away. Almost always it will go away if I wait long enough with the sub's power off. And sometimes it doesn't develop for hours.
I understand enough about electronics to know: a) I know nothing; and b) how an earth loop arises, but I'd love to know why the issue is intermittent and particularly why the old "turn it off and on again" often works. By the bye, this happens without any new power draws (or former draws being switched off) on the circuit, so I guess I'm asking why there's a change in potential without anything causing it, from a layman's perspective.

Thank you,
S.

 

[AdamG]

Is the sub connected to a shared breaker circuit? Maybe someone else or even you are flipping a light switch or some appliance that is connected to the same breaker?

Just a stab in the dark.

 

[Sashoir]

Thank you, Adam. So far as I can tell, no: it arises with no changes to the load on the circuit that I can see (including when there's only "idle" draw). It never disappears on its own, however, so far as I can recall.

 

[AdamG]

Do you have a cable tv box in the loop? Either conned via an AVR/AVP or HDMI cable? Disconnect the cable box from wall power and any interconnects. Does that fix it? Does the sub hum with the RCA Jacks unplugged?

If disconnecting the Cable Box fixed it. It’s because of a mixed up pin configuration with the HDMI port on the cable box. This product lifts the offending pin and can fix the ground loop. It’s cheap and worth a try.
HDMI Adapter

 

[Sashoir]

Thank you, Adam. I very much appreciate your help.
Alas no television or set-top device.
The sub has never hummed with its line-in disconnected. I suppose it *could* though, having not tested extensively (which I shall do in the near future). In which case presumably the problem would not be an earth loop, but some issue with the amplifier's power supply, and the "fix" with the trafo was just coincidence.

 

[pseudoid]

Any heat issues as you seem to indicate it is a long delay before unset of hum?

 

[Andysu]

it seems like a capacitor that has gone faulty and easy repair , just which capacitor , need device to detect faulty , multi - meter / volt meter

i have psu for d-theater d vhs needs repairing with new capacitors as its producing some under-voltage not enough to power the video/audio processing on the player so have change ( have all the parts just not been motivated for 4 or so months to start de - soldering soldering )

so what you have is common capacitor fault issue

 

[Sashoir]

Any heat issues as you seem to indicate it is a long delay before unset of hum?

Thank you, pseudoid. You could well be on to something: the sub-woofer's plate amplifier is reasonably well-ventilated, but I imagine that the heat conduction is asymmetrical, so that heat builds up within the enclosure. What is then the thermal/electrical mechanism for changing the potential difference to earth? The heat buildup in the sub-woofer's amplifier increases resistance, and the earth level at the subwoofer relative to the source's earth level rises? I apologise for the total ignorance: I've bought and read a basic electronics book, but it doesn't seem to cover this sort of "practical" thing.

 

[Sashoir]

it seems like a capacitor that has gone faulty and easy repair , just which capacitor , need device to detect faulty , multi - meter / volt meter

i have psu for d-theater d vhs needs repairing with new capacitors as its producing some under-voltage not enough to power the video/audio processing on the player so have change ( have all the parts just not been motivated for 4 or so months to start de - soldering soldering )

so what you have is common capacitor fault issue

Thank you, andysu. I'm not yet confident enough to open a mains-powered device (at least, if I ever want to plug it in again).
Does anyone know whether you can get an e.g. low-current, 2.3V AC power supply to use for learning, where if you get shocked, it's at 1% of what would happen in real life? Training wheels, if you like?

 

[Andysu]

open it up , now and do video yt it , lets have a look , there be ether a leaking capacitor kinder like volcano of larva or it would look bulging like its been on steroids those are some of visual signs to look for other is volt meter have to de-solder and test the pins on the cap should give a low reading of sort , someone else may explain that better

 

[Andysu]

open it up , long as you don't go sicking your fingers all over capacitors i guess few on there will have some huge amount of voltage still stored in ( cap ) enough to give a jolt or can be lethally fatal , have to show it some degree of respect then you be confident , but always ( exercise extreme caution ) if unsure

caps be common parts on ebay cheap and soldering iron solder de sucker , spool of solder , but i won't encourage if not confident enough

 

[MarnixM]

Is the sub connected to a shared breaker circuit? Maybe someone else or even you are flipping a light switch or some appliance that is connected to the same breaker?

A scheme:

 

[JSmith]

develops a hum

If it's a low pitch level hum, it might not be a ground loop as often ground loops are more of a buzz at the 2nd or 3rd harmonic of the AC frequency (i.e. 100/120Hz 150/180Hz) and a bit of a higher pitch. This could be EMI, especially if intermittent... you're not running any interconnects close to an AC cable are you? Maybe the shielding is damaged or poor on some of your cables.


JSmith

 

[DavidMcRoy]

Do you have a cable tv box in the loop? Either conned via an AVR/AVP or HDMI cable? Disconnect the cable box from wall power and any interconnects. Does that fix it? Does the sub hum with the RCA Jacks unplugged?

If disconnecting the Cable Box fixed it. It’s because of a mixed up pin configuration with the HDMI port on the cable box. This product lifts the offending pin and can fix the ground loop. It’s cheap and worth a try.
HDMI Adapter

A modem can be a culprit, too In my experience.

 

[Sashoir]

If it's a low pitch level hum, it might not be a ground loop as often ground loops are more of a buzz at the 2nd or 3rd harmonic of the AC frequency (i.e. 100/120Hz 150/180Hz) and a bit of a higher pitch. This could be EMI, especially if intermittent... you're not running any interconnects close to an AC cable are you? Maybe the shielding is damaged or poor on some of your cables.


JSmith

Ah. Thank you, JSmith. It's definitely lower in pitch than that. The phono cable to the sub by necessity terminates near the IEC inlet of the sub, so perhaps that's it. Would the 1:1 trafo "fix" electro-magnetic interference, though?

 

[Sashoir]

A modem can be a culprit, too In my experience.

Thank you, David. MoDem (and an unholy assortment of switches) are all on the same circuit as the stereo. Alas I can't move any of them (I'm in a small flat, and everything is in our "everything" room (i.e. the room which is not the bathroom, bedroom, or kitchen).