[andyr]

I recently bought a new TV and found it produced a fairly nasty hum when I selected "TV input" on my pre-amp. The TV and associated paraphernalia is in another room from my hi-fi and is plugged into a different mains power circuit.

However, my old TV didn't have this problem!

I tried unplugging the cable from the cable set-top box but the hum remained (so it doesn't seem to be caused by the cable connection, which I understand is a common reason for hum).

Then, as I happened to have a 1KVA mains isolating transformer lying around unused, on a whim I decided to see whether plugging all the TV units into it (TV, DVD player, VCR, cable set-top box, digital set-top box) would take away the hum.

Lo and behold, it did!! )

But I wonder why, since every one of the "TV components" has a 2-wire mains cable ... ie. they have no earth wires to cause an earth loop!!??

Can anyone explain why the isotrans solved the problem??

Regards,

Andy

[ctviggen]

Somewhere you have DC coming into that power circuit.  The transformer stops DC, thus no hum.  Now whether the DC is actually on the power circuit or a component is placing DC onto the power circuit, I don't know.

[andyr]

Somewhere you have DC coming into that power circuit.  The transformer stops DC, thus no hum.  Now whether the DC is actually on the power circuit or a component is placing DC onto the power circuit, I don't know.

Thanks, Bob ... makes sense.  Wonder how a component would place DC onto a power line, though?

Regards,

Andy

[Uptown Audio]

Hi Andy,
It could be caused by 'back emf" from a fan motor in a DLP or CRT set. I doubt that the flyback transformer on a CRT could cause this unless it was seriously defective or the yoke was broken. If it is a CRT set, do you hear a super high pitched whine? Take a look at the following link to see what "back emf" is: http://www.newton.dep.anl.gov/askasci/phy00/phy00351.htm . I may be completely wrong here, but if this is the case, you could avoid the hum by isolating only the TV?
-Bill

[andyr]

Hi Andy,
It could be caused by 'back emf" from a fan motor in a DLP or CRT set. I doubt that the flyback transformer on a CRT could cause this unless it was seriously defective or the yoke was broken. If it is a CRT set, do you hear a super high pitched whine? Take a look at the following link to see what "back emf" is: http://www.newton.dep.anl.gov/askasci/phy00/phy00351.htm . I may be completely wrong here, but if this is the case, you could avoid the hum by isolating only the TV?
-Bill

Thanks for the explanation, Bill.

The isotrans has 6 outlets so I may as well plug all the TV devices into it but, one of these days, I will experiment by plugging/unplugging them into the isotrans to see if I can isolate the culprit.  (There are no other devices on this power circuit.)

Regards,

Andy