2. amps-preamps
  3. 1347156354

Amp hums in standby mode when TV is on

Hi, my Sherbourn 5/1500A power amp starts humming whenever the TV in the same room is operating. This occurs even when the amp is in its standby state, and also happens when using the amp(although you can't typically hear the humming at that point because of the sound from the speakers). What's more, as the image on the TV varies in brightness, the intensity of the humming/buzzing also varies. If the TV is off, the amp is dead quiet. I am assuming this is some kind of ground loop problem.

Should I try to plug the amp in with a "cheater plug"? There is a sticker on the back of the amp that indicates that the amp must be used with a grounded power cord.
mtrot
gbart
290 posts
 
Jim, my e-mail address is sae_2300@optimum.net. You forgot the underscore. :)
jea48
4,405 posts
 
Gbart,

My bad.
Thanks for the heads up.
sae_2300@optimum.net.

Gbart, I have a question for you.... Assuming the TV's SMPS is putting DC back out on the mains what would happen if a DC blocker was installed between the TV and mains power receptacle? Would it block the DC offset voltage from the TV from going back out on the mains?
.
Jim
gbart
290 posts
 
Jim,you asked an interesting question about something I hadn't thought of before. I tried a quick test with a pair of back-to-back caps bypassed by diodes, between a known source of DC offset (400mV) and mains. I connected a toroid transformer that is susceptible to DC on another branch circuit. The DC blocker reduced the audible level of transformer noise by about 2/3.
jea48
4,405 posts
 
Gbart,

Thanks for setting up and running the bench test experiment.

The reason I asked the question is so the user of a DC blocker could install the blocker on the known offending device or piece of equipment. In this instance the OP's TV instead of the power amp.

For those out there that are feeding their amp from a high dollar after market power cord would not have to add the DC blocker into the mix. The extra connections of another plug and receptacle plus the power cord or in the case where the DC blocker has an IEC inlet connector, another high dollar after market power cord.

The DC blocker reduced the audible level of transformer noise by about 2/3.

The million dollar question?

How does that compare to the audible noise lever of the toroid xfmr connected directly to the DC blocker?

Thanks,
Jim
gbart
290 posts
 
Jim,
I plan on doing a comparison of the blocker at the source of DC versus at the toroid. I could not do a thorough test the other night because I was working with exposed mains connections and wanted to make it quick. I will try again when I have a blocker properly assembled and enclosed in a box.

Glen
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