Low level hum in head unit

[Papahyooie]

My father gave me his Sony home theatre system that he just replaced because there is a low level hum that is almost always present. When you turn the volume up, the music overpowers the hum, but with the volume low, you can still hear it. I'm sure it's some kind of interference, but I can't seem to find out what.

Things I've already tried:

Moving the unit (radio/power interference from outside?)
Different outlets (dirty power?)
Rerouting interior power cables (power cables next to signal cables causing hum?)
Switching signal sources, moving speaker wires around to different connectors, switching input methods (surround sound, etc) and general troubleshooting.
Looked at the boards for obvious problems, bugs, bulging caps, loose connectors, etc.

So far, no dice. Some more info:

The hum seems to be lessened on certain inputs (phono and FM radio are lower than CD player for instance)
The amp has two speaker "rails" (dont know what else to call them) A and B. B seems to have a little bit less hum.
It seems to have a little less hum on the surround modes, and moreover, a little less hum on specific types (dolby mode has less hum than simulated mode for instance) but I'm sure that just has something to do with the different sound configurations lessening it and not an actual reduction in the interference.
It has several power plugs in the back for the other componants (cd player, phono, FM radio, Tape deck) but unplugging those and using wall outlets does nothing to lessen the hum. I'm fairly confident it's not an outside source of interference.

Anyone got any ideas? I'm usually good with the basic stuff like above, but this one has me stumped.

 

[Champ]

if this were car audio, I would be all over this I want to get into home audio thou.

 

[hat]

Why not give it a shot anyway? The worst that can happen is whatever advice you give him is totally irrelivent.

 

[Steevo]

http://www.audioholics.com/tweaks/c.../ground-loops-eliminating-system-hum-and-buzz

This was the whole reason I added a new ground rod to my home, plus soldered the connection, and added a ground point at the water pipe, and grounded the OTA/FM antenna to the same post. In my case the old grounding rod was eaten away and it would switch back to the old plumbing ground by the water heater that was also corroded.


Add a new ground rod, and test you current grounds for proper power dispersion levels. A easy test with 120V for a ground is to use a 20AMP fuse and see if it blows the fuse with a hot lead attached to the ground and NO ground wire attached to the ground rod. Then to test for ground point difference use a multimeter that is capable of dealing with a bit of electricity and see if the ohms of resistance are close to one another on the two points. Attach the hot lead to the positive side of your home wiring, a outlet works fine, disconnect the ground from the other and test each one with NOTHING ELSE on the whole home circuit on.


If the ground potential is almost nothing or your readings are a few hundred miliohms of each other you are OK. It should bleed off the ground potential difference at the closer of the two grounds instead of backfeeding and causing interference.


I had cable entering the house, going through a amp box on one end, and it was plugged into wiring that met the copper plumbing first, my amp was a new wire ran from the breaker panel to the theater just for that purpose. It was connected first to the grounding rod.

In the case of internal hum, there must be a bad solder connection, so unless you can get a wiring diagram and start tracing leads you are screwed.

 

[department76]

sounds like 60hz noise from a ground-loop, probably due to crappy wiring in your house and/or power strips. this occurs when the resistance to ground varies across where all of your components are plugged in. the end result being that for one device plugged in, instead of ground being 0V it will actually be some small non-zero voltage. hope that makes some sense, it's the same exact problem seen in car audio, no difference between AC and DC in this case. the easiest solution in HT i've ever found is to plug in your sensitive devices (ONLY your stereo or other components) with a 3-prong to 2-prong adapter, bypassing the ground connection alltogether. this is a solution that isn't possible for car audio (DC).

or you could try what the guy above suggested

 

[BazookaJoe]

Another very real possibility is that the unity is just faulty.

It CAN happen.

If its still under warranty - I'd get the store to have a look at it. Not knowing how recent the device is, If the store still carried them - you may be able to compare it to another unit at the store and see if they ALL hum, or is it just your device.

 

[SabreWulf69]

Yeah my best guess ground-loop problems.

 

[qubit]

I don't think it's a ground loop problem, because his dad's new unit doesn't do it and this one does regardless of how he connects it.

The most likely reason to me, is that it's internal to it: the AC power smoothing from the transformer is insufficient and is likely caused by marginally faulty components such as capacitors, resistors, transistors etc going out of spec. The most likely candidates are the caps though.

If it's under warranty, then RMA it, otherwise only poke around inside it if you're confident with this sort of stuff (it's not for the faint hearted and can be dangerous with high voltages).

If it helps any, I've got two small audio amps, both Aiwa MX-70, one hums just like you describe and the other one doesn't at all, so one is faulty. Both are about 25 years old.

 

[SabreWulf69]

mm makes sense

 

[Papahyooie]

Thanks everyone for your replies!

I will try the grounding rod method first, but qubit makes a good point. My fathers house is pretty old (1950 something maybe?) so I would assume the wiring is nasty in the first place. However, after bringing it to my house, which is much newer, it still does it. Of course that doesn't mean anything, as my wiring could be bad too. However, like qubit said, my father's new unit doesn't do it atall in the same house. Dunno, I guess we'll give it a try.

I have no problem poking around inside the unit. I've actually resurrected several amps myself that had obvious problems (busted caps, etc) but this one seems to be pretty solid on the inside. I can't seem to find anything wrong visibly with the boards atall. One person on GN suggested I listen to the caps, (sticking your ear right next to amp equipment has stupid written all over it, btw) but I couldnt really hear any bubbling or hissing or whining or anything.

The unit is probably 10 years old or so, so warrenty is moot. However, it didnt do it 4 or so years ago when I still lived with them, so it has happened over time if that helps atall.

If I could give you two thanks, I would steevo. That article is perfect. Unfortunately after thinking about the troubleshooting tips they talked about, it seems like I might have multiple forms of interference, lol. (eg, there is a hum that is not affected by signal, and there is a higher pitched hum that is affected by what signal I am inputing)

Another thing that I realized after reading this: I don't have cable TV. However, I do have cable internet. This would amount to the same thing right?

 

[DonInKansas]

Why not take it to another house and see if it does the same thing? that wil take power out of the equation.

 

[Papahyooie]

Already did.