Need help with System "Hum"

[Cliff]

I've been tracing a hum in my system over the past two weeks (when I first noticed it), and I *think* I've found the problem, but I'm not sure. The hum is like a "PA", or "Metal Halide" sound. It is only present in my mains, and it's audible once the power amp is turned on (everything else can be off).
I think it's in my Lexicon preamp's L/R main speaker outputs (RCAs which lead to my power amp). But it doesn't make sense to me. When I turn on my TV, the hum gets VERY loud. So, I thought that meant AC hum, and not RF hum. Anyone know a lot about hum and RF/AC interference? If I switch the main outs into the "Zone 2", which is analog bypass, it's completely gone! I am so friggin' confused. I don't want a professional to take advantage of me 'cause I'm so clueless. I'm sure I left off some important information, so if you need more "test" results let me know. Thanks in advance for any help!

[sgraham]

Does it go away if you disconnect the inputs to the amplifier?

Try disconnecting your antenna or cable from the system and/or TV to see if the hum goes away.

Try plugging *everything* that is hooked up to your stereo in any way, including TV, VCR, etc. to the same AC outlets.

[Tomcw]

Could this be the infamous 'TV Cable ground loop'?
Tom

[Cliff]

Originally posted by Tomcw
Could this be the infamous 'TV Cable ground loop'?
Tom

I've unplugged the satellite cables completely, and it was still present.

[Cliff]

Originally posted by sgraham
Does it go away if you disconnect the inputs to the amplifier?

Try disconnecting your antenna or cable from the system and/or TV to see if the hum goes away.

Try plugging *everything* that is hooked up to your stereo in any way, including TV, VCR, etc. to the same AC outlets.

It is gone when I unplug the inputs from the preamp to the power amp. It's also gone when I plug the outputs into "Zone 2" and run them into the power amp (this is for the front mains only, and I use Zone 2 for analog bypass). I've plugged the rears into the L/R main jacks, and they then have the hum. So, that's why I thought it was the main output jacks. It's really puzzling!

[Cliff]

I should mention that I also have a Monster HTS2000 surge protector, and everything is plugged into it EXCEPT the power amp. When I unplug the Monster, the hum is gone, even if the power amp is still turned on.

[Tomcw]

Could it be that the output jacks on the pre amp have lost thier common (ground) connection? Can you pick up a common somewhere else on the unit as a test, perhaps with a jumper lead?
Tom

[sgraham]

This begins to sound like it has something to do with that surge protector. What happens if you also plug the power amp into it?

(If there's a ground loop problem adding more ground connections may actually make it worse.)

[Cliff]

Originally posted by sgraham
This begins to sound like it has something to do with that surge protector. What happens if you also plug the power amp into it?

(If there's a ground loop problem adding more ground connections may actually make it worse.)


I'll give that a try. I've not yet tested the Monster thoroughly, but the proper ground LED is lit, so I assumed everything had to be OK (that's what I get for assuming).

[Cliff]

Originally posted by Tomcw
Could it be that the output jacks on the pre amp have lost thier common (ground) connection? Can you pick up a common somewhere else on the unit as a test, perhaps with a jumper lead?
Tom


This could be it, Tom. This is my best guess as to the problem. How do I test the RCA ground though? Thanks.

[Tomcw]

Take a jumper lead, just a piece of wire will do, and touch one end to the outside of your rca's that are connected to the main outs, and touch the other end to the outside of one of the zone 2 output rca jacks. If the hum quits, the outter 'common' on your main outputs has become disconnected. Probably a broken solder joint inside the preamp. This can happen if you stress the connectors to the side too much.
Tom

[Cliff]

Originally posted by Tomcw
Take a jumper lead, just a piece of wire will do, and touch one end to the outside of your rca's that are connected to the main outs, and touch the other end to the outside of one of the zone 2 output rca jacks. If the hum quits, the outter 'common' on your main outputs has become disconnected. Probably a broken solder joint inside the preamp. This can happen if you stress the connectors to the side too much.
Tom


Bingo, Tom! I just used a spare RCA patch cable, and plugged one end into the Zone 2, and the other side, I touched the main output where it was exposed, and the hum totally disappeared! Now, I need to rip apart the Lex tomorrow, and resolder the ground. Hopefully an idiot like me will be able to do this. Thanks Tom, and all others for your help :thumbsup:

[Tomcw]

Glad to be of help. Be sure to unplug the power and you may want to wear a wrist strap, with all the ESD sensitive components used today.
Let us know of your success!
Tom

[Cliff]

Hmmm. Interesting. I pulled apart the Lex tonight, and found a few solder joints were cracked. Redid them, plugged everything back in, and the hum still exists. I guess tomorrow night, I'll pull apart the power amp, and see if all the RCA grounds look good... :(

[Tomcw]

Hmm...
I don't think you'll find the problem in the amp.
I'd look for more broken solder joints in the pre.
Tom

[Cliff]

I called Lexicon today, and explained what was going on. They were very helpful, despite it being almost impossible to trouble-shoot over the phone. He asked me to use one of those wall plug adapters that have 3 prong inputs, to two outputs. Thus covering the ground of the component it's attached. I tried the Lexicon, and the hum was still there. Then I tried the Acurus Power Amp, and it disappeared completely.
I also unplugged all RCA cables from the power amp, and it disappeared. Weird... I'm going to borrow my dad's ancient SAE Power Amp and give it a try, and see what happens.

[cosmikdebris]

Is it a 60hz hum?

[Cliff]

Originally posted by cosmikdebris
Is it a 60hz hum?

The Lexicon guy is saying it is the dreaded 60Hz hum, but how do I REALLY know?

[Tomcw]

Cliff
Could it be that you fixed the original hum when you repaired the broken solder joints in the pre, and have introduced a 'new' hum in the tearing down and re-connect? I'd go back and check all the cabling, particularly the polarity of any non-polar ac cords.
Tom

[Cliff]

Well, I still have the "hum" issue. I called some of the "high-end" power conditioner manufacturers, and they thought it was a wiring issue. So, I went and ran a dedicated circuit for my A/V Room, and ran new romex, etc. But, it's still there! Cheater plugs which I thought worked at first, make no difference. I know the ground into the house is up to code, and done right, because I did it myself (along with my brother who's an electrician :)). I have read that there are two types of sound - a 60Hz "hum", and RFI/EMI "Buzz". After describing the sound to another manufacturer, he thought it may be RF/EMI. I just ordered some cheaper, well made ICs, and I'll hook them all up and see if one of my RCAs is bad - even though I've swapped them all out before. I've also heard about "shorting" out all the unused RCA jacks, as this could introduce EMI into the system. It doesn't make sense to me, but so far, nothing else has either! Anyone tried these RCA "Shorting" plugs before? Also, every once in a while, I can hear what sounds like faint radio, and I don't even have an AM/FM Tuner! This totally sucks. I always like a challenge, but it's more fun when there's a solution... Anyone have any other suggestions? TIA.
EDIT: I should also mention that the buzz is not consistent. Some days, or even minutes, it's VERY loud, then it gets quieter at other times. Never traceable to lighting, or dryer, fridge, etc though. It just seems random, and I can sometimes lessen it when I move the ICs around (without unplugging them), so that's why I ordered new sets.

[Jerry]

maybe it just doesn't know the words...........