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Ground Loop - New Twist?

In a recent discussion I pointed out having lost a 60Hz speaker hum from my stereo after moving to a place with a better electrical system. Well, that claim was premature. The cable company was out yesterday, did their thing and OUCH! things are worse than ever. With the TV and digital cable box in the loop the 60Hz hum is LOUD. Putting a ground isolator on the cable connection helped, but doesn't completely remove the noise. The installer tried to ignore the problem, even said it is normal, but finally relented after much insistence on my behalf, called his supervisor and a technician is supposed to be assigned to investigate the problem. What I'm hoping for with this post is a little guidance on the best path(s) to take when the technician comes out.

Before the cable installation there was a *minor* speaker hum audible from a couple of inches away. It was about as loud as the tube rush from the pre-. Floating the ground on the amp or pre- does remove it. With the pre-'s ground floated there is a lack of life to the sound, so that isn't an acceptable fix. I tried floating the amp's ground this morning, against my better judgement, and while the hum is gone this is really not my preference. Too unsafe; the ground is there for a reason.

In conjunction with this the ballasts for the flourescent lights in my office and garage all started buzzing, too, right after the installation. Something is plain not right.

The installer said the cable connection is grounded to "the power mass". This was not a familiar term, so had him show me the ground connection. It is attached to the outside of the breaker box next to the power meter. Somehow that doesn't seem right.

What are the available options and what should I say to the tech to get this fixed? BTW, I do have a basic understanding of what causes ground loops. Also, the equipment is a McCormack DNA-1 and the pre- is a Sonic Frontiers Line 1. Thanks in advance for walking me through this one.
fpeel
sean
6,231 posts
 
Unless you have a digital power amp, i would ground everything BUT the amp. Some digital components such as CD's, DVD's, Transports & DAC's, maybe even tuner's, etc... are VERY ground senstive and get "squirrelly" when you remove ground. The good thing about this is that you already know that removing the ground from the amp itself takes care of the problem, so there is little guesswork on this one.

As to removing the ground from the amp, you can do several different things. My suggestion is to simply open up the IEC ( if it is screwed together ) on the amps' power cord and simply disconnect the ground wire. Before doing a "permanent" type modification, i would simply bend the wire back away from the terminal and wrap a layer or two of electrical tape around the bare wire. This will lift your ground, get rid of your hum / noise problem and still give you the benefits of what is called a "telescoping ground". This approach is FAR better than leaving ground connected at the IEC and lifting it at the wall outlet.

By leaving the ground wire connected at the IEC and using a cheater plug or lifting it inside the wall plug, the ground wire is still connected to the component. As such, you now have a long wire antenna connected to chassis ground. This opens the door to increased RFI or EMI based problems. While "cheating" the ground may have gotten rid of your initial hum / noise problem, you've now left yourself open to another form of "noise" and aggravation.

Most people don't think about this aspect of "lifting" ground and that's why i took the time to mention it. Sean
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fpeel OP
455 posts
 
Thanks, Sean. Your suggestion makes sense even to a non-techie like myself. Along that line, it would seem lifting the ground at the IEC inside the amp would accomplish the same thing. This is just me looking at options. The PC has heat shrink over the IEC that would have to be cut away, something I am reluctant to do. Not sure about the connection in the amp, but it may be easier to return to stock form than the PC. Is there a reason not to disconnect at the amps IEC?
sean
6,231 posts
 
No reason at all not to disconnect inside the amp at the IEC and thanks for bringing that up. I forgot to mention it. I would caution that even though an amp might not be plugged in, you can still get "whacked" from the power supply reserve stored in the filter caps. As such, you can simply play some music and turn the amp off while maintaining a low to moderate volume setting. The amp will continue to play until the power supply is drained. It is then pretty safe to "poke around" inside the amp. Besides this, it will let you know just how good of a power supply reserve the amp has. Obviously, the longer the amp plays, the bigger ( & better ) the power supply reserve. Amps with relays in them will not do this, so don't worry if an amp of this type "dies" instantly when turned off. Sean
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fpeel OP
455 posts
 
Update: The tech from the cable company never called as promised. BUT he did visit. There is now a ground connection from the cable entry to my power meter's junction box instead of the former daisy chain to my neighbor's. Removing the cable ground isolator at the digital cable box and the cheater plug from the amp power cord as a check of the results showed there is a very minor hum audible only if I put my ear next to the speaker. This existed before and is a known system issue. It's better now than it was at the last house, albeit just barely so. Overall, an improvement. I also learned that the digital cable box has audio outputs. Using these is quieter than plugging into the back of the TV. Could be the age of the TV, could be a difference in ground potential. Either way it's quieter.

Thanks for all the ideas, guys. I'll most likely float the ground at the amp in one way or the other to create as low a noise floor as possible. Hopefully this discussion will serve as good reference for others having similar problems in the future. Again, thanks for helping sort this one out.
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