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In the Forum: Audio Discussions
In the Thread: It’s mad, mad, mad... electricity.
Post Subject: Ground loops and humming and AC powerPosted by PurePower on: 4/19/2013
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 SOS wrote:
What about this issue of humming, this is a reoccurring theme that has not been addressed. Even my own 2000+ unit I sold to a gentleman that owned an original 2000 unit who said he would be able to figure the issue out had the humming issue and to date has not been able to stop the new 2000+ from this concern. I've yet to hear or see a report from any customer who has been happy with one of these newer plus units. 

Hum is one big nuisance in audio.  It is frustrating that we have sometimes been blamed for hum that occurs after the installation of a PurePower unit. The typically scenario is that a customer installs a new PurePower and is happy about the performance, but when they play from their phono sources they encounter speaker hum.
When they go back to their old utility power, it disappears. When they plug all their components into the PurePower and leave the phono on the wall outlet everything is perfect. They jump to the seemingly obvious conclusion - something in the PurePower AC causes the hum. But it isn't the correct technical conclusion. The actual cause is a faulty ground system. Put the PurePower unit into the system, ground loop hum appears. Remove it, ground loop hum disappears. 
We see this phenomenon occasionally - usually with high efficiency speakers and very sensitive sources. Anecdotally I can report that we have seen three cases in our last one hundred installations. One of them has been thoroughly reported here by Steve. Typically he found the PurePower performed to his liking - except for the speaker hum when he used his phono input. The hum did not appear when he plugged the phono source into the wall. He eventually sold his PurePower unit because of the hum - in spite of the advice we and others provided suggesting he had a grounding problem that needed to be resolved.  Another was in the studio of a high end audio (and PurePower) dealer who immediately recognized the cause and resolved the problem by adjusting his interconnects. A third was a customer who felt his interconnects system couldn't possibly be the cause and decided not to buy our product. 
There are numerous proofs that the actual culprit is a ground loop. The first is the fact that we have successfully solved the problem in customer systems by finding and curing ground faults. It can be easier said than done - but with a methodical enough diagnostic procedure it is possible to find and remove the problem ground faults. 
In one case it was easy to show the AC power was not the cause because the problem only occurred in one channel. It was unlikely that something in the PurePower output affected only one channel.
Another proof is that the addition of a second PurePower unit feeding the phono side will also cure the problem.  If the cause of the hum was somehow a function of the PurePower AC output the hum should have remained.
Another definitive test is to place a small isolation transformer in front of the phono section. It often cures the problem by creating a new ground.
There are numerous good references to help understand and eliminate grounding problems. A search for ground loops on Google will yield so many results that the subject can be studied for days on end.

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