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advice re: hum and passive distribution bars.

Need some advice from the experts please. I've got two dedicated 20 amp lines installed ( on circuits 1 and 3 on the same side of my electrical panel , hopefully in phase?) they travel across my garage into my living room ending with 4 available power points. When I use both lines(normally one for the cd the other for the amps. see system for more details) I have a very very low level hum only heard with ears near to speakers. However when I connect all my system to just one of the dedicated lines using a two way adaptor to help accommodate the three pieces in my system. I have no hum . Any ideas to what I should get my electrician to change with regards to the two lines installed ( for example. widen the cable runs further apart or twist them together? , change earthing at circuit box? ) which will allow me to use both of them with no hum. Lifting the ground (tried on the cd or pre but didn't try on amp) which I'd rather not do anyway didn't help. I use XLR cables throughout and don't have any tv's/ cable boxes in the same room. Also what are your thoughts on the possible advantages or not on using a high quality passive power distribution bars such as, but randomly selected,Supra, ESP, Nordost ( thor) or Acoustic revive (RTP-4) which always get rave reviews where one is only using a single electrical output but obviously mixing digital and analogue components on it. As compared to using the separate dedicated lines. Thanks.
128x128pcoombs

Showing 3 responses by kirkus

kirkus
502 posts
 
I don't know how your particular electrical panel is numbered, but if the two breakers for your stereo circuits are physically right next to each other and in the same column, they are almost certainly on OPPOSITE phases, not on the same one.

This can easily be checked at your outlets with a voltmeter. Insert the voltmeter probes into the hot side of the one plug (the narrower blade) for each of your dedicated circuits . . . that is, you are measuring the voltage difference between the hot leads of the two circuits. Set the meter to AC volts, and if you get a small reading (i.e. 0.2 volts) or nothing, then they're on the same phase. If they're not, the meter will read double your line voltage, that is, 240-ish volts. PLEASE NOTE that it is generally very possible to shock yourself doing this if you're not careful . . . so if you're unconfomfortable with the safety issues of doing this yourself, then DON'T.
kirkus
502 posts
 
Okay, this sounds much more likely to be on the same phase . . . but maybe you still want to check to be sure.

If they are indeed on the same phase, then the problem is likely to be caused by the ground resistance between the two circuits. Are they wired to two physically separate electrical boxes, or have you used any ground-isolated outlets or such? If this is the case, then the actual ground connection between the two circuits is at the electrical panel, and there's likely to be a few ohms of resistance between the ground connections at the plugs, which can cause significant ground currents to flow through you interconnects.

The best way to solve this would be to have both circuits brought into a single, metal multi-gang electrical box, and all the ground connections for the plugs and circuits be connected together inside. This will reduce the ground resistance between your circuits to a tiny fraction of an ohm, and should remain code-compliant.

Now, it's true that a properly designed balanced interconnection system should reject noise created by slight AC ground currents (common-mode), but it's been my experience that the presence of an XLR plug has little to do with whether or not the particular component was properly designed with common-mode noise rejection as a goal. And this is a very low-level hum you're talking about.
kirkus
502 posts
 
Yes, a voltmeter comes with two probes . . . and you can get the cheapest of the cheap, analog voltmeter to perform this check.

It's very unlikely that you have any isolated grounds if you didn't specifically ask for them. Also, all of the grounds MUST be connected together at the breaker panel to meet code . . . so the grounding at the breaker box is fine.

As far as the "two physically separate boxes", I was referring to the electrical boxes that have the outlets in them, in your listening room . . . that is, does each circuit have its own box & wall plate, or do they share the same? If they share the same box & wall plate, then all the electrician needs to do is to make sure that all of the ground wires, from both circuits and all outlets, are physically connected together here (in your listening room).

If they're completely separate boxes, plugs, and wall plates in your listening room, then there's not a whole lot you can do without being very questionable in terms of electrical codes. If this is the case, then simply using one of the circuits is better, with a high-quality power strip or conditioner. Or if you have any components which have a two-prong plug, you can power them from the other dedicated line, and have no grounding issues.
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