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, I don't think I explained myself very well, The two breakers I'm using are both on the left hand side but have a different breaker between them. So its the top one and third one below it. Thanks again for interest. Philip.
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.
thanks for the post Kirkus , I'll try and get a voltmeter soon . I'm presuming that they come with two probes so I can insert them into both 'hots' at the same time.? Not quit sure what you mean when you asked "Are they wired to two physically separate electrical boxes, or have you used any ground-isolated outlets or such?" All i know is my main breaker box has 14 separates breaker box/switches on both sides with the two ones used for my system amongst them (in the top and third positions as I described before) How would I check if these where ground -isolated or not? If I didn't want to go to all the hassle of having new electrical boxes installed as you suggested would is help or make any sense to just get the electrician to put the two sets onto just one circuit sort of like bi-wiring speaker cables? and just free up one of the 20amp circuits for something else. Or even just use one of the lines as is for the whole system with an added distribution bar of some sort. Thanks again Philip.
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.
It sounds like a typical ground potential difference to me. It's possible that the ground wire is not making good connection in one or more of the outlets. You can remove each outlet and check the ground wire connection -- attempt to tighten the green ground screw.
Ok, heres the update, tested lines with volt meter. Definitely both circuits on same leg. which is good .All screws in outlets where fully tight. I tried all three components on one circuit only, both with and without cheater plugs throughout with same results, no audible hum. ( just a super faint sense of some noise compared to nothingness when I de-mute my line stage , which I'm presuming is perfectly normal) I've also put two components, for example amp and cd on one line and the line-stage with a cheater plug on the other line , again no hum. So the hum which is, I should stress very very low in nature can only be heard when I use both circuits without any cheater plugs being used. I went up into my garage loft to look at the cable runs. they are stabled together along various joists There is in the middle of the loft floor possibly the garage a door opener power supply/transformer? sticking up which the cables run to about 6 " at its closest . Would it be worth spreading the cables apart from each other and further away from whatever that is? Even if i manage to somehow remove the last vestige of hum from when using both lines I'm still wondering if I should focus on only using one anyway along with a quality distribution bar.
One other thing you might try.....
I assume the two dedicated circuits and duplex receptacles are somewhat next to one another. A few feet apart at most.
You will need a length of bare #12 or #14 copper wire.
(Bare solid wire from a piece of Romex would do the job.)
The wire needs to be long enough to run from one duplex receptacle to the other.
* Turn off both circuits at the electrical panel.
* Pull the cover plates from the duplex receptacles.
* Loosen the 6/32 screws that fasten each duplex receptacle to the rough-in boxes. Bottom screws more so.
* Bend a hook on each end of the solid bare wire.
* Install the hook of the wire behind the duplex receptacle supporting strap, (bottom of recept.)
* Tighten back down the 6/32 screws making sure the bare solid wire exits down from the duplex receptacle supporting strap.
Basically you are tying the safety equipment grounds of each dedicated branch circuit together. Creating a closer star ground point ahead of your audio equipment.
Same thing Kirkus was describing in one of his posts
* Reinstall duplex cover plates and turn back on the two circuit breakers.
* Re-check for hum....
If the Hum is gone..... best you call back your electrician and have him tie the two safety grounding conductors of the branch circuits together in a more permanent concealed manner.
Thanks for the advice Jea48 I'm going to try it. Just to be certain I understand the bare wire should attach between and connect to both the receptacles by just pressure after (tightening back down) onto the bottom metal body work ( supporting straps) that has the holes used to screw duplex into wall. Rather than the actual earth wire screw that are located on bottom left of each receptacle. At least as a temporary test method. Thanks again Philip.
Just to be certain I understand the bare wire should attach between and connect to both the receptacles by just pressure after (tightening back down) onto the bottom metal body work ( supporting straps) that has the holes used to screw duplex into wall. Rather than the actual earth wire screw that are located on bottom left of each receptacle. At least as a temporary test method.
But to be a little clearer. The hook, or open eye, of the solid wire will hook, hang, on the 6/32 mounting screw behind the strap of the receptacle.
The wire will be sandwiched between the strap and the receptacle rough-in box.