Tuesday, August 26, 2014

Sonos Hum caused by STP cables

I'd started to think it was inevitable that there would be hum in the 2nd bedroom Sonos unless I ground lifted the amplifier.  The previous version of the Parasound Zamp V.3 did have a ground lift switch, but mine does not.  It seemed possible that it had been shipped with a 2 wire power cord, but I have been unable to get that confirmed.  I tossed the original power cord into my spare box without checking until later, and when checking later the cord that most likely came with Zamp has only two connectors on the plug (but three slots in the IEC female portion that connects to chassis).

Ground lifting the amplifier might not meet electrical code UNLESS the amplifier was supplied with the ground lifted cord.  And even then, a paranoid electrician might say you are better off having the actual ground connected, which the 3 connector IEC on the chassis permits.  Some equipment is deliberately designed with 2 wire IEC connectors which can't be grounded no matter what kind of cord you attach.  But not in this case, grounding is clearly an option, and an option which would be better used if possible, any good electrician would say.  It protects both against breakdown of the insulation in the amplifier transformer, and current which might be carried inadvertently by the network cables (say, if they had an insulation failure in the attic).

An extra line of defense in my case is that the circuit to which the amplifier is attached has an upstream GFCI outlet which provides GFCI to this outlet.  I tested it with a GFCI tester to be sure.

However, despite all my precautions, my friend who is interested in staying in the Queen's room is never convinced I'm a safety freak (compared with most audiophiles, anyway, who often ground lift and neutral reverse with enthusiasm).  She's constantly convinced I'm creating some kind of electrical safety hazard, even though the truth is I'm constantly thinking about safety issues that hardly anyone else thinks about.  "Everybody does it" would not be an acceptable alibi if she discovers something arguably substandard about the wiring.  This would prove, finally, that I'm a reckless fool to be watched constantly.  So I do really want to do things "the right way" especially in this room.

I have not had a hum problem with any other of my Sonos connections.  I suspect Sonos uses ethernet transformers for the network connections.  But it occurred to me that a ground loop could be caused by the shielding in my Cat6a STP network wiring.

So I tried isolating the ground by using an ethernet coupler to attach a second piece of unshielded network cable.  Sure enough, that fixed the hum!

(Other "fixes" like using short and stout RCA line cords didn't help.)

Now I had been persuaded that shielded network needs to be shielded everywhere.  And I think that's generally desirable.  But this is only one line that goes straight back to the main fast switch in the kitchen.  I believe it won't cause any harm to the rest of the network to have the very end of this line unshielded and ungrounded.  The ethernet line is way shorter than the maximum run of 330 meters, more like 40 meters.

In fact, I've seen this specifically advised for home networks.  Only shield ethernet cables at one end, some people say.  Others, a whole slew of professionals, recommend avoiding shielded cables altogether, and especially in the home, saying that it's incredibly complicated and difficult to terminate the shielding correctly.  But my thinking now is that the solution is easy.  When ground loops occur, lift the cable shields at one end.

The problem here must be that the Sonos modules wouldn't be correctly designed for use with shielded cables because they have no ground connection themselves.  It's funny, however, that this has never caused a problem before, especially for the last year (that's how long it's been) since I installed an all new home network with all Cat6a STP cables.  When an ethernet shield enters the Sonos chassis it becomes, for all purposes, the effective ground.

I've decided to make the permanent solution a new 10 ft length of unshielded Cat6a cable, from the wall panel to the Sonos box in the queen's room.  Using a coupler is ugly (and the current coupler is especially ugly because it's a Cat6a Shielded coupler, and I think all exposed metal is connected to the shield, making it no good for wet noses.

I don't think the 10 foot length of unshielded cable on one dedicated line will adversely affect my overall network at all.  And the Queen's room is a relatively low RFI/EMI area anyway.  The main purpose of the shielding was to protect the long runs of cable in the attic.

I did also order a 1 foot length of unshielded cable and could use that with a nice unshielded coupler.  That way I could limit the amount of unshielded cable to 1 foot.

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