Earth Loop Hum

Given the lengths you and the dealer have gone to with no avail would it be worth setting the system up  in the dealers listening room and see if the earth loop hum is still present?

Does it hum with no interconnects ?

PCD makes a very good point. The other thing i'm not clear about is whether the 300 has been substituted during the investigation - if it hasn't then i have a couple of easy things you can try yourself that may help you track down the issue. 

From what you describe i'm wondering if you have an unintentional second signal ground to chassis ground occurring due to a wiring fault somewhere. If you disconnect all the components from the mains and each other, use a DVM on its continuity setting and do the following.

With the CD555 connected to its 555PS and not plugged into the mains. Measure between mains plug Earth and the outer ring on one of the signal output RCA's - you should have continuity as this is where signal ground and mains ground is connected in a Naim system.

With the 552 connected to its 552PS and not plugged into the mains, measure between mains plug earth and one of the RCA input connectors outer rings - there should be no continuity. 

With the 300 connected to its 300PS and not plugged into the mains, measure between mains plug earth and the -ve socket on one of the XLR inputs - there should be no continuity. 

Remember when doing these checks, apart from the Burndy cables connecting the PS units to the head units, do not have any other interconnects between the 555 / 552 / 300 head units. 

James

Several diagnostic questions:

1. Do you plug any auxiliary components into your system? If yes - this is the most likely culprit.
   
   
2. Are are you plugging all the NAIMs into one, earthed mains? Different mains rings / circuits may have different voltage (minute differences) which will in turn manifest themselves as hum.
   
   
3. Are all mains cables properly earthed?
   
   
4. Does the system hum when it's on but no sources are connected? As I understand the system hums when CD555 is connected so this question will narrow down where the problem occurs. If the answer is NO, than the culprit is CD555 and it's wiring.

Ahhh - just saw that you wrote 'unplugging the CD resolves the problem'...
Do you have other sources that you could try connecting with the CD being disconnected?

Correct me if I'm wrong, but do the CD555 and the NAC552 not require power supplies?

If so then you haven't fully described your system.

That suggests that there may also be other components connected that you also haven't mentioned.

If you haven't described your system accurately, how do you expect us to attempt diagnosis or throw any useful light on the situation?

Huge posted:

Correct me if I'm wrong, but do the CD555 and the NAC552 not require power supplies?

If so then you haven't fully described your system.

That suggests that there may also be other components connected that you also haven't mentioned.

If you haven't described your system accurately, how do you expect us to attempt diagnosis or throw any useful light on the situation?

Bryce mentions substitution of the PS for both 555 and 552 in his original post and i'm assuming it's just those 3 components (including PS's) mentioned....

Ah yes, quite correct, I see now.

Being dyslexic, when some things are stated directly in one place and other things stated by inference in different places, I often miss the inferred things.

Good point though Huge. It's easy to assume (as i have done) that all the info is correct in the original post so perhaps some additional clarification (the 300 doesn't seem to get much of a mention) on some points would be welcome from the OP. 

Thank you gentleman, much appreciated.

James N - have not tried substituting the 300.  And what's a DVM?  I'm guessing a voltage meter?

Adam:

1.  Rega TT and Arcam 850 (I use my loudspeakers as Front L and R channels in my home cinema setup.  The Arcam passes through those two channels to the Naim system).

2.  The Naim equipment is all into one earthed mains.  The Arcam is on a different circuit but we did the experiment using a long extension cable of putting it on the same circuit and the problem persisted.

3.  They're Chord Sarum Super Aray power leads so for the money I've paid they should be properly earthed which isn't the same as saying they are of course.

4.  The hum goes the minute the CD555 and 552 are disconnected.  When connected the hum is present on CD and AV input.  I don't think it is on Phono input.

Huge - sorry, 555PS and 552PS.  Both have been swapped to see if the hum is eliminated without success.

Most likely the Arcam is also earthing the signal 0V creating an earth loop with the CD555/555PS.

The signal earthing in the Arcam needs to be set to floating.

They should most definitely NOT be connected to different earth circuits.

Yep that's it Huge - now it all makes sense. Surprising the dealer hadn't sussed that.

So with the 555/552/300 ONLY (nothing else connected to it !!) i take it there is no hum Bryce ?

Funny what happens when you start to get a complete description of a problem!

Huge posted:

Funny what happens when you start to get a complete description of a problem!

Yep 

"I suggest ensuring that the Arcam doesn't earth the signal circuitry."

"Make it so."

Bryce Curdy posted:

 

Adam:

1.  Rega TT and Arcam 850 (I use my loudspeakers as Front L and R channels in my home cinema setup.  The Arcam passes through those two channels to the Naim system).

2.  The Naim equipment is all into one earthed mains.  The Arcam is on a different circuit but we did the experiment using a long extension cable of putting it on the same circuit and the problem persisted.

 

Bingo - that's why I've asked question no 1

A very common problem and I'm surprised your dealer did not pick up on that (mind you - mine did not spot it either in my setup - Yamaha Aventage 5000 into 252).

As Huge wrote you have a perfect ground loop for several reasons: Arcam is not earthed (the 3rd pin on the mains is not connected); you are powering your Naim and Arcam from different mains circuits.
Even if they were on the same circuit, you'd still most likely get an earth loop.

Solution:
An isolating transformer (stereo version) costing a princely sum of GBP 10 or so connected between your Arcam output and your NAC 552 input. It will break the galvanic connection between the Arcam and NAC but will maintain the signal's purity. Search for 'stereo islation transformer'.  
This should cure the problem.

Adam

james n posted:

Huge posted:

Funny what happens when you start to get a complete description of a problem!

Yep 

Live long and prosper

An isolation transformer

Adam Zielinski posted:

james n posted:

Huge posted:

Funny what happens when you start to get a complete description of a problem!

Yep 

Live long and prosper

And get a new dealer !

Very common problem.  I added a CD5X to my AV-connected Naim system and got the hum -- so I blamed the CD5X for my issue.  

Per my dealer's recommendation, I tried disconnecting CATV at the point it enters my home, which eliminated the hum, as predicted.  Solution was to add the isolater between my Denon amp and Naim preamp, just as Adam suggests.

Nick

Very grateful but don't want to get my hopes up.  In terms of confirming the above unplugging the interconnects from the Arcam 850 from the 552 should eliminate the hum when on CD source?  I thought we had done that but I've lost track.  And assuming it does just add the isolation transformer.

Bryce - the thing with an earthing issue like this is to start with a simple configuration - ie just 555CD as source and then add the other sources back in to see when the hum comes back. It sounds likely to be the Arcam so if it is then add the isolation transformer as Adam suggested. 

First step - 555/252/300 only nothing else (TT, Arcam) connected. Any hum ?

The key thing is that the signal must be earthed only once, and that's through the CD player in your case. If you then connect the Arcam and the hum returns, see if there is an earth switch on the Arcam, and if so set it to 'floating'. If not, get one of those things Adam pictured. 

OK, you still haven't told us the full story...

1  To what is the TT connected?

2  What else is connected to the Arcam receiver?

HH, there's no switch on the Arcam Receiver, it's double insulated and there's no earth pin in the IEC plug on the back - hence I know he's still not telling us everything!