DC blockers have a role to play if you have DC on the mains, most commonly manifesting itself as a buzz, often variable from time to time, coming direct from torroidal mains transformers in amplifier power supplies (not from the speakers). DC blocking is the normal way to stop that if someone can’t identify and resolve the cause of the DC, which often though not always is local within the home. Whilst the buzzing can be an annoyance, especially during quiet music passages or if the amp is kept powered when not playing music, I’ve never come across anyone saying the DC affects the quality of sound through the system.
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Oh now that’s interesting because it’s not what I’ve read on some other sites elsewhere however what you say does make a lot more sense. 
In your explanation then would an isolation transformer help ‘clean’ things up?
Thanks to you both, I’m sure you have probably already read the many reasons I have collected into this humming topic in the attached thread. In my case, the primary reason for looking at dedicated mains is for SQ, as everyone seems to agree it’s one of the cheapest upgrades. My HiFi is currently on the same Ring Main as my Kitchen, which is not ideal. If it does also resolve or help reduce humming then it’s a bonus, and might prevent the need for an additional box.
In terms of DC offset, I understand that the only reliable way to measure DC offset is with an oscilloscope. I have done the N-E test out of interest and read 0-0.002V DC, ~0.03V AC, but as said not sure how relevant that is. I do feel it’s about time someone build a DC Offset plug-in meter. I’m sure all the HiFi shops would buy one, then loan it out, and the result would be potentially more sales of DC Offset devices. Or perhaps companies like IsoTek/AudioLab/iFi could lend them to their suppliers to help boost sales.
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Just so I get my head around this… so, for example, the area of the +ve half of the cycle is larger than the area -ve half of the cycle?
sorry, there was a typo (now corrected). meant to say DC Offset plug-in Meter
oh that’s a lot harder
YES - assuming you mean a balanced transformer such as Airlink
Basically YES - if the one half of the sine wave is different in voltage or timing as a whole or in part.
Yes exactly that BUT this is where I get confused because I asked Airlink this question and they said no… hence why I purchased the Supra product.
Saying this though, I’ve searched my emails and I can find the email where I asked them the question but not their email reply 
A balanced isolation transformer will correct wave form distortion on its output, but it will not fix the source of the distortion & might become the noisy transformer in the room. So it depends on where its located.
Also we can never forget, transformers can be naturally noisy.
My SN & NAT are both quiet enough to live with hummers despite having a ‘DC’ Offset filter.
The noise went down a few months ago with the power supply voltage was reduced from a high 248-252vAC to (measured to date) 238-240vAC
If you have the space, then from what I have read, an AC generator would trump the Balanced Power supply, but have never tried either. The generators are much more expensive. But again I would say if only we had some sort of meter that you plug in to help determine what state your mains is in.
Interesting the latest Signals newsletter mentions this as a good tweak on moderate priced amplifiers etc and as mentioned many are selling with a money back offer if not happy, so pretty good potential tweak for £99.
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This only appears to be good for 600va your Naim kit may be higher…I think the Nap500 is around 800va… or perhaps higher…
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The Audiolab unit is rated up to 920VA (4A @ 230VAC).
I’ve bought one of these - it’s sitting on my monitor “running in” before it goes near the hifi stuff. I’m sure the picture is rather sharper with the blocker in place than without. Can’t think why that would be though. Expectational bias ?
The downside of the Audiolab is that the output socket is IEC female so you need to either use the supplied cable or make up one with one of what appears to be only two types available, neither looking anything other than budget in their construction.
I used a Isotek EVO3 Syncro Uni with the SN3. Although it did not eliminate the slight transformer hum, it did improve the sound quality.
I made my own DC Offset filter, its based on the well established series capacitor net with bypass diode bridge circuit, which I suspect is the case with most commercial devices.
I designed mine to have more than enough capacity for my total system with aprx 1000VA which means its normally running with something like >90% reserve. The max load is always at start up with transformer inrush, the bypass diode bridge is designed to handle any overload; in my case my diodes are well over spec with 400v 35amp.
As noted before on this & other threads on this subject; a DC Offset filter can slightly improve SQ, and although its does very effectively correct any DC offset, (if indeed you have it) its not the cure for a naturally noisy transformer.
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