Power cords and the importance of "last meter"

I want to try adding a good DC Blocker for my Supernait 3 to reduce transformer hum. The amplifier is currently connected directly to the wall socket with a Powerline.

Assuming i will add a Supra DC Blocker in between, does it still make sense to use a Powerline from the Supernait to the DC Blocker as the blocker itself will be connected by a Supra power cord into the wall socket?

Is it safe to say that it is really the last meter (in this case the Powerline) which will be the most important cable here? Or will the added power cord after the DC Blocker neglect the performance of the powerline?

What you add from the Supra DC blocker to the amp indeed will change the character of performance. But I very much recommend you getting a different C19 power cord from wall to Supra DC Blocker. That’s your weak link. I use an Audioquest Monsoon C19 myself and have used Isotek EVO Premier too with good result.

How do you know you have a DC Offset problem? Transformers will all hum to some extent, including almost silent.
If the hum tone & volume changes at different times, then you most likely do have a DC problem
If the hum is unchanging in tone & volume, then you most likely do not have a DC problem. OK to try a DC blocker but you might want to think about buying it on a trail & return basis

I have been living with it for a few months and there definitly is a change in volume depending on the time of the day. I have tried a DC Blocker from HMS but it had very little effect. After speaking to another person who had the same problem with a 300DR he told me that he also tried a few and the only one who worked for him were Supra and Isotek, as they can handle higher offsets than many other DC blockers.

In Germany it is very easy to try and return, so this is not a problem.

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FWIW a power chord is not the last metre. It’s the first metre. It’s all from the point of view of the power supply. Often misunderstood when people ask how on earth can the last metre of miles and miles of cable to the power station make a difference.

It’s the first metre en route to a much shorter run to your consumer unit. A non audio power cable’s function is merely to provide coupling between the power supply and your mains. An audio cable’s function is to provide the best, low resistance coupling of just the conductors, and if possible provide decoupling of everthing else (mechanical, electrical interference).


You are so right Bravo! To the component it is the first link to Power

Thanks for the clarification. With that in mind, what would be your opinion on my question regarding Powerline and the cable going from the DC Blocker to the wall socket?

Well PowerLines are mostly about decoupling mechanical interference entering the audio component via the cable and plug, so absolutely from the DC blocker to Supernait. But even if not, always ensure the power supply sees the good cable.

But, I’d first evaluate the cause of the hum. Don’t guess at DC offset, get it confirmed. You might have another problem like over voltage instead. A cheap parallel DC blocker that just plugs into an adjecent socket would be preferable by far to a serial DC blocker that sits in the circuit. If you’re putting anythin in the circuit, a balanced power transformer would be the way to go. But as I said, don’t waste money on a solution for a problem you might have. Spend on a specific solution for a problem you know you have. Remember, hum isn’t the problem. It’s the symptom.

On my big Naim system, currently in storage, I use a very substantial, but affordable, mains lead (Furutech Empire from their pro audio range) into a power strip and PowerLines from there.

Thanks, i’ll try measuring the voltage during the day and see if it is above tolerance. From my understanding anything above 240v could also cause transformator hum, correct? The allowed tolerances seem to be 230V ± 10%

As for the parallel DC Blocker, could you show me an example? I have never seen these…

Sadly I’ve only heard mention of them on the forum but never seen one.

I don’t know of an effective DC blocker that plugs into an adjacent socket. These are typically voltage noise clamps.
To remove DC (actually its asymmetrical AC wave distortion) the current must pass through a capacitor, or a centre tapped (balanced) transformer

That’s very possibly true @Mike-B. A few members have mentioned them but I’d do balanced transformer or nothing.

Still, may be getting ahead of ourselves here. In my opinion, the root cause is yet to be confirmed.

One thing that is popular in Japan is lower cost in line noise (both mains and earth) blockers like ones made by NTT and rather ghan put them on the hifi, put them on whatever in the home is bleeding offset/earth noise/etcl to mains. Because they work both ways. I use one in my office but put the other crap like routers and wallwarts on that and the UQ2 direct to mains. My problem was awful earth bleed. Cleared right up.

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Measuring 232V which is frankly quite good. So it is not the overvoltage causing the trouble.

Last resort tomorrow is the DC Blocker. Hope that does something.

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