DC Blocker - Suggestions?

I have in periods issues with humming transformers on my Naim equipment. Naim seem to be rather sensitive to DC/bad voltage and since it is changing over time it is most likely due to something in my house turning on/off creating this (heat pump, fridge, etc). I’m therefore looking into evaluating DC-blockers to see if this helps out. Most of what is heard is coming from the transformer itself but when it’s peaking I get some through the speakers too at levels around 10 and up meaning. So it’s mainly the mechanical hum I want to reduce since I have a small room and this can be disturbing on low listening volume.

  1. What is your experience of DC-Blockers?
  2. Any suggestions on DC-Blockers?

I’ve looked into this one so far.
jenving dc-blocker

1 Like

I use an Isol-8 Powerline Axis. I dont have the problems on the same level as you seem to but it certainly quietens things down.

1 Like

Would it be possible to treat this problem close to the source instead if one find out what’s causing it? I mean letting an electrician solve it for my whole house instead of just my Naim amp.

First of all measure your mains to see if you have a DC offset. Measure DC between neutral and earth - assuming you are competent to do so :wink: It should be zero or pretty close to it. Mine is about 1.6V… enough that you can feel a tingle in your fingers if holding both wires. I tried the Supra LoRad MD01 DC Blocker (cheapest place was from Germany and I sourced my own UK plugs) to try and reduce mains hum on my power amps. Had zero effect! Apparently they (my P1s) have a low level hum and that’s it… live with it :wink:

Just realised your link is to the MD01… that’s the one I tried.

1 Like

When the noise starts, turn off everything else in the house. Does the noise stop? If so, turn things on one at a time and you’ll find what is causing the issue. That item can then be dealt with. It’s worth considering dedicated mains with its own consumer unit. It should reduce the problem, though it’s not guaranteed, and increase sound quality.

Hi Slamdam,
All the Naim transformers were humming in our system, so after a lot of searching we tried one of these highly regarded units:

It stopped the humming but also took the life out of music. For circa £2,500 at the time, this was not a great solution, so it went back to the dealer.

After more research, we tried one of these:

An Airlink Transformers Balanced Power Supply. This one worked for us and is still in use to this day. It cost about £500 at the time.

Hope this helps, BF


I am not sure that is a good, safe or effective way of measuring DC on the mains… and it’s effectiveness will depend on the distribution earth arrangement provided by your electricity distribution company.
The way I would recommend (assuming you are competent) is to use a DC coupled ‘scope on the mains and compare the amplitudes of the upper and lower cycles… the difference is any residual DC. It should typically be than a volt… but will vary with loads

1 Like

I have my own design & made DC blocker, its a derivative of a well established design & it does work. I’m not sure how you can determine what any of the commercial designs are based on or is included in the circuit. Mine is a large series capacitor polar pair & a bypass diode bridge, plus some other features, but does not include X & Y capacitors which can (can) be detrimental to SQ. The added & unexpected bonus was it had a subtle positive effect on SQ.
However in retrospect I would focus more on the cause of the problem, if its in your property, fix it. In my case it was from a nearby property (since moved on)
A Balanced Power Transformer will do the same job & probably better, but its a big lump to find a home for.
The suggested way of measuring PD between N & E is not indicative of DC on your power supply, it can be measured but you really do need to know what you’re doing.

1 Like

Aside from Mike-B’s observation, this really must not be attempted by anyone not absolutely sure they know what they are doing and competent to work with live mains electricity. Get it wrong (e.g. finding your wiring has been reversed and what you think is neutral is live) can mean a very abrupt end to you hearing hum -or anything else.

(BTW, 1.6V DC is insufficient to cause a tingle, at least with intact dry skin, and if that reading was a correct measure of DC between neutral and earth it suggests there may be a higher AC voltage also present.)


I’m not going to risk my life for this hum so either a product solving it or an electrician doing so :blush:

I also have a Balanced Transformer which, although it’ll help some issues, won’t do anything about a DC offset.

A balanced power transformer output does not have DC offset on its output. If your transformers hum on a balanced power transformer supply, it’s the transformers that naturally hum.

1 Like

Hi Steve,
Are you really sure?
Curing DC offset is the main purpose of installing a BPS.

The large transformer inside the BPS allows the AC to transfer through but it cannot transfer DC from one set of windings to another set.
Partial attenuation of higher frequency mains pollution is a secondary and lesser benefit.

Best regards, BF

1 Like

The difference between neutral and live will be 230V but ±115V. The cross over point will be offset from earth by the DC offset. So the offset wrt earth is still there.

Hi Steve,
Perhaps we are talking at cross purposes here.
As Mike-B indicates above, the output of a balanced power supply does not have DC offset on its output. Otherwise, it would not be balanced.

The Naim power supply transformers then see a supply from the BPS with zero DC offset, which is why ours then stopped humming.

While the earth is at 0V, the BPS output live and neutral become +/-115V AC waveforms, so there will be a voltage difference between the output neutral and the 0V earth. Is this the voltage difference that you are referring to?

Best regards, BF

1 Like

Correct - through Galvanic isolation DC can’t pass through a transformer - so a transformer will remove DC on the mains.

The noise the transformer or more accurately the slats make from DC is them saturating hard on and hard off because of the DC present - this will occur more on efficient transformers with less resistance.
This noise is different from the ‘smooth’ hum a transformer will produce with correct operation.

The output of a saturated transformer does not look neat - it can be quite a horrible waveform with many frequency components.

The 0 volt reference of earth is kind of irrelevant - its the asymmetry of the mains duty cycle that is pertinent. (Both voltage and current)


The waveform is ±115V with respect to each other but, for example, live might be 1.6 to 118.6 wrt earth and neutral then +1.6 to -113.4V. So still 230V AC but the waveform is offset from earth. Hence a net DC element. tbh I also thought the BLT would fix DC offset issues but then it was explained to me by Airlink.

I did a small repair to the organ in the church where I’m organist. I switched off the mains ring and started to replace a small electric motor. Got a serious shock. Some idiot did connect this motor to a different mains ring, and did put the switch in the neutral line, not the live line. The good thing is that I now know how to use an electricity screw driver.

Thanks for that. I remember this from the old forum. It is surprising that many of us spend loads of money on fancy cables whilst a power supply like this is likely to have way more positive impact on the sound.

How did you do the calculations which model you needed?

1 Like

+1 I have the same, and it cured humming in our previous house.