How Much DC is OK on a Power Line?

The other night my Classe amplifiers started producing a substantial mechanical hum. Classe told me that it was likely from DC on the power line. The hum was there even when the preamp was switched to standby, and even when I plugged the amps into different sockets.

My questions:

1) how much DC on the powerline does it take to cause problems with audio equipment?

2) How does DC get into the AC signal on the power line?

3) Do the power companies have any spec they need to acheive for maximum DC?

4) Or is it more likely appliances within my house causing the DC.

Thanks, Peter
DC can come from your neighbors on the same feed as you....I notice my transformer will hum only occasionally at different times of the day...has to be from something my neighbors are running...have tried DC removal devices and none ever worked
The PS Audio Humbuster III is working well in my system. It's main goal is to eliminate as much DC as possible coming into the system.

My system isn't humming.
Peter, what model Classe amp do you have ? A lot of later production CA-XX0 series amps have DC blocking circuitry, and for earlier production models that did not have it, the DC blocker circuit board was available as a retrofit.

A couple hundred millivolts of DC is all it takes to cause transformer hum. Abnormal high voltage and harmonic distortion can also cause hum. DC blockers will not eliminate hum caused by harmonic distortion.

DC can be caused by a variety of devices, including but not limited to light dimmers, appliance speed or temperature controls, washing machine motors, refrigerator and air conditioner compressors, etc.

The PS Audio Humbuster and Channel Islands XDC-2 AC filter should eliminate most transformer noise caused by DC. There is also the DIY option. I've done DC blockers for a few Audiogon members at a fraction of the cost of the "big boys".

As mentioned above, DC can come from a neighbor's house. It can also be generated in your own house. I would investigate in-house sources before looking for a solution. Look for possible sources, shut off all breakers and see if the noise goes away.

I don't know of any power company DC spec.
whould a power regenerator (Purepower, PS audio) remove the hum/buzz caused by the DC on the line?
Thanks Gbart. I'm using Classe Omega amplifiers. I just measured my outlet for DC, and am seeing about 200 mv. I had an air compressor plugged in that night, and maybe it went on and the motor increased my outlet DC?
Power regenerators do NOT remove DC. I use a PS Audio Premier Power Plant in conjunction with the Humbuster III to eliminate the DC on the line that is not addressed by a regenerator.
I've been told that a buzz/hum from an amp can be caused by "odd harmonics on your home AC". Is this the same as 'DC on the line' as Peter in first post was told ? And if different what sort of conditioner is best to 'cure' this?
By the way, I'm using an Environmental Potentials home surge protector / conditioner as discussed inthis thread. As it is on a breaker, I can take it out of the system.
Oh - and the question still remains - how much DC offset is problematic on an AC line for equipment?
Pcoombs, harmonic distortion and DC are two different phenomenon. A UPS or power regenerator like one of the PS Audio units should cure harmonic distortion issues.

Peter_s, I said in my post above that as little as couple hundred millivolts of DC can cause a transformer to hum or buzz.
I struggled with this issue for quite some time a couple years back. IMO Gbart has summed the situation up nicely.

'being an electronics hacker, I constructed a DC offset blocker for AC line, and all was fine.

I have noticed for instance that in the case of Bryston, at least the amps I have looked at in fact have this DC block built in. ( I don't own Bryston, it's just that they publish schematics on their web site, and the circuitry is shown )
It is my understanding you cannot measure DC on the AC mains just by connecting your digital meter, set on DC voltage, across the mains.

Not trying to start a flame but

Blocking Mains DC Offset
Jea48, its easy to measure DC on top of AC if one uses a Fluke RMS meter. My Fluke 77 measures it accurately. I see less than 100 mV at most friend's homes.
I have seen a few noisy power transformers (including toroidal ones) when DC measures more than roughly 200 mV.
Best Regards
Most hum on power transformers is caused by DC on the power line. The source could be anywhere. It could be caused by a poorly designed switching power supply in a Television or a light dimmer. You can DIY a DC filter to take the DC off the power line before the power transformer or purchase something like PS Audio's Humbuster 3.
Does it matter, since the power supplys converts to DC anyway? ***** NEWS FLASH, All your stuff really works off of DC ********

How much AC is okay on a battery power supply?
It matters if you don't want your power supply transformer to hum loudly. If you are trying to be a smart ass, all you have done is made a ass out of yourself.

Quote from article.
How DC Appears on the Mains
There are any number of different machines that can create a mains supply DC offset. Most will be totally outside your control, many DC "events" will be transient in nature, but one common theme applies - they will all load the mains supply asymmetrically for a period of time that ranges from a couple of cycles to minutes at a time. Figure 1 shows a typical (small) example that you may even have in your house - the transformer (shown within the dotted line) is your toroidal transformer. Many older hairdryers (and some heat guns as well) had a switch for "half power" that simply switched a diode in series with the mains. For a 240W element at 240V, that equates to a resistance of 240 Ohms (example only - actual power will vary widely).

If a diode is switched in series with the heating element, this reduces the voltage and hence the power (actual power will be almost exactly half). However, by half-wave rectifying the mains in this manner, there is an inevitable interaction with the mains impedance.

Figure 1 - Half-Wave Rectified Appliance, Transformer & Mains Wiring
The arrangement shown above assumes that the mains has zero impedance. Actual impedance is shown as R1, which varies from one house to the next. The value of 800 milliohms was chosen because this is what I measured at my workbench. Your mains may be better or worse than this.

After the asymmetrical load has done its job, a simulation shows the positive peaks of the 240V AC waveform reach 338.35V, but the (unloaded) negative peaks reach the proper value of 339.28V. This is a tiny bit less than the theoretical value of 339.41V because of the transformer load resistance and simulator resolution. The difference between the peak voltages is 0.93V, but the mean (average) DC voltage is -275mV. It is the mean value that appears as "DC" on the mains. It can also be measured, but to do so requires that one works on live components. This is not recommended as it is inherently dangerous. However, if you must (and PLEASE take extreme care), you need a 100k resistor and a 10uF non-polarised capacitor, wired in series. Connect this circuit across the mains (power off!), and connect a DC voltmeter across the capacitor. This attenuates the AC enough to prevent the front-end of the meter from being overloaded, and the DC voltage is easy to measure. Expect to see the DC vary around the zero voltage, with a normal variation of +/-25mV or so (typical - residential areas). The alternative method is to measure the DC across the diode/capacitor network in the circuit of Figure 3. Do not connect or disconnect the meter with the circuit live, and use alligator clip leads to make the connections.

With a half wave rectified load, the mean DC level is 275mV as described above - polarity is not important, because either polarity will be as bad as the other. If a transformer has a primary DC resistance of 2 ohms, there will be an effective DC current of 137.5mA in the primary. This is many times the current needed to cause the core to saturate during the negative half cycle of the AC waveform. Remember that with a toroidal core, saturation is a "hard limit". Because there is no air gap (intentional or otherwise), when the saturation limit is reached, inductance falls and current rises rapidly.

Tests were done using a 500VA toroidal transformer with very close to the example values given above. With 240V AC mains, 50Hz, 264mV DC offset created by DC injection (see Figure 6), and at no load, the current was seen to rise from 16mA to 218mA. The test was done at no load because this is the worst possible case. As load increases, the effective primary voltage falls - the voltage dropped across the winding's resistance is "lost" to the transformer. 264mV DC offset causes a current of 132mA DC in the transformer primary. This is probably the maximum offset that you will encounter in real life, although some areas may be worse. I have no data on this.
End of quote.
^^^Give me your address and i will send you a royality fee..sounds like you already own the market for being an a-hole.

That is exactly the type of response I would have expected from you. Sad, just very sad.
Say what no DC is best.
nothing that an isolation transformer can't fix.
All my low current stuff goes thru a small, 400va iso transfomer and it and the gear are dead quiet.
I have to take umbrage with the quote above. Unless there is a center-tap on the primary side of the power transformer (and there never is) this DC thing is of no consequence whatsoever as the power transformer will simply see that total voltage across its windings.

It may be a distorted waveform, but there won't be any DC. Keep in mind that one thing transformers get used for all the time is to block DC, and values at considerably higher levels than just a few millivolts.

However, *something* does happen, the distortion I just mentioned. Fluke Instruments published a very nice white paper about 20 years ago that explores this phenomena. It turns out that the primary (no pun intended...) cause of mechanical noise in power transformers is the 5th harmonic (300Hz in the US). The paper also gives a formula to allow one to calculate the distortion on the AC line, if you know the current drawn and the source impedance of the line transformer (perhaps the one on the power pole outside your house) winding.

The 5th harmonic can cause power transformers to become mechanically noisy, power rectifiers to radiate mechanical and electrical noise, and creates forces in hysteresis motors that can make them run backwards.
Not sure what a center-tap on the primary would have to do with DC voltage on the mains.

Nelson Pass
The one and only

If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +.
Nelson Pass


I've done some measurements that show what's really hapening when you connect a transformer to mains

Using a lowpass filter [100k + 47uF] I have measured about 50mV average over time of DC on my mains supply

I also have an old electric heater that in half-power mode uses a diode in series with the heat element to pass only half of the mains waveform. When I plug this heater in half-power mode I get an additional 1V of offset on mains supply

To test the need and the efficiency of DC filtering, I've done some measuremens of the current through the primary of a 750VA toroidal transformer

This oscillogram shows what happens when I connect the transformer to mains and let it deal with the 50mV DC offset
diyAudio Member

Eva gives actual hands on test data....
Hmm. So, with the AC line having one side at ground, there's supposed to be a DC level? The idea that a hair dryer can shift the ground potential to a DC level seem far fetched- it would require that the neutral side of the AC line be not connected to ground.

Distortion, OTOH, is the sort of thing that both Nelson and Eva were describing.
What does one AC line conductor, (neutral conductor), of the electrical service intentionally connected to earth have to do with it.

The main reasons for connecting the service neutral conductor to earth is for lightning protection and to establish one common ground plane.

Distortion, OTOH, is the sort of thing that both Nelson and Eva were describing.
How does the DC blocker filter block out the distortion?

Eva has many follow-up posts throughout the running of the thread. Like I said in my earlier post she ran hands on experiments and tests.

If I get a chance tomorrow I will visit the archives on AA. I remember John Curl posting about DC voltage on the mains.
Well Jim, I'm certainly open to finding out I've been wrong about things. Its how we learn :)

As far as the earth connection, what I am talking about here is that the hot side of the line is measured with respect to the neutral (ground). So if there is a DC component present, the AC voltage will be shifted with respect to the neutral. *That*, since the neutral is tied to the earth connection, will take some current and will likely heat the wiring up in the process.

Now if you have as an example Eva's hair dryer, running on half power so it its only conducting when the diode conducts, I can see that that might be interpreted as DC. But, it can also be interpreted as harmonic distortion. I'm not arguing, IOW, that the phenomena does not exist, just the explanation. One could as easily argue that a 2nd harmonic in the output of an amplifier is a DC phenomena as well.

I would like to see Eva's scope readings, but the images don't load off of DIYaudio.

I am not dismissing what you are saying about odd harmonics. They can and do play havoc on AC power systems.

They can cause the secondaries of power distribution transformers to run hotter than normal. And yes be mechanically noisier than normal.
Gbart, could you make a dc line filter / blocker for me? If so, can you email me at: white 78 911 at hotmail dot com

Is DC offset the product of harmonics?
My balanced power conditioner (Isoclean) was humming/buzzing loudly, sometimes for hours, sometimes for days, with no predictable pattern, and this was on a dedicated line. I put the Channel Islands XDC-2 between the wall outlet and the Isoclean unit, and the problem immediately and permanently disappeared.
Forgot to post that @Gbart built me a very nice custom DC blocker that allowed me to not only address my transformer hum in my preamp but also use the outlet I wanted, which is one of Al Porter's Ports.

Great service and product all around. :)

would love a dc blocker as well if you're willing:) I'm unsure how to get a email to you through the new agon system.
Would you please send me an email to if possible? I'm also interested in your DC blocker, and I have another question for you as well. Thank you.
Actusreus, I sent you an e-mail message.

Chris, you can reach me by e-mail at