How Much DC is OK on a Power Line?

Actusreus, I sent you an e-mail message.

Chris, you can reach me by e-mail at sae_2300@optimum.net

Gbart,
Would you please send me an email to actusreus13@hotmail.com if possible? I'm also interested in your DC blocker, and I have another question for you as well. Thank you.

gbart,
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.
Thanks
Chris

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. :)

Scott

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.

Harmonics.

http://www.fluke.com/fluke/twen/solutions/pq/troubleshooting-power-harmonics.htm

http://www.psihq.com/iread/harmonic.htm

Is DC offset the product of harmonics?
.

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

Ralph,

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.

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.

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.
.
Jim

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.

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

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

>>>>>>>

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
Eva
diyAudio Member

http://www.diyaudio.com/forums/solid-state/2080-dc-filter-6.html
#57

Eva gives actual hands on test data....
.
Jim

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.

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.

Say what no DC is best.

6550,

That is exactly the type of response I would have expected from you. Sad, just very sad.

^^^Give me your address and i will send you a royality fee..sounds like you already own the market for being an a-hole.

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.

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?

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.

http://www.musicdirect.com/p-2845-ps-audio-humbuster-3.aspx

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

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

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

Eva
.

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 )

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.

Oh - and the question still remains - how much DC offset is problematic on an AC line for equipment?

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.

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?

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.

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?

whould a power regenerator (Purepower, PS audio) remove the hum/buzz caused by the DC on the line?

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.

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.

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