What is DC offset on the AC power line?


I realize this question may be more appropriate in the miscellaneous section so please accept my apologies in advance. This may be somewhat amp related though, as in "don't use it with an amp".  

Can someone comment as to the pros/cons of a  DC offset suppression device, such as Emotiva CMX-2.

https://emotiva.com/products/accessories/cmx-2

I haven't seen this feature in other power strips and/or surge suppressors.

Thanks.


Convert?fit=crop&h=128&rotate=exif&w=128gdhal

Here are a couple of Links for you to read on the subject.

You can not measure for DC on the AC mains with a regular multi meter.

http://sound.westhost.com/articles/xfmr-dc.htm


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


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Quote from Nelson Pass:

"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."




Thanks. The article is rather comprehensive, and while it explains a common cause could be an old household appliance, the article does not indicate what the disadvantage (if any) would be in using a DC offset filter if you didn't have an old appliance or otherwise need to. There must be some disadvantage in using this type of filter and that is what I'm looking to understand. If there is no disadvantage, I'm curious why the filter wouldn't be more widely available.

Did you read the DIY Audio thread I provided?

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


As for the Web Link you provided, my computer says it is corrupted and advises not to open it. I suggest you provide another link for the product.

Both my SS (for remote speakers) and tube main amp are plugged into a PS Audio Humbuster III…the tube amp wasn't humming but there was an extra outlet there so I figured it couldn't hurt…so…the SS amp had that pesky transformer hum sometimes and now it doesn't, both amps are dead quiet. The end.
the link u are providing is a forum thread from 2002 with many pages. I glanced over it. I do see one notation " What I'm trying to say is that the capacitors could limit the current and thus prevent the fuse to trip which may lead to dangerous situations. " 

jea48, here is an alternate url
http://emotiva.com/products/accessories/cmx-2

thanks for your continued feedback

gdhal,

Lots of stuff inside there. I do not have any experience with the Emotiva device.

Are you experiencing a loud mechanical buzzing/humming sound coming from a torrid transformer in a piece of your audio equipment?

There is a member here on Agon that used to build DC blockers. I might be able to find his username if you need a blocker.

Edit:

Gbart is the Agon member's username. If interested you might try sending him a PM over the Agon message system.

wolf_garcia, thanks for that post. Obviously use of that kind of filter can "work" (have the intended effect of reducing hum). Actually, they describe rather well what I was looking to understand.
http://www.psaudio.com/wp-content/uploads/2011/11/HumbusterIII_-Manual.pdf

jea48, no, I am not currently experiencing a loud mechanical buzz or otherwise think I would need such a device. However, in the past using a Carver MXR130 on the same outlet I have my current M6Si the Carver transformer did have a hum. Besides the PS Audio Humbuster III posted by wolf I wasn’t sure if this was something perhaps widely used in a audiophile grade system. But it’s because I do not have a problem that I’m looking to solve by virtue of a DC Filter that I’m asking what the drawback of using one is. Perhaps my electric system emits a "noise" of some kind amplified by the audio system and I merely cannot hear it. I doubt it, nevertheless, a possibility. Its also seems conceivable that a filter of some kind could eliminate or reduce something "unwanted" whether or not a person can detect it from an audible perspective. So again, this begs the question that why wouldn’t a DC Filter be used more widely in an audiophile grade system?
My SS amp stopped humming (note this is a physical transformer hum not audible in the speakers) on its own even before the Humbuster was added…DC gremlins taking a break? Some amp designer do put DC filtering of some sort in their stuff as my addled brain seems to remember reading about that in reviews…but hey…they certainly all should do some damn thing. I use a couple of "after market" power supplies for my DAC and phono pre that claim to have that feature.
NP nails it. It’s a little unusual to have DC on the AC line but it does happen, and as he points out, it happens from a device that draws current only when the voltage is going one direction, but not the other.

Dimmers, but also some digital power supplies may cause this. Any oscilloscope will show this problem, Usually you won’t just see a shift int he AC signal, but you’ll also see some nasty noise along with it.

The traditional fix is an isolation transformer, which are usually not toroids and present an AC at the output with little or no DC. DC may occur in cheap isolation transformers as a result of uneven winding if they are center tapped at the secondaries. One approach that is sometimes cheapest is to put the offending device itself on an isolation transformer.

Other fixes are to move to another circuit, or especially to a circuit that’s on a different phase than the one causing the problem. More expensive solutions MIGHT be to improve the wiring to the service transformer, allowing the device (dimmer) that is causing the problem to draw more current without a voltage sag.  If the house is old, uses fuses, etc. it's not a surprising thing to have, there may be other reasons to seek a wiring upgrade from a qualified professional.

Best,

Erik


DC off set is a much bigger problem than most audiophile's realize ... the humming transformers are merely the symptom ... the real deliberating disease is the loss of transformer efficiency 

DC off set is caused by the half wave bridge rectifiers used in all switching power supplies that draw current in uneven pulses .. this drives the sine wave off it's proper zero degree crossing point when the phase turns from positive to negative in the cycle

The humming in the transformers is caused by the transformer drawing an extra cycle and the plates snapping  back ... but the real issue is the loss of efficiency your equipment experiences  

Just about every appliance in your home including that flat screen TV ... cable box all your computer devices and some of your audio equipment uses switching power supplies and cause this problems with the harmonics they create  

The issue is that the loss of efficiency is not very audible like noisy RF can be ... your equipment just doesn't play with the authority or robustness it could or should  

I have a Emotiva CMX2... which also has some  common mode filtering ... on my wife's plasma  

I never A/B-ed it just bought it and installed it when I bought her a new 60 plasma .. along with the Oyaide R1 outlet and matching carbon fiber plate .... happy wife happy life 

I also use two PS Audio Humbuster III which I wouldn't be without in my 2 channel set up ... they had a profound impact  

Just today someone on Audio Circle put a used  Emotiva  CMX2 up for sale for $70 

I would have posted the link or sent it to the OP but the last time I tried to be helpful I was blocked  

Enter your text ...
My amp activates its protective system, and shuts down if it sees DC offset.
thanks to all who have responded.

davehrab, so here is a question perhaps you can provide an answer for. Your explanation is very good. Suppose that one does NOT have any electronic device with half wave bridge rectifiers and therefore would not have a DC offset problem. What is the disadvantage (if any) in that case if one were to use a DC offset filter (i.e. use the filter in a scenario where there is no DC offset to correct)?

I've seen schematics on the Bryston web site of their big amps and they definitely implement a DC off set circuit ... it's two  electrolytic caps in series by passed by a diode bridge ... the caps center the wave and the diodes prevent damage from reverse biasing ..

Bryston also uses a ground blocking circuit on the safety/ground wire with a diode bridge that allows a fault to be conducted to the ground rod but blocks any voltage being conducted on the ground wire in the opposite direction towards the amp

 Not sure who else or if any employ these circuits

Stringreen ... I'm going to go out on a limb here but I think your amp's protection is against DC (direct current ) riding on the the line and most likely the analog ICs ... the amp shuts down when it senses the DC current so it is not pass onto the speakers were it can damage the drivers ... DC offs set does not harm the speakers only reduces components efficiency

gdhal     if you could develop a perfect world scenario with no switching power supplies and half wave bridge rectifiers there would be no DC off set to defeat and no use for a blocker

But the blocker can serve another important role if and when not defeating the off set

I think the worst offenders are the commercial and industrial users that use 3 phase power ... they create a lot of harmonics that get back on the line and pollute the grid that you draw your power from

The  power your components draw from the grid  is loaded with these line harmonics that do the same thing in reducing transformer efficiency and you have the same issues ... reduced dynamics and head room

So even if you can evade the DC off set scenario some how ... you still have a big issue with the 3 phase users distorting the line and here is where a blocker like the Humbuster can be beneficial    Enter your text ...

"Just about every appliance in your home including that flat screen TV ... cable box all your computer devices and some of your audio equipment uses switching power supplies and cause this problems with the harmonics they create."


Add to that the micro processors in appliances.

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"The  power your components draw from the grid  is loaded with these line harmonics that do the same thing in reducing transformer efficiency and you have the same issues ... reduced dynamics and head room

So even if you can evade the DC off set scenario some how ... you still have a big issue with the 3 phase users distorting the line and here is where a blocker like the Humbuster can be beneficia"


I would think anything ahead the primary winding of the utility transformer feeding the house/s, the secondary winding of the Power Company's transformer would not be affected. An isolation transformer will not pass DC.

Of course if your neighbor next door is fed from the same transformer it is possible something in their house could cause a problem in the other houses fed from the same transformer. It would/could depends on the appliance/equipment. In the majority of household appliances/equipment though I would think it would not be a problem to other houses fed from the same transformer. Now if your next door neighbor has something like an arc welder in his garage that could cause a problem.