Hope this has not been mentioned earlier. I had a hum in a Bryston 4ss2, called Bryston, they would get back, blah, blah, blah. I went online and found a post where the OP suggested testing how tight the transformer was bolted to the chassis. Not real tight, just good and snug. I took the cover off the amp and used a wrech to see how tight the bolt was torqued for a reference. It was not even finger tight, so I snugged the bolt down and the noise is gone. Easy fix, no thanks to Bryston.
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.
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.
16 responses Add your response
This is fascinating stuff. I came to this thread after reading about the
Burmester 948 Power Conditioner which is specifically designed to eliminate DC offset. I live in an apartment building in a large city and I find that my system often sounds bad (anemic, glassy, cold, strained, two-dimensional, lack of power, no extension into the room) in the early evening (5pm to 8pm). I think davehrab may have hit the nail on the head with this: " the real issue is the loss of efficiency your equipment experiences... 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" |
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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 ... |
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)? |
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 ... |
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 |
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. |
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? |
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. |
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 |
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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. |
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