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What is common mode noise?


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We often cite common mode noise as being bad for audio, well it is but how bad is it, where the source comes from, and find a way to work around removing the source. Where noise ends up being audible is in the DAC, that's why this topic is in the DAC section of CA. DACs are very noise sensitive, and their SQ can be greatly improved by trying to remove common mode noise from the signal path and the power supply.

 

There are quite a few good papers to grasp the concept of common mode noise.  Here's three that are well written, not too much detail to be lost in the topic, but to find cures needs a little detective work, between all of us, we could find and report and publish the results.

 

From Gryphon and Emerson. From Maxim integrated. From Pulse Electronics.  The Maxim paper has drawings of cabling systems that will be familiar to CA users and where common mode noise can cause problems just from cabling. Texas Instruments have produced graphs of a few noise sources we use everyday, and effect of that noise is astounding.

 

The papers are not too arduous to understand, but not all recommendations are accurate.... For instance, the Emerson paper recommends a UPS for common mode noise reduction, but only if there are isolation transformers fitted. Not any transformer will do, as we know, Pulse Electronics mentions on the last two pages, how common mode noise can jump transformers, and how a balanced network can cancel out common mode noise. The drawings are mainly for @Cornan to appreciate why the centre point is earthed (grounded). There's not that much in the way of reducing differential mode noise on AC networks, other than a differential choke which could be worth looking at in some detail.

 

I wish it were easy to measure and apply an xyz and bingo it's gone. Not that simple. The best tool we have is the power of observation and listening.

 

 

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I will surely read those links when I’ve got the time @One and a half even though I have already read a lot about it before.

 

Meanwhile to give you a greater understanding I post a couple of pictures just how my *floating* and balanced isolation transformer is wired when you look at it without the switch. Note the star-point on the chassi where all the grounds is connected. The center-tap is grounded but the secondary is floating. This sounds vastly better than if the center tap is grounded through the secondary winding in my setup. I know because I can easily switch between the two in an instant.

 

 

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54 minutes ago, Ralf11 said:

Let's back up briefly - common mode noise is noise that is in common to both leads ...

 

Yes, but what is your point?

 

The crud can either find its way through the transformer and be cancelled there or take the back-door to where it came from. Common mode noise usually originate outside the isolation transformer AFAIK. That’s my theory anyway! ?

 

 

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8 minutes ago, Cornan said:

 

Yes, but what is your point?

 

The crud can either find its way through the transformer and be cancelled there or take the back-door to where it came from. Common mode noise usually originate outside the isolation transformer AFAIK. That’s my theory anyway! ?

 

not everyone has the sam background, so I thought it might be useful for some

 

pity that <2 can't seem to control himself

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