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In the Forum: Audio Discussions
In the Thread: It’s mad, mad, mad... electricity.
Post Subject: Common mode noisePosted by JOHNR on: 8/26/2019
One of the things that I picked up in this thread is the fact that if you use 2 PSUs in series, the sound degrades.This is typical of one unit generating common mode noise and this adds to that produced by the second unit.Even if you run them in parallel and driving different parts of the system, the cumulative effect will remain.Common mode noise passes through all electronic systems unless properly treated.
Let me explain common mode noise first.When diodes are used to convert a.c. to d.c. they generate this type of noise.The self capacitance of the device is charged and discharged with each waveform transition.  This creates  a tiny current pulse in series with the output voltage.This pulse has no energy and appears on the two output rails as a pulse having the same polarity on each rail.  Therefore, each rail goes up and down together thus the voltage does not change.  An oscilloscope does not show this pulse.  A capacitor does not respond to this pulse either since the voltage on each terminal goes up and down at same time.
The only way to filter this noise is to use a current compensated choke; aka a common mode choke.  This device will absorb the pulse appearing on each winding therefore self cancels.If you take any commercial product you will only find one of these on the a.c. input.  You will rarely find any on the d.c. side of a circuit.What you will also find is that the choke that is used meets a very rudimentary specification and is less that useful in keeping noise out of other circuits in a  system.  This explains why switched mode systems have a bad name, they corrupt everything else.
I will continue this in a follow on email.RegardsJohn

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