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In the Forum: Audio Discussions
In the Thread: It’s mad, mad, mad... electricity.
Post Subject: Common Mode is a static eventPosted by rowuk on: 8/27/2019
 JOHNR wrote:
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

As you describe it, common mode noise is specific to architecture and would always be present. The phenomena described in this thread comes and goes. It is as if the power companies change the supply - which they do to meet demand during different parts of the day. During those times, we also have heavier use of devices that require more power. So does the problem come from operating generators with heavier load or is the problem the devices using power? Is the problem garbage being induced to the grid by RF or other means? Is the problem described in detail by Romy (East Coast)and Paul (West Coast)the same as that mentioned by Alex (Iran). How do we explain less mention of these issues by western European audio practitioners?

Alex, I have a APC RT UPS which is a regenerator and UPS in one. I use it for on site recording as you never know what you get from the wall socket. Dimmed lights, PA and all sorts of other devices like air conditioning and refrigeration are plugged in randomly at those venues. For especially bad situations, the device has enough charge for operation of the recording devices and mix board for about 11 hours. In the last 15 years, I have only needed the DC operation once. The APC RT successfully „fixes“ these problems - at least in the context of a mixing board, multi channel ADC/DAC, PC for recording and kilometers of microphone cable. Perhaps the problems in the recordings are not as prevalent because power amplifiers are affected not small signal devices? I do not know.

This thread is many years old and it seems that no one that has the problem gets universal relief from the „PurePower“ or other device. Seeing how Alex introduced the idea that power cords between the PurePower and mains change bass shows me that this is not a „solution“ but a deficiency in the PurePower device.

If I did have this problem of dramatic variable sound quality due to grid influences, I probably would make notes of when it is not so bad and move my listening to those times.

In any case, my personal opinion remains. This phenomena should be solvable by „better“ power supply design. Until we know what causes the problem, there is no way to define better. As we leave coal and atomic energy, I suspect that things will get worse - maybe so bad that some smart people will do something ingenious.

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