X1/X2 Capacitors AC in, adding noise to the system!? - diyAudio
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Old 16th December 2010, 10:32 PM   #1
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Default X1/X2 Capacitors AC in, adding noise to the system!?

It occurred to me that as I got noise in my own system from more capacitance than the typically .01-.1mfd people use that there is something afoot.

I had capacitors (X filter caps) wired in parallel to the AC receptacles on bus bars of my power distribution box. Switching from being connected just in parallel to putting the capacitors on the taps where the AC in (from wall) connects to the bus bar eliminated noise that I experienced and reduced outside noise from devices switching on and off (motor driven devices that I have to live with).

Now that I look back on it I understand what was happening. Because the capacitors put the noise 90 out of phase forward, when it dumps it back into the circuit it makes available an extra amount of electrons that other equipment can and will take advantage of because the draw is there and the cycle from where you get your power (power station) from is providing less... Essentially you can increase noise by attenuating it with capacitors on one piece of equipment and dumping it back into a bus bar, especially at a moment when the draw of equipment is higher.

You can have a filter capacitor on all of your equipment which might work or might end up amplifying noise to one device, or you can only put them on the taps of the AC in to your power distribution device before the AC receptacles.

P.S. my theory of why the noise is going back into the system could be faulty but I promise is does act that way. This idea just occurred to me that many builds of many items on here have the common practice but they are actually making more noise in their system.
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Old 17th December 2010, 01:25 PM   #2
DF96 is offline DF96  England
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Your theory is wrong. You should not need to parallel lots of capacitors. Instead, use series chokes too i.e. make a proper power filter. Failing that, buy a ready-made proper power filter.

Your equipment is not drawing power from the filter capacitors, as they are far too small in value for that unless you paralleled many thousands of them.
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Old 17th December 2010, 06:39 PM   #3
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Then back to the drawing board for why the behavior is thus.

They do introduce noise when not hooked up how I described. That is true. Why is a good question.

Commercial power conditioners are garbage at any price range if you ask me. Too many suck the life out of music or do very little.

By the way I do use CMC's. They help but I would not choose them over the proper amount of capacitors configured correctly, never. I am not convinced of isolation transformers at all for music. The few units I have heard sound a little flat.
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Old 17th December 2010, 06:47 PM   #4
AndrewT is offline AndrewT  Scotland
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Interesting discovery !
Moving the X cap along the buss bars reduced the noise that has been troubling your system.
Any chance you could take the time to sketch the before and after layout?

How about reversing the experiment and see if the noise comes back?
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Old 17th December 2010, 07:40 PM   #5
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I will try and make some pictures.
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Old 17th December 2010, 08:37 PM   #6
Elvee is offline Elvee  Belgium
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Capacitors are passive devices.

There is no way they can create or amplify interferences.

What they can do is provide low impedance paths for said interferences.
This could create relatively low-impedance loops, perhaps resonances, and the resulting currents could affect some equipments in the vicinity.

This normally would not happen in a "clean" environment, but if the return wire doesnt follow the same path as the feeding wire, or if neutral and ground are intermixed in a fancy way, or anything similar, parasitic voltages could be induced in the shielding and grounding conductors.

Remedies are are good wiring practices, common mode and differential inductors, and common mode ferrites on critical paths (with more than one turn wherever possible).
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Old 17th December 2010, 08:46 PM   #7
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Quote:
Originally Posted by Elvee View Post
Capacitors are passive devices.

There is no way they can create or amplify interferences.

What they can do is provide low impedance paths for said interferences.
This could create relatively low-impedance loops, perhaps resonances, and the resulting currents could affect some equipments in the vicinity.

This normally would not happen in a "clean" environment, but if the return wire doesnt follow the same path as the feeding wire, or if neutral and ground are intermixed in a fancy way, or anything similar, parasitic voltages could be induced in the shielding and grounding conductors.

Remedies are are good wiring practices, common mode and differential inductors, and common mode ferrites on critical paths (with more than one turn wherever possible).
That must be it. I will have pictures soon. Whatever the case we can all do ourselves a favor by not putting X capacitors in the equipment or inappropriately for how we distribute power.
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Old 17th December 2010, 09:05 PM   #8
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Quote:
Originally Posted by DF96 View Post
Your theory is wrong. You should not need to parallel lots of capacitors. Instead, use series chokes too i.e. make a proper power filter. Failing that, buy a ready-made proper power filter.
.
Trying to filter with just a capacitor doesnt work very well.
You need an RC or an LC.
The LC solution works best but is more expensive.

A mains filter should have a 2u2 across LN and then 2 inductors in series in the L and N legs, with to a 2u2 across the output of the filter.

I used this combination with great success in some industrial equipment.
It sailed through EMC tests.
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Old 17th December 2010, 09:29 PM   #9
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Quote:
Originally Posted by nigelwright7557 View Post
Trying to filter with just a capacitor doesnt work very well.
You need an RC or an LC.
The LC solution works best but is more expensive.

A mains filter should have a 2u2 across LN and then 2 inductors in series in the L and N legs, with to a 2u2 across the output of the filter.

I used this combination with great success in some industrial equipment.
It sailed through EMC tests.
A capacitor sure, but I use a lot more than one. For good attenuation you need a fair amount. I want to link you to an old article by Magnapan but it is gone.

I am not sure what 2u2 means but I found this,

"Disadvantages of LC filters:
~ Must be designed for specified source and load reistances.
~
~ Inductors that are close to ideal inductors are difficult to design, so the final design must be "tweaked" to allow for finite Q, distributed capacitance."

There is no specific calculation in what I make. Picture soon.
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Old 17th December 2010, 09:37 PM   #10
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We bought in a stack of mains filters from various different companies to start with but they were very expensive.
I was given the task of coming up with an in-house filter that would do teh same job but for a lot less money.

I stripped down the bought in filters and they were basically all the same with 2u2 X2 capacitors across the mains and Y rated capacitors from l+n to earth. They also had 1mH indcutors in series with the L and N.

I tweaked the filters to give best results on our EMC testing kit.

In the end I came up with a 7 filter to replace the 50 ones we were buying in.
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