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Old 26th March 2003, 05:11 PM   #1
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Default AC mains filtering question

While searching the archives, I found two debates.

Some people say that filters on the 115 VAC 50/60hz
(input to your transformer) is used to filter out
noise on the mains to create a cleaner power source
for your AC-DC power supply.

Some people say that those filters are actually used to
prevent noise generated by your amplifier from
leaking into the mains.

OR.. are both statement correct ?
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Old 26th March 2003, 05:47 PM   #2
Jeff R is offline Jeff R  United States
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Both statements can be correct. It depends upon the filter topology.

The source impedance presented by the AC input is very small. The load impedance seen by the filter (i.e. the AC transformer) is relatively high. Thus, depending if are looking from the filter into either the load or into the source, your filter will have a different frequency response.

While you may think that providing filtering for the line is a dumb idea, remember that you likely have other audio equipment plugged into the AC line so that noise generated in one amplifier can be conducted to those other devices. So, I believe a good filter to prevent conducted emissions is a wise thing to have. Most the filtering to eliminate power line noise may best be located on the secondary side of the transformer.
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Old 26th March 2003, 06:17 PM   #3
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If your designing a power supply and you add one
of those emi/rfi filters (like computers have) on the primary
transformer, this helps reduce noice that is sent back
into the mains? Then you add your Pi filter on the secondary
to clean up the signal/reduce ripple, etc. ?
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Old 26th March 2003, 06:22 PM   #4
Bricolo is offline Bricolo  France
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filters often are used on swiching power supplies (look on a computer one) to avoid noise from going to the main

this don't say that audio supplies don't generate noise
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Old 26th March 2003, 08:28 PM   #5
Jeff R is offline Jeff R  United States
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Quote:
Originally posted by thylantyr
If your designing a power supply and you add one
of those emi/rfi filters (like computers have) on the primary
transformer, this helps reduce noice that is sent back
into the mains? Then you add your Pi filter on the secondary
to clean up the signal/reduce ripple, etc. ?

That is how I would do it if I wanted such filtering.

If you chose a commercial EMI filter, I would get one oversized for current. The smaller ones, like for 5 amps, can certainly handle the power for a typical amplifier, but the internal wiring is of relatively small guage. If you get one with a higher rated power, the internal wiring will be larger, and this may have less of a sonic impact (I don't know for certain, but for an amp that otherwise has a big honking, 12 AWG power cable, running all that current through a little 24 AWG wire in the filter assembly and inductors therein can't be a good thing)!

Oh, if you go to the trouble of doing all this, be sure to enclose the filter in a grounded, metal box. Actually, the entire power supply should be in a grounded metal box to prevent the energy you are filtering from radiating and still getting on the wires. Feed through capacitors on the DC output are standard in low noise power supplies.
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