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Old 18th July 2002, 07:20 PM   #1
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Default ac filtration

First off, I'm sure there's been a discussion of this at some point, but the search engine won't let me enter "ac" as a query. If someone can point me to that thread, great.

Second, I'm not a hard core DIY, but a minor (well, not to some of my audio buddies: they think I'm nuts) tweaker. So, please excuse how ignorant I sound.

I've got one of those iec ac filter boxes on my amplifier. I don't like what these do to the sound. On a previous amplifier (of the same manufature, but different model) the sound with the filter is pinched and a little hard/bright, although cleaner sounding than with no filter. I've tried using just quality film caps shunted across the lines helped a little to clean up the sound, but it's not enough.

I was thinking of taking the filter out, measuring the inductance on each leg and using air core (if they're not TOO humungous - I don't mind housing them outside the amp) foil inductors instead. I know there are some other parts that those boxes use: capacitor shunts etc. but not sure what to do about those.

Any suggestions greatly appreciated.
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Old 18th July 2002, 08:09 PM   #2
tiroth is offline tiroth  United States
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I think it is unlikely that air core inductors have any qualities that will make a difference in this application.

It is possible that the impedance of the filter is too high for the PSU of the amp, causing the bass to be weaker and thus making the amp sound brighter. Probably you could try inductors with a smaller DCR.

An easy way to experiment might be to buy a chassis-mount type filter as opposed to a tiny IEC filter. Comparisons could be difficult though if you can't find one with similar topology/values.

I don't know what kind of film caps you are using, but if they are not rated for AC line use (X, Y) you have a potential fire hazard.
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Old 18th July 2002, 09:47 PM   #3
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The filter is a chassis mount filter with an iec receptacle that pokes through the chassis.

The caps are 600V Auracaps.

I was just thinking that high quality inductors would be better than the ones that are in the filter. I'm guessing the ones in that silver box can't be that great. I know that just changing the wire in the ac line makes a difference in the sound........

The amp has good bass, definitely not weak or lean. There's just a band of "hardness" in the treble. When I removed the ac filter on my last amp, that went away, however the resultant sound wasn't as clean/quiet.

Thanks
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Old 18th July 2002, 10:04 PM   #4
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The power line filter was engineered to meet certain specifications for attenuation of different frequencies of noise coming in via the power line (actually, it was probably designed to keep noise from your equipment from going out into the line). As long as your filter is rated for the sort of current and voltage your amplifier uses, it will have NO effect on the sound other than preventing power line crud from getting to your active circuits.

Even if it is under-rated, it is extremely unlikely that the filter would cause any audible effect whatsoever. The circuits in the amplifier have a certain amount of power supply rejection. Low level circuits are surely regulated and the possibility of crud going from the power line through the transformer, past the filter caps, through the regulators and then into the low level circuits is about 0.

If you have a problem with the way your system sounds, look elsewhere. Don't kid yourself or let someone else fool you into thinking that anything happening at the power line has anything to do with what you are hearing or imagining you are hearing.

MR
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Old 19th July 2002, 01:59 AM   #5
tiroth is offline tiroth  United States
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Those caps are probably unsafe in their current role. Although the chance of failure may be 0.01%, if failure means burning your house down, it is not worth it in my opinion.

X and Y rated RFI caps are relatively inexpensive and work well for their purpose. Buy the fancy Rifa dry paper caps or Wima polypropelynes if you must, but please use suppression caps. They are specially designed to avoid faults.
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