Iso trans and ac mains filtering advice please - diyAudio
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Old 14th November 2011, 12:33 PM   #1
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Smile Iso trans and ac mains filtering advice please

Hi,

I am looking for advice about the best way to implement a 1KVA iso trans into my system (it has 0-220V in and out and one earth connection on the input side). Mains here is 220V AC. The system is 2 class-T amps for bi-amping (100VA trans for each), a pre-amp/crossover (80VA), two DACs and a few sources (maybe 100VA total) so about 400VA absolute max load.

I have some objectives - eliminate DC, reduce or eliminate transformer buzz, remove smps noise from other devices in the flat, stop the ocassional crackle when a strip light turns on, etc - ultimately to improve sq of course.

I also have some 140uH 5A toroids and a whole bunch of 275V y-rated caps and some 630V metallised poly caps too.

If anyone can direct me to circuits / methods they've tried and know work well I'd be very grateful !

Thanks to all.

Last edited by KlipschKid; 14th November 2011 at 12:35 PM.
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Old 14th November 2011, 02:22 PM   #2
AndrewT is offline AndrewT  Scotland
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An earth on the input (primary).
Are you sure?
A centre tap in the secondary is very possible.
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Old 14th November 2011, 02:41 PM   #3
vuki is offline vuki  Croatia
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Earth connection is probably static shield. I suggest to try it as it is and from there you can always go this way.
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Old 14th November 2011, 03:01 PM   #4
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Hi,

Thanks for the replies. Here is a picture :
Click the image to open in full size.
Click the image to open in full size.

The input dc resistance is ~1 ohm and the earth reads open circuit to either/both input (and chassis) so it isn't a centre tap. Static shield seems more likely but I'm not sure...

Croatian site looks interesting - I was thinking of using a filter (LCLC) on the output and just some small value caps on the input but I don't know if this would achieve my aims ?

Click the image to open in full size.

cheers

Tom

Last edited by KlipschKid; 14th November 2011 at 03:28 PM.
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Old 14th November 2011, 03:42 PM   #5
DF96 is offline DF96  England
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Don't forget that interference can be radiated as well as conducted. The most perfect mains filter in the world won't affect radiated interference. You may need RF filters on your equipment inputs too.
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Old 14th November 2011, 03:42 PM   #6
vuki is offline vuki  Croatia
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Since you already have xformer, caps and inductors the best way is to try it yourself.
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Old 14th November 2011, 04:37 PM   #7
AndrewT is offline AndrewT  Scotland
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Your open circuit result confirms the GND is a screen between primary and secondary.
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Old 15th November 2011, 11:16 AM   #8
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Hi,

Thanks for the help guys. A screen is good news and it can also serve as a star earth.

I already have filters on the individual gear and interconnects, and the power and pre-amps also have their iso trans but still the crackles and noise can come through. I've tried using a surge suppressor normally used for PC supplies but that still lets the odd one through too. I'm hoping a big iso trans will free the other trans of DC and reduce buzz (already inaudible but I can feel it through the case), and adding a filter will clean up the supply.

I'll have a closer look at the parts I've got and do a schematic so you can see what I'm planning. I think putting the filter on the input side is a better idea to avoid losses on the output.

I'd rather get some comments before I build anything so it isn't a complete ****-up.

Thanks again :-)
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Old 15th November 2011, 11:24 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by KlipschKid View Post
A screen is good news and it can also serve as a star earth.
No. The Screen gets tied to the chassis. The chassis is the outer screen.
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Old 15th November 2011, 12:26 PM   #10
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Now you know why I want advice ... ;-)

So if I tie the screen to the chassis, doesn't that then have to be tied to earth to work as a screen (helmholtz type thingamy) ? So I should use a different point as a star earth ?

Thanks
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