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83 posts

Master Geek


# 51688 30-Nov-2009 22:30
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Will a Stepdown Transformer act as an isolating transformer at the same time?
I have a earth hum and I am using a stepdown. It is coming from a pc connected to the pre-amp. the pre-amp is running off the stepdown. I would think a stepdown might isolate at the same time. but This might not be.
Can someone confirm this.
Thanks

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  # 278147 1-Dec-2009 08:21
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Some do, some dont, typically I would expect them to isolate, however there is most certainly a earth connection from the input socket to the output socket, but unless its an auto transformer (which is not uncommon) then the neutral and phase should be galvanically isloated primary to sec.

Cyril

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  # 278168 1-Dec-2009 09:38
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Most of the time, yes, a step down does isolate.
Have you used the transformer in a non-PC environment to make sure that it is not the source of the hum ?
I also have a hum issue between my PC and AV system that is proving to be a bit of a bugger to eliminate, but haven't invested too much time in resolving, just doing the basics first. It maybe similar to your problem, but I would check the transformer first.

The PC sounds good with locally connected Creative 2.1 speaker system, but when I run a lead from the sound card output to the CD input of the AV system, the volume is reduced, and there is a significant amount of hum introduced.
Thinking it may either be a faulty cable or a hum-loop between two power points, plugged in a regular portable CD boombox into the same power point as the PC, and used the same lead from the boombox to the AV system. Clear as a bell, no hum, etc.
So now I have both ends that work individually, but don't work together.

I have a couple of other steps to try yet:
Remove PC from UPS - earth isolation issue perhaps ?
Try the on-board Audio rather than the Soundblaster card

If those don't work, then I'm stuck - perhaps an audio isolator might work ? These are 1:1 transformers used in car systems to eliminate hum.




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  # 278343 1-Dec-2009 15:16
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I have bought many stepdowns, and the only which isolated specifically said so on them, and had no connection to the earth on either side. all the others were autoformers, and often wired so that the output was sitting at 240 on one side and 120 on the other side which would make noise worse.




Richard rich.ms



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Master Geek


  # 279536 5-Dec-2009 12:39
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Thanks guys, the stepdown was illegal to NZ standards. I think it was that neutral and earth went straight through and the phase was coiled to change the voltage. This resulted in a voltage difference between the the pc - preamp. We still got a different hum between pc and pre's which after getting a proper stepdown a car audio isolator fixed.
Now the only noise is the right noise(music)
Thanks Again!!

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  # 279556 5-Dec-2009 15:38
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Last I checked autotransformers were fine for NZ specs so long as they were labled as such and it was definatly the neutral that was passed thru.

The illegal ones (and rightly so) are the ones with 2 female sockets on the transformer and a male-male cable that you put into the socket for local voltage and then the other will output the varyed voltage.





Richard rich.ms

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