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Old 4th March 2008, 12:11 PM   #1
jkeny is offline jkeny  Ireland
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Default From 230V 3pin to 120V 2pin?

Here's a simple (& probably dumb question - I'm sure when I know the answer I'll be red faced):

I just imported a 120V Panasonic SA-XR57 HT receiver from the US to Ireland.

In order to step down the voltage I'm going to use a 1KVA site isolation stepdown transformer which gives balanced out 55-0-55.

Now this US amp only has a 2 pin 110V input connection.

In order to take advantage of the balanced power how do I connect my ground to the amp?
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Old 4th March 2008, 12:31 PM   #2
Dan2 is offline Dan2  South Africa
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i don't think you can - unless you want to open up your amp.

inside the amp is a step down transformer (usually) from 110v to a balanced +- voltage - which then gets rectified to DC which runs your amp.

So now you will be connecting a transformer to step down mains (220V??) to 110v - then the transformer in the amp will (probably) step down again. you will be using 2 tranformers to do one job.

The ground (0V) is set up using the transformer in the amp, so you would just connect the outputs your transformer to give 110v out. that is, of course, if you want to use both transformers.
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Old 4th March 2008, 12:58 PM   #3
mag is offline mag  Switzerland
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If you need the ground you can just connect the earth terminal to a chassis screw of your amp.
If it comes with no gnd connection it means that is has double insulation so no gnd is needed.

Gnd is needed only for safety reasons.

What do you mean by "balanced power"?? in any case the connection between the insulation transformer and the amp transformer will float about GND.

PS: What the power of that amp? Probably a 1kVA insulation transformer is quite an overkill....
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Old 4th March 2008, 01:38 PM   #4
gootee is offline gootee  United States
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You might also want to first open the amp and have a look around. Perhaps there is already some provision for running it from your mains voltage.

If there is a single main transformer which has dual primary windings, check to see if the primaries are wired in parallel or in series. If dual primaries were wired in parallel for 115VAC, you should be able to change them to series wiring for 230 VAC and still get the same secondary(s) ratings. If you're lucky, there might even already be a switch, or a set of jumpers or pluggable primary wiring, for that purpose.
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Old 4th March 2008, 01:39 PM   #5
jkeny is offline jkeny  Ireland
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Ok, your answers & my thinking about it further have clarified it!

Firstly, I'll answer your questions - the amp is a home theatre pure digital amp (equibit) with 7channels of 100W, so for current delivery I will probably need a 1KVa trafo. Anyway, I think this is the smallest size that the trafos come in. They are meant for building sites, for power tool operation, as a safety precaution they have 55-0-55V output. In other words 110V between the 2 live pins (balanced power) but only 55V between any live pin & ground - so electrocution is minimised.

There is a trafo inside the amp but it's SMPS so no hope of just substituting this I'm afraid! Anyway, I think there is some attenuation of mains noise in the balanced power approach.

I don't think I need to connect the earth to anything - the amp has a floating earth, I believe
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Old 6th March 2008, 02:51 PM   #6
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If your receiver is powered by an SMPS, check if it is a universal input (85-265VAC). If it is, then you should only have to adapt the plug, and not the voltage. If it is a half-bridge topology, then there might be a 115-230V switch that changes the full-wave rectifier coming off the AC line into a voltage-doubler, generating the ~330VDC needed for the primary of the main power xfmr. Looking at the manufacturer's specs should answer this. If not, try contacting them to see.

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Old 6th March 2008, 03:00 PM   #7
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You simply need to connect the amp to the two hot output of the transformer. In fact you are lucky the amp only needs live and neutral as if it needed earth as well you transformer would be no good, as it's earth is electrically and galvanically connected to one of the phases.
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Old 6th March 2008, 05:32 PM   #8
jkeny is offline jkeny  Ireland
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OK, thanks guys,
I hired a site stepdown isolation transformer (230V to 110V) & plugged it in - too much of a buzz to use. Luckily, it made me search through my surplus trafos & I found a EI trafo with 53-0-53 that I had pulled from a Sony HT receiver.

This works great!

No there is no internal switch for 230V operation - PS schematic attached!
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File Type: pdf xr55 ps schematic part3.pdf (78.4 KB, 30 views)
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