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-   -   Rewire 240V toroidal transformer for 120V (http://www.diyaudio.com/forums/power-supplies/210163-rewire-240v-toroidal-transformer-120v.html)

tsibbs 4th April 2012 08:38 PM

Rewire 240V toroidal transformer for 120V
 
Having moved from Asia to Canada, I am attempting to convert my (British) 240V equipment for use with 120V mains.

I was able to rewire the toroidal transformers in the monoblocks (got lucky: the toroids were labeled with wiring instructions), but wasn't so fortunate with the preamp.

It is an LFD LS2 linestage preamp, and I have taken a few snapshots and uploaded them here:
Photo Album - Imgur

The transformer on the preamp is about 4 1/4 inches wide and 1 1/2 inches high. The label on it reads:

B: 3301 8
S: 3252 sec
oxfo

op ts

and is stamped twice with 26 in a circle.

Is there a way for me to rewire it for 120V or, if not, what kind of transformer should I use to replace it?

Thanks,
Tim

CharlieLaub 4th April 2012 09:40 PM

Quote:

Originally Posted by tsibbs (Post 2973028)
Having moved from Asia to Canada, I am attempting to convert my (British) 240V equipment for use with 120V mains.

I was able to rewire the toroidal transformers in the monoblocks (got lucky: the toroids were labeled with wiring instructions), but wasn't so fortunate with the preamp.

It is an LFD LS2 linestage preamp, and I have taken a few snapshots and uploaded them here:
Photo Album - Imgur

The transformer on the preamp is about 4 1/4 inches wide and 1 1/2 inches high. The label on it reads:

B: 3301 8
S: 3252 sec
oxfo

op ts

and is stamped twice with 26 in a circle.

Is there a way for me to rewire it for 120V or, if not, what kind of transformer should I use to replace it?

Thanks,
Tim

Why not get one of the very common and inexpensive (relatively speaking) step-down auto-transformers that will convert 220 to 110V AC? This seems much simpler than rewiring and replacing all the transformers in your equipment. I can see why you would do that for the power amp(s), but for line level equipment, I think that the step-down transformer would be the quickest and simplest way to solve your problem.

-Charlie

tomchr 4th April 2012 10:11 PM

Something like the Antek AS-2T230 would be perfect. 2x115 V primary, 2x230 V secondary. Hook the two primaries in parallel and the two secondaries in parallel (observe correct phasing) and you have a 115->230 V converter trafo that can handle 200 VA. At the normal 120 V AC line voltage in North America, you'll end up pretty darn close to 240 V on the output of the trafo.

You could probably also find a trafo at Antek that'll fit in place of the original trafo. Then you can rewire it for 230 V if you move to Europe at a later date...

~Tom

sreten 4th April 2012 10:27 PM

Hi,

From what I can tell your preamplifier transformer only has a single primary,
the two orange wires. Ideally what you want IMO is a step up transformer.

1:2 or 115V to to 230V, it will cost less than a replacement transformer,
that won't be easy due to the apparent number of secondaries, and the
two in tandem will be better at rejecting vagaries in the mains supply.

They are quite common, doing 115V to 230v or the other way round
as a step down, depending on how they are wired, get one one size
bigger in VA than the one fitted internally in the preamp.

rgds, sreten.

CharlieLaub 4th April 2012 11:03 PM

Step up, step down... it's all the same if you use an autotransformer (they work both ways). There is no need to buy a transformer with an isolating core and independent primary and secondary. Get a $50 1000W auto transformer (it has only one winding, tapped at 50%) and power all your equipment from that. These are very commonly available.

-Charlie

tsibbs 5th April 2012 12:11 AM

Thank you all for the responses.

The equipment had been hooked up to a 1000w step-up transformer up to this point, but I kept getting a rather annoying buzzing sound from the speakers whenever it was connected, hence my desire to remove it from the chain.

Now that I no longer need it for the amps, I will try to either find a less buzz-inducing transformer or might just bypass the pre until I can figure out what/how to replace the transformer in it. =)

AndrewT 5th April 2012 10:44 AM

do you have dual phase power that seems to be normal for the North American countries?

110/120Vac to most of the house?
220/240Vac to the utility room?

Use the 220/240 to power the Audio Room. Get an electrician to do the cabling and sockets.

KatieandDad 5th April 2012 10:57 AM

Try speaking nicely to the manufacturer. Although they may be reluctant to release the full schematics you might be able to get the wiring diagram of the transformer.

At line level its a relatively simple job to replace multiple secondaries with regulators.

SOMETIMES its not possible but it's worth considering.

My idea would be to replace the transformer with a suitable alternative and then to regulate out the various supplies that you need.

You could also do this yourself by measuring the AC voltages of the secondaries yourself. You already have an auto-transformer to power it up with.

KatieandDad 5th April 2012 11:22 AM

1 Attachment(s)
This is based PURELY upon what I can see in the photo and the gauges of the wires.

I would remove the transformer and ascertain which wires form which windings.

A centre tapped winding is easy to ascertain, you will have three wires that seem connected to each other, the centre point will be the one that has the same (low) resistance between the two ends and it...

Once you have drawn out the windings you can connect all the cables to connecting block (just to stop anything shorting).

Ascertain the Primary (which is almost certainly as shown - but will be the highest resistance winding. Connect this with a bit of cord, through the block to your step down transformer.

The rest is a simple measurement using a DVM.

Once you have the AC voltages we can make some suggestions.

KatieandDad 5th April 2012 11:33 AM

There is another method which is perfectly valid but not recommended unless you have some experience with transformers and high voltages.

1. Wind 10 turns of cable around the toroid core - ie another secondary.
2. Measure the voltage generated across this secondary - for example 8 VAC.
3. Now - CAREFULLY calculate how many turns are required to work at 110V
- In my example that would be 110/8 x 10 = 138 Turns.
4. Now - CAREFULLY wind another primary of 138 Turns on the transformer, making sure that the turns are equally spaced (within reason) and then wrap the finished article in good strong PVC (Electrical) tape.
The 240V Primary will thus be redundant and will need to have its terminations insulated.

That looks like a 120VA transformer so 1.5A (16 AWG / 18 SWG) would probably suffice, thicker wire will do no harm here.


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