Can I reduce transformer voltage by unwinding?

[Tripmaster]

Hi

I am trying to put together an amp that requires a 25-0-25 VAC transformer, one per channel. I have two 30.6-0-30.6 VAC transformers I acquired a while back, and would like to use these if possible as I am trying to keep this build to the minimum.



Can I unwind the transformer until I achieve the correct voltage?

Many thanks

[cyclecamper]

That depends entirely on how it's wound. The answer is yes, if you can do it. You need to unwind the same amount of both secondaries. Sometimes primary and secondary wires are wound on together (with more prmary wires than secondary, then the primaries are put in series after it's all wound). Other times they are in layers, in which case you'd have to unwind layers to get to the previious winding. Good luck.

[awkwardbydesign]

I wound a transformer for a valve amp years ago, and the secondary voltage was proportional to the number of windings. So, yes, in theory, but can you unwind without it all coming loose? I must add that I am no expert.

[DF96]

I would just ensure everything can cope with a 20% overvoltage, then use it as it is.

[AndrewT]

30.6Vac transformer?
Is that stated on the label?
or
did you measure the unloaded output voltage?

I suspect you have a 25Vac or 28Vac transformer, not a 30.6Vac transformer.

[SoNic_real_one]

From the looks of the picture I think you have both secondaries winded simultaneously. You need to unwind that type of transformer without removing the cores, like threading the wire.
How many turns? Measure the voltage before and after you take 10 out to figure out how many turns per volt you have.

[simon7000]

You can easily add windings! Use a similar gauge wire and wind on two pieces at a time. i suspect you will need about 15 to 20 turns. That should give you 5 volts which when wired out of phase with the existing windings will drop you to 25 volts!

Or just keep winding and make your own secondaries at 25 volts!

[Tripmaster]

Hi

Thank you for all of your replies.

I'll answer Andrew's question.

The transformers were taken from a MF MA65 power amp. I Googled the transformer model number but could not find anything online.

http://www.ilpelectronics.com/images...mers_mains.pdf

Judging by the dimensions it appears to be a 120VA 30-0-30 transformer

Diameter 95mm (A) x 45mm (B)

I measured the voltage from the transformers output whilst disconnected from the PCB. This gave a reading of 30.6VAC. If you take a look at the PDF they made a 30+30 transformer.

I'm trying to cobble together a couple of MyRefC mono blocks without spending too much money. The preferred transformer is 25-0-25 or slightly less if used with a 4 ohm load.

I hope this helps

Regards

[wrenchone]

It may be simpler to use a bucking transformer to reduce the input voltage - that way, you wouldn't need to touch the main transformer. ~20V of bucking would get you what you need.

[Tripmaster]

Originally Posted by simon7000

You can easily add windings! Use a similar gauge wire and wind on two pieces at a time. i suspect you will need about 15 to 20 turns. That should give you 5 volts which when wired out of phase with the existing windings will drop you to 25 volts!

Or just keep winding and make your own secondaries at 25 volts!

Sounds good. I'm intrigued by the way transformers operate. The more I hear...