Transformer primary winding question

[gary h]

Hi all,

This is as simple as it gets, but then again, so am I;

I have a transformer that supplies 40-0-40 to a 260W amp. It's primaries are wired in parallel and are connected to a 120V main.

I would like to use it in a circuit requiring a 20V supply. I assume I can wire the primary in series to achieve this? That is, wire the primary for 220-240V but give it 120V?

If I understand correctly I will get half the voltage but what about current considerations, will this harm the transformer?

Thanks,

gary

[Elvee]

Hi

You won't harm anything, but you'll halve the available power (the maximum current remains the same), and the regulation will be rather poor.
On the other hand, the no-load power input will be very small.

[gary h]

Thanks. I'll give it a try.

[AndrewT]

hi,
wait.
The transformer will be capable of half VA when run at half voltage.
The 40-0-40 gives ~+-58Vdc after the rectifiers.
When run at half voltage it will give ~+-29Vdc not 20Vdc.
You could regulate down to 20Vdc, but you are wasting a good transformer that is capable of so much more.

[gary h]

Hi Andrew,

the circuit I have in mind is a gainclone using the lm3886 chip. (I built one of these recently using Peter Daniel's boards and am very happy with it.) I think that 26VDC is right in the chips speced range so I wouldn't need to bleed off any more voltage there. I am concerned about losing the power though. If the transformer is rated at 260 Watts, I have been told that I will end up with about 30-35 Watts if I run the primary in series. This might just be enough for a small pair of 4 Ohm speakers.

PD mentioned possibly keeping the 40V rails and employing active regulation to get the voltage down. If I'm correct this will probably equal the cost of a new 22V transformer I should just order.

But I'm trying to salvage what I can from this fading amp I have, it would be so nice to just replace the circuit and leave the (pricier) chassis, tranny, etc. as is.

cheers,

gary