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Old 20th December 2008, 11:19 PM   #1
gary h is offline gary h  United States
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Default Transformer primary winding question

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?


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Old 21st December 2008, 07:59 AM   #2
Elvee is offline Elvee  Belgium
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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.
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Old 21st December 2008, 11:03 AM   #3
gary h is offline gary h  United States
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Thanks. I'll give it a try.
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Old 21st December 2008, 11:26 AM   #4
AndrewT is offline AndrewT  Scotland
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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.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 21st December 2008, 06:21 PM   #5
gary h is offline gary h  United States
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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.


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