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Old Yesterday, 01:47 PM   #1
Banne is offline Banne  Sweden
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Default One transformer per rail?

Hi! I have salvaged some old ei transformers that i want to use in
an power amp.
The only problem is that like most amps it requires dual rail supply, like example
30-0-30 volts and these transformers are of the single secondaires type.
The transformers are identical to each other, ie same fabric and power ratings.
Would there be any ground issues or other problems related if i would use them as a Full Wave Rectifer pwr supply per transformer and connect them to each input sharing PGRND at the amp?

Last edited by Banne; Yesterday at 02:00 PM.
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Old Yesterday, 02:15 PM   #2
badman is offline badman  United States
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It is very important that they're well-matched (or you use loading in the power supply to match them), as any voltage difference between the two supplies will show up on most amps as a DC bias on the output. Beyond that, this method can work, and is one I've used myself when I had a couple hunks of good iron that could be shoehorned in this way.
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Old Yesterday, 02:33 PM   #3
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If you use a single full bridge with "identical" transformers balance is pretty well guaranteed, particularly the split output average voltage.
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Old Yesterday, 02:37 PM   #4
Banne is offline Banne  Sweden
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Thanks for your quick reply!
The main concern was if they would be very well-macthed as you mentioned in your reply and if the waveform could behave strange after rectification and for the risk of voltage difference .
I guess it would be worth a try.
Have an old diy amp lying in my lab that i will test it and measaure on, experiment with different loads, maybe try out some regulators too
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Old Yesterday, 02:39 PM   #5
Banne is offline Banne  Sweden
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Hi Andrew! If i use a single full bridge, wouldn't it be issues with the ampere output instead of using two full wave supplies?
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Old Yesterday, 02:41 PM   #6
sangram is offline sangram  India
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I've done this before and there's absolutely no problem even if the transformers are not 100% matched as long as the reference for the input and output are the exact same potential - star grounding is one way to achieve this. What happens is that the amp has lesser available swing in one direction than the other, by the difference in the rail voltages.

Any amplifier can be operated this way. A few volts difference between rails is not a problem, at least not from the output offset point of view. Obviously if using a servo this changes quite a significantly and a trimmer is required restore DC balance.
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Old Yesterday, 02:50 PM   #7
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Quote:
Originally Posted by Banne View Post
Hi Andrew! If i use a single full bridge, wouldn't it be issues with the ampere output instead of using two full wave supplies?
Per transformer VA duty is the same either way (assuming sine amp output with no offset), but the single bridge sends each half cycle load to alternate supplies.
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Old Yesterday, 03:03 PM   #8
Banne is offline Banne  Sweden
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Thank you for your replies!
I'll get to work on this in a moment and tryout different solutions.
I promise i will post the results later this week
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