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Old 9th July 2004, 09:58 AM   #1
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Default transformer in series

Hi guys,

I was trying on an amplifier circuit that requires the transformer having an output 50Vx2 625VA (for stereo). To have a special order for such transformer was way over my budget. I happen to have 2 pcs of 24Vx2 150VA transformers. What I am thinking of is to put their output windings in series (forget about the individual CT) with the common point as the CT. I'll have the DC output supply to one channel at a time (so it needs only 300VA). If I can put it together and want to have the amp. for good, I might make a tailor-made order. Will this work or am I just nuts? Forgive me if this sounds stupid, it's only my third diy project. Anything I have to look out for ?

Thanks for your input and advice.
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Old 9th July 2004, 02:03 PM   #2
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Two transformers will work. Just make sure that you wire them in phase so that the secondary voltages add rather than subtract.
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Old 9th July 2004, 02:28 PM   #3
moamps is offline moamps  Croatia
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Quote:
Originally posted by Frank Berry
Two transformers will work. Just make sure that you wire them in phase so that the secondary voltages add rather than subtract.
Hi,
Ratio Vac/Ptransformer(VA) looks little to high for usual solid state amp.IMO
+/-70Vdc on 8 ohms produce about 300W; 300VA transformer doesn't look appropriate.... IMO
Regards
Milan
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Old 10th July 2004, 04:47 AM   #4
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Tks guys,

Frank :
I do understand the term 'in phase' but how to achieve this? Can you elaborate further on this, please.

Moamps :
That is what it's spec. says : 400W with 4ohm load and 200W with 8ohm load. As I mentioned earlier, I am just trying on this. I won't be (and would never) going anywhere near full load. Since you brought this up I would like to put another Q : how can we 'burn in' an amplifier at full load. I hope I don't have to use my speakers (and my ears) although they are not expensive.

Tks for your input.
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Old 10th July 2004, 11:47 AM   #5
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Connect primaries in parallel to mains the standard way, use a fuse of course, then connect the secondaries in series. Now measure the voltage you get from one end to the other, if the transformer are connected correctly you will get twice the voltage of a single transformer if you get aproximately zero you will have to reverse one winding, you must reverse one and only one winding; you may reverse a primary or a secondary.

Remember that the phase of the output winding is linked to the phase of input winding, so to keep the output in correct phase the input winding connection should not be changed.
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Old 10th July 2004, 05:58 PM   #6
moamps is offline moamps  Croatia
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Quote:
Originally posted by bigpanda
[B]
That is what it's spec. says : 400W with 4ohm load and 200W with 8ohm load. As I mentioned earlier, I am just trying on this. I won't be (and would never) going anywhere near full load. Since you brought this up I would like to put another Q : how can we 'burn in' an amplifier at full load. I hope I don't have to use my speakers (and my ears) although they are not expensive.
Hi,
One amp (class AB) has eta ca 50-70%. Then, add more 10-20% and you have nominal transformer power.
I use high power resistors for "burning in" and checking amps parameters. (automobile bulbs in correct parallel-serial connection are also good solution)

Regards
Milan
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