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Old 2nd October 2003, 04:31 PM   #1
DIY_Peter is offline DIY_Peter  Belgium
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Default P3A question

I'm going to build a P3A amp. Today a friend of mine gave me 2 new toroid transformers, that he couldn't use.

2x30 V 225 VA, (made by Transduktor from sweden)

30V 's gives a power output of about 100W with a P3A

How do I best apply these 2 transformers?

One for each channel? Isn't there a chance that the voltages are going to be a little bit different between the two? or doesn't that matter? They are exactly the same types, so I guess the difference wont be great.

Can I use them in parrallel? Will this give me 450VA?
Or 2 bridged amps?
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Old 2nd October 2003, 04:55 PM   #2
Andypairo is offline Andypairo  Italy
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The best solution, if you need a 2-channel amp, is a toroid per channel (dual-mono configuration). If you need 4 channels a toroid (and capacitor bank) can be enough for 2 channels, which will share the same PS.


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Old 2nd October 2003, 09:38 PM   #3
Greg Erskine is offline Greg Erskine  Australia
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P3A question
I have built my P3A with one 25VAC 160VA toroid per channel.

If I have done my maths right 30VAC will give you about 42VDC. I think this the maximum voltage Rod Elliott recommends. At this voltage pay careful attention in choosing your output devices.

BTW, Rod now has a forum for answering these questions.
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Old 2nd October 2003, 10:35 PM   #4
Jean is offline Jean  United States
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my 25-0-25volt tranny gives me 36volt dc when everything is connected. I would use one per channel and double up on the output transistors
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Old 3rd October 2003, 08:10 PM   #5
Mark25 is offline Mark25  United Kingdom
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My p3a's are dual mono and run 30-0-30 160va tr's, which give +/-45vdc with no load, i have used mje15003/4 op devices with very large heatsinks and have no prob's to date, after 1 yrs use. i think you should be ok with high power o/p devices, and a h/sink on the class A driver, althoughi don't have one and the bd140 only gets warm.
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Old 6th October 2003, 07:18 AM   #6
Cradle22 is offline Cradle22  Germany
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Use one toroid for each channel, and decide between single-bridge or double-bridge rectifier setup for each channel (Rod and I myself use single-bridge setup).

Voltage differences shouldn't matter much, as they would be very small, and you will regulate the bias independently for each channel, anyway.

I myself use a single, 30-0-30 500 VA transformer, but with seperated rectifying bridge / caps setup for each channel. I wun the 42 V into a 6 ohms load, works alright for me, even 4 Ohm (which Rod strongly advices against) works alright, although I did not try playing at high levels for a long time into that load.


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