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Old 13th January 2004, 06:26 PM   #1
Hitz is offline Hitz  Canada
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Question P3A transformer

I have a 300VA 32-0-32 transformer and I was wondering if this could be used for the P3A, or will the rails be too high, I'm guesing that this transformer is too large for the output trnasistors I have. They are the MJE3055/2955 pair. And if I can't use this transformer any recomendations would help thanks.
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Old 15th January 2004, 04:24 AM   #2
paulb is offline paulb  Canada
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Maybe, mercator built a P3A with a 30-0-30 and he had to add a fan; he plans to move to a lower voltage and ditch the fan.
It sounds great, though.
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Old 15th January 2004, 04:50 AM   #3
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Well, If I had a 32-0-32 AC supply I'd would use a higher voltage output device such as mjl21193/94 pair or others in this range, then the mje2955/3055 pair. Although cheap and easy to get the mje2955/3055 pair being on a to-220 platform don't have the surface area or the vceo to make good use of your supply unless paralleled.

I have always like the 2sb688/2sd718 combo but I'm finding these hard to get lately. These trannies would work well with your supply.
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Old 15th January 2004, 05:18 AM   #4
Jean is offline Jean  United States
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I would use it but change the output and maybe even driver transistors to something else. Also, you might want to think about what speakers you going to use, 4 ohm or 8 ohm.
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Old 15th January 2004, 06:31 AM   #5
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I think it's not a good idea to build the p3a with that transformer. I tried it with a 2x30V one, and the outputtransistors got really hot. Now I'm using a 2x25V and everything stays really cool.

You should try Rod's new Mosfet amp (P101) . You can use up to 2x70Vwith that design
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Old 15th January 2004, 10:50 AM   #6
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Hi, if you have a look through this thread I started a while ago you should be able to see the changes I made to the circuit (mainly to do with altering the driver/output transitor types and then adding a couple more output devices so that the amp should be able to drive up to about 250W into a four ohm load) as well as the board alyout I finaly came up with for it. Unfortunately I haven't gotten round to building this up yet, but hopefuly I might be able to do that soon.

With regards to the amp getting to hot, you could always just use larger heatsinks instead of adding a fan to keep things quiet.
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Old 15th January 2004, 11:19 AM   #7
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I used a really large heatsink and it got pretty hot. about 45°C with no load on the amp...

Now (with 2x25V transf) it stays at room temp.

The p3a just isn't designed for high power. If you want to use that transformer, I suggest trying a different design.

Why push the p3a to it's limits, if you can build another amp that you don't have to push?
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Old 15th January 2004, 01:09 PM   #8
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If you were to use the components listed in Rod Elliot’s design then you are right this amp is not capable of much more then it's rated at.

You could parallel up 10 output devices if you really wanted to given the driver stage is adequate and achieve vary high output power. The design is solid and easy to alter for much higher power.
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Old 16th February 2004, 02:40 AM   #9
Rodrigo is offline Rodrigo  Brazil
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Hi...
Sorry i dont speak english


I You send for me, PCB Layout Amplifer P3A .. ???

Very Thanks

Rodrigo
São Paulo - Brasil

rmp2706@hotmail.com
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