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Old 11th May 2003, 10:04 AM   #1
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Default P3A transformer question

I'm planning to build the P3A.

On the construction site: http://www.anidian.com/audio/construct/p3a_1.shtml

there is a little calculator that lets you fill in the power you want and then calculates the transformer.

What is meant by VCT?

My local store sells toroid transformers with a secondary voltage of 2x30 V. I guess that's the rail voltage? so 2x30 = 30-0-30 ?

They also say that this is the voltage under load.

Another question:

Has anyone here build this amp with the MJE2955T/MJE3055T transistors?
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Old 11th May 2003, 02:04 PM   #2
moamps is offline moamps  Croatia
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Hi,

with 2*30Vac (or 30-0-30) you can get after rectifier and caps about 2*42Vdc. This mean that RMS power on Z=8E is Prms=ca 30*30/8=112,5W.
Power rating of transformer must be higher then 200-250VA.

2N3055 is old transistor. Try 2sa1302/ 2scxxxx.

Regards
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Old 11th May 2003, 02:23 PM   #3
PassFan is offline PassFan  United States
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VCT is the -0- in your 30-0-30. This represents two transformer windings side by side with the tie in the center. As Moamps implied when you convert this from ac to dc you will gain almost 40%. Multiply the 30 by 1.414 to wind up with around 42 to 45 volts dc. When you put this under the load of a circuit (your amp) it will sag (drop down to a lower value). Perhaps your article gives you an allowable % to factor in for this sag. If it is scaleable (bigger power supply voltage for more power out) then it is no big deal just take what you get close enough to what you want. If not, wait for someone smarter than me (there are plenty), or do a search on power supplies. Good luck
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Old 11th May 2003, 03:52 PM   #4
paulb is offline paulb  Canada
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Default Re: P3A transformer question

Quote:
Originally posted by DIY_Peter
I'm planning to build the P3A.
...
Has anyone here build this amp with the MJE2955T/MJE3055T transistors?
Yes (actually 2N3055/MJ2955) , and it sounded fine. But when I switched to the MJ15003/4 pair, the difference was significant. Well worth the cost.
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Old 11th May 2003, 04:45 PM   #5
sam9 is offline sam9  United States
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Even if you understand the idea of a center tap transformer, it is easy to get confused with the way they are listed in many catalogs. For instance, you can find listings that just say 30V but mean 15-0-15. on the other hand if you take it the other way around, you can easily order a 60V unit (30-0-30) when what you wanted was 15-0-15. The Plitron and AvelLindberg websites are unambiguous but some other sources are a pain.

Built a TIP3055/2955 version to use for some experiments. One was cheap and the other was sitting unsused in a drawer here. Not bad but power is limited. I keep the rails at 30VDC or less. If I was going to use it for listning, I'd use 2sa1943/2sc5200
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Old 12th May 2003, 01:15 PM   #6
saurus is offline saurus  United States
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I used a Plitron 400 VA transformer with 30-0-30 secondaries for my P3A. After rectification, they measure at 43.7 Vdc unloaded, and 42 Vdc loaded. According to Rod, this gives 90-95 wpc.
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Old 12th May 2003, 01:23 PM   #7
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Ok, thanks, I think I understand it now.

What would be the best choice for the powertransistors?

I can't find the ones that Rod recommends.
Are TIP3055/TIP2955 a good choice? Or is there really a big difference in soundquality between those things?
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Old 12th May 2003, 02:07 PM   #8
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Ok, thanks, I think I understand it now.

What would be the best choice for the powertransistors?

I can't find the ones that Rod recommends.
Are TIP3055/TIP2955 a good choice? Or is there really a big difference in soundquality between those things?
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Old 12th May 2003, 02:13 PM   #9
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Default p3a

The toshiba 2sa1302/2sc3281 are in stock @ digikey, after being out for a while. These are excellent flat-paks and they are way easier than the T03 to locate and use.
for that matter the lower rated toshibas in the digikey lineup, (a 1497 & and its complement, I think) would make great drivers (if a bit of overkill) for this amp.

I would ignore what rod sez about these being counterfiet.
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Old 15th May 2003, 04:00 PM   #10
sam9 is offline sam9  United States
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Default Try www.donberg.ie

DIY_Peter is in the EU. Mailorder from digikey might not be his best choice. I looked at Donberg (no umlauts in theURL). They have these as well as the newer replacement parts 2sa1943/2sc5200. I would think it would be cheaper and faster to use an EU source.
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