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Old 11th January 2004, 11:56 AM   #1
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Join Date: Aug 2003
Location: Hong Kong
Default Powre Transformer for 2A3

Hi all expert,

I am building a SE 2A3.

The power output is around 3.5W.

The input voltage is AC 280 before regulation.

1. How many mA is required (or enough)?

2. When compare 280V - 0V or 280V - 0V - 280V, which one is better?

Thank you for your help.
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Old 11th January 2004, 02:54 PM   #2
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Location: Macedon NY
The 2A3 needs about 60 mA at 250V, according to the data sheet. The driver stage will need about 10 mA. So you need 70 mA total. Now the tricky part - the transformer needs more current capacity that that, because there is "ripple current" flowing in the filter capacitors - so for a 280 VAC transformer secondary,you'll need at least 100 mA - 200 mA for a stereo amp. For a 280-0-280 secondary, the current is split between the halves, so a 70 mA (140 mA for stereo) is enough.

Hammond has this on selecting transformers: http://www.hammondmfg.com/pdf/5c007.pdf

Or you can use the free PSU designer program from Duncan Amps: http://www.duncanamps.com/psud2/
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Old 11th January 2004, 06:36 PM   #3
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For a stereo amp design:

Is that mean for 280-0-280 secondary trans, it need 140mA.

For 280-0 secondary strans, it need 280ma?
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Old 12th January 2004, 01:20 AM   #4
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Location: Macedon NY
No, actually 200mA is the minimum, as the full-wave (4 diode) bridge makes more efficient use of the transformer than the two-doide full-wave or half-bridge used with the center-tapped winding. Another way to put it is that the VA (volts*amps) rating of the transformer can be 30% less if a full-wave bridge is used.

But you have to use a transformer you can GET... not necessarily what you WANT... and larger than the minimum is better - less heat, less voltage drop, less crosstalk between chgannels...
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