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Question about transformers 300VA

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Member level 1
May 19, 2002
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I´m building an audio power amplifier(60W+60W RMS).
The transformer for power supply is specified on parts list as 300VA only (not mention Secundary volts and not mention secundary Ampers).
The power supply requerided by amplifier is simetric, +30V / GND / -30V.
What is the secundary volts and ampers that the transformer need to be?
Thanks in Advance

capacitive input filter

If you use a center tap secondary and full wave rectification and capacitive input power filter, you will need about 31 V peak which is about 21 V RMS on each side of the center tap.

300VA Transformer

If amplifier is made with transistors, you must take a good look in the specifications of power output transistors ( that Vce and Ic let them become the necessary ) in order that this don't hoverheat.
A transformer with the necessary characteristics for 60W RMS'S stereo amplifier, it must have 25+25 Vrms's secondary voltage to 30+30 Vrms (depending of power output transistors) with 5 Ampers.

If voltage and current increase, also potency increases.

The rectified voltage will be approx 1.414 * secondry volts, so any value betwwen 20 - 24 sould be ok. EG: 22 * 1.414 = 31.108. The absolute value on the amplifiers power rail is not super critical. There is also the transformers regulation to take into account, which is the off load voltage compared to the full load voltage. Just make sure your devices have adequate breakdown voltage ratings.
A transformer will supply as many amps as you ask of it, the limiting factor being temperarure rise due to the resistance of the windings. Transformers are rated in VA, which is volts * amps, and is the continuous power that the transformer can supply without overheating. The overheating temperature is about 70 degrees C. So, if you had 20-0-20 secondry with a 300VA transformer, the continuous amps you can saftly draw is 300/40 = 7.5 amps.
In my experience you get about 1 watt of output power into an 8 ohm load per VA. So for a stereo 60Watt per channel amp, a 120VA transformer would be sufficient, add a little safty margin and a 150VA transformer would do the job. Unless of course you are a HiFi freak, then 300VA would be about right.

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