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power supply for 150w 8 ohms

Posted 3rd May 2010 at 01:01 PM by klewis
Updated 19th January 2011 at 12:53 AM by klewis

Hi,
a dual secondary 40+40Vac transformer will give ~58 to 59Vdc.
With good smoothing and a well built amp you can easily achieve 150W into 8r0.

If you want to have the ability to drive a 4ohm load then you will need to double the smoothing capacitance and double the VA rating of the transformer.

For 150W into 8r0, a 300VA with +-20mF will do.

My 230:40+40Vac gives +-58.5Vdc when fed with 240Vac and the amp bias is 220mA. It gives 170W into 8r0 from a 3pair output stage.

The power supply application note is available at http://www.national.com/an/AN/AN-1849.pdf

build the right half of the PSU in the app-note (Fig.1) and instead of all the soft-start/relay fun, put a thermistor, CL60, in series with the primary leads of the transformer.

transformer http://in.farnell.com/united-chemi-con/esmh800vnn103ma50t/cap-alum-elect-10000uf-80v-snap/dp/1363662

Andrew T notes on sizing fuses.

If the amplifier is to run on +-20Vdc you do not use a 20+20Vac transformer.
20Vdc/sqrt(2) = 14.14Vac Use a 14Vac or 15Vac transformer.

Power distribution.
The power distributed either from the mains or within your amplifier is governed by the formula Power = Volts * Amperes.
Sometimes Volts and Amperes are not quite in phase. So we use VA = Volts * Amperes.

If you have a 600VA transformer and want to connect it to 110Vac mains then the maximum primary current is defined by the same formula but rearranged.
Max I = Max Power / applied voltage.
Imax = 600VA / 110Vac = 5.45Aac (this calculation assumes transformer is 100% efficient. If we know the 600VA is 93% efficient then actual primary Max I = 5.45 / 0.93 = ~5.9Aac
Use a T5A or T6A fuse to run this transformer up to it's maximum continuous rating.

But, there is a problem.
The fuse will often/sometimes blow when you start up the amplifier.
You need to add a soft start to allow the fuse to pass starting current without nuisance blowing.
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