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Old 25th August 2010, 09:18 AM   #1
ervinl is offline ervinl  Indonesia
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Join Date: Mar 2010
Default Choosing transformer for desired wattage

Hi,

I would like to know how to estimate maximum of power amplifier can be delivered/expected with known power transformer.

For example, if we have 400VA transformer, published spec 2 * 40V AC secondary (means 5A each by my simple calculation), roughly it can deliver up to 200VA/channel max for stereo. But in what speaker/load impedance?

If we have 4 ohm speaker, watt is the expected wattage/channel by this transformer?

Could we get more current just by adding more capacitors, or smaller capacitor with cap multiplier, or really needs more current from transformer? By rough calculation, the max current is only 2.5A per channel per rail, and in push pull, max power for 4 ohm is just (2 * 2.5)^2 * 4 = 100VA/channel? (50% of transformer max rate)?

Thanks,

Ervin L
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Old 25th August 2010, 09:23 AM   #2
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Join Date: Aug 2005
Capacitors only increase the available current for short periods, i.e. the amount of time it takes to deplete the charge in them, this will also cause undesirable sagging of the voltage, they are really only useful for smoothing and for short increase in current demand, any current draw that is continuous will deplete them faster than the trafo can "refill" them.

A cap multiplier increases voltage, but lowers available current, so that is not an option.

You can't have your cake and eat it too.

The current available thru the trafo is what it is. If you need more, you need a bigger trafo.
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Old 25th August 2010, 10:52 AM   #3
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
Hi,
a 400VA transformer could be used to power any size of amplifier from 100mW to 10000W.
Into small amplifiers it would perform very well but not provide good value for the money spent or the space/weight it takes up.
Into big amplifiers it will not perform well to the point it might overheat and certainly impact on the performance of the amplifier it feeds.

The recommended range of amplifier power that 400VA can supply economically is 200W to 400W of maximum total ClassAB output.

Your 40+40Vac will give you +-57Vdc to +-60Vdc depending on mains voltage and smoothing cap bank and amplifier quiescent current.

Let's assume the Voltage at the smoothing bank drops by 4V when delivering maximum power. That leaves +-54V (58-4) at the amplifier supply rails.

Let's also assume that the amplifier clipping level, when driving the load to maximum power, is 3V below supply rail

This fixes Vpk @ 51Vpk into your load.

Pmax=Vpk^2/2/Rload

8r0, 163W
7r0, 186W
6r0, 217W
5r0, 260W
4r0, 325W
3r3, 394W
3r0, 433W

The maximum output power varies with the load.
If you want one channel, you could attach a load between 6ohm and 3.3ohm and the transformer will perform well and return good value for your money/space/weight.
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regards Andrew T.
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