How big a Toroid Transformer

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How big a transformer do I need to power a 100 watt FET power amp board.?. I will be using two 250 VA toroids to create separate power supplies for each channel.. The transformers are rated at 3 amps each. I have been using the formula that 1 watt equals 1 VA but I have been told that this should be double that amount which means the two transformers I have should be okay with a little headroom there. The power supplies will be using Shcottsky diodes rated at 125 amps. I recently completed an amp using only one of these transformers to power a similar amp and I was promptly criticized for using such a small unit and that the amp would fail.. I have been using it for the last 4 months and it is driving AR3A's without any issues. Is there a good and valid rule of thumb to follow on this? Thanks
 
In general, a 250VA transformer can power both a 100W class AB and class D amplifier. Whether it has the right voltage and current for the type of amplifier and speaker impedance is another matter.
I am not convinced you will have any benefit from Schottky diodes though they have a reduced forward voltage drop. I have been told that the very rapid switching speed can cause ringing on the supply lines such that you may need snubbers for damping. Schottky diodes are very useful when you have low voltages and high currents (which is not the case) or for high frequency switching, evidently.
 
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Mr. FauxFrench says it well.

250 VA ≈ 250 W, which is about right for an amplifier rated at producing 100 W output, continuously.

My personal rule-of-thumb is 1.2 max( peak watts, 2 × average watts ), or 20% more than the larger of… twice the average watts (2 × 100) and the peak pre-clipping watts.

A competent 100 W amplifier — into an 8 Ω load — needs to have a power supply of approximately 1.5 × √(100 W × 8 Ω) = +42 : GND : –42 (if working off a split power supply), or in H-bridge form, just a 42 volt supply.

Competent means, being able to handle substantial peaks above 100 W to follow the dynamics of expressive source material.

Anyway, I think you're good to go.
Don't worry about people criticizing your toroid choice.
It should be plenty.

BTW — since you've got 1 channel working, how hot does the toroid get? Too hot to touch (after an hour), or warm-but-not-painful?

If the later, then you are JUST FINE. No matter what the critics say.
GoatGuy
 
Dual power supply project

Thanks for all the info on the Toroids. Just found a chassis that I can use for the amp. It was an TOA 906 MK II which I have gutted and cleaned up for use. I have gone to a smaller power supply board rather then the large one with the Schottsky diodes due to space limitations. It is rated at 25 amps bridge rectifier. Right now I am in the preliminary stages of putting everything in place to make sure it all fits.
For the power supply I have caps rated 100 volts dc and 5000 mfd each, two in each rail. Are these going to be enough filtering to do the job.?. I used them before in a single power supply project and the amp sounded great with no apparent issues, at least to my ears... DC offset was very low around 3 mv each channel. I tested the amp with my AS103 speakers, ( Kit version of AR3A)
and they sounded clean and very transparent. I like using these speakers as they are 4 ohms and really suck up a lot of power which hopefully will reveal any issues with the amp and of course not blow the speakers. ! Hope everyone had a great Christmas Cheers Wayne
 
Generally I figure a static load in watts of around 50~60% of the rated VA will be fine, so long as it is warm (even very warm is generally fine) but not hot. You figure that average AC current (while charging caps, etc) is 1.4X the standing DC load current, so this gives a bit of headroom assuming reasonable ventilation.

GoatGuy's recommendation is, as always, very good design rule :)
 
Let us be clear. A 250VA transformer will provide about 100-150W of continuous DC via a normal linear PSU. This may or may not be enough for a 100W amplifier channel.

For a Class D amp (efficiency almost 100%) it should be fine.

For a Class AB amp (peak efficiency 78% in theory, nearer 50% in practice?) it may or may not be fine. It depends on duty cycle. If you are listening to normal music at domestic sound levels then it may be fine. If you are listening to heavily compressed music at very loud sound levels then it may not be fine.

For a Class A amp (peak efficiency 50% in theory, nearer 0% in practice?) it will not be fine. A 100W Class A amp needs something like 250W DC input continuously, which needs a 400-600VA transformer.

So the right answer to the original question 'can I run my 100W amp from my 250VA transformer?' is 'we cannot tell you because of insufficient information'.
 
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