Actual PSU size needed?

[Boscoe]

So from the things that I have read and that on here and elsewhere this is what I have concluded with transformer size.

Say our target is for this amplifier to be able to run 24/7 100W into 8ohms, not peak or instantaneous, without distortion introduced by the PSU.

So for 8ohm at 100W we need 28.3Vrms or 40Vp so for a little headroom we should use 45V plus PSU regulation typical for a 250VA toroidal traffo is 6% so ~48Vdc rails closet secondary voltage is 35Vrms. At 40Vp into 8ohm peak current is 5A. We need two lots of 35V secondary windings so 70V total * 5A = 350VA traffo needed.

This worries me because it's a great deal more than we need. We're not using 350VA just 100W 43% of our power is just for headroom and 50% isn't being used for half the time!. Is this really necessary? What can we get away with without increasing distortion? I haven't even factored in quiescent current for bias and capacitor charging. So we need more like 400VA!

I may well be quite mistaken here but I can't see how.

Thanks
Boscoe

[KatieandDad]

You're about double the VA rating that I would expect but I can't remember why ?

220VA should be fine and reliable.

[Boscoe]

Originally Posted by KatieandDad

You're about double the VA rating that I would expect but I can't remember why ?

220VA should be fine and reliable.

Assuming the degree regulation from pulling all power from one secondary.

[AndrewT]

Ignore the the transformer current rating.
Use the target maximum power output of the amplifier/s to determine a suitable transformer VA.
Use the target Vpk into your load to determine the transformer voltage.

100W into 8r0 is equivalent to 40Vpk.
Assume the losses through the PSU and amplifier and cables/connectors are about 5V to 8V. A 35Vac transformer gives between 49Vdc and 52Vdc when lightly loaded.
That is close enough to your 45 to 48Vdc target.

For a two channel amplifier the total maximum power output would be 200W.

Choose a transformer that is VA rated from 1times to 2times that target power.
Select a 200VA to 400VA 35Vac+35Vac transformer. For a single channel you can halve the VA rating.

Current rating of a transformer becomes very important when the load draws a continuous current. Eg any ClassA amplifier, even the Pre-amp can be quite demanding since it too usually operates in ClassA.

[eem2am]

Boscoe:- you say it seems overrated but thats just how it is with non power factor corrected power supplies.............about half of the power being shipped by that transformer is going to do NO useful work at all.............because the power factor of transformer/rectifier/smoothing-cap systems is terrble.....about 0.5.

[DF96]

Two issues complicate things, but they push in opposite directions.

The RMS vs. average current in the transformer secondary means that you need a much bigger transformer.

The Class B duty cycle issue means you can probably use a much smaller transformer. (This does not apply to Class A, of course).