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Old 23rd March 2008, 04:11 AM   #1
dfdye is offline dfdye  United States
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Default Transformers: How big is TO big??

Due to the rise in copper prices and the coincident rise in transformer prices, I got to thinking: how much of a transformer do we really need for an audio PSU? I have always erred on the side of "bigger is always better" since I have always been lucky enough to find "overkill" transformers surplus (thanks ApexJr!).

Still due to the transient nature of audio signals in conjunction with the often large rail capacitances we use for amp PSU's, how would you go about actually calculating what you really NEED for an amp as opposed to simply using the maximum peak amperage and getting a transformer that can handle that? Google wasn't really any help on this front, and I figured a ton of you EE types probably solved this problem a long time ago as undergrads!

I would love to hear any insight people may have on the subject.
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Old 23rd March 2008, 04:17 AM   #2
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1.414 x rated RMS out ....
Round it off to 1.5 ....
VA ..
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Old 23rd March 2008, 04:39 AM   #3
dfdye is offline dfdye  United States
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Wait, so you are saying a 100W amp * 1.5 = 150VA transformer? That doesn't really sound too insane, but is it really that simple? Am I just obtuse for never seeing this before??
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Old 23rd March 2008, 05:35 AM   #4
sangram is offline sangram  India
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I've gone from 0.8 to 2.5, the sweet spot (price/quality) seems to be in the 1.5-1.8 range. This varies wildly with designer's tastes, application and speakers' abilities/demands, so take this IMO.

Overkill never hurts though, and you'll see recommendations of 300VA for a 60-watt amp. I have no access to toroids at reasonable prices, so cannot verify that such a thing could sound better, but I'll assume that may also be true.
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Old 23rd March 2008, 06:14 AM   #5
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In a normal capacitive power supply, conversion between Watts and VA is approx. 0.68:1

A typical real world class AB amplifier has an efficiency of maybe 65%.

Music has a duty cycle that is highly dependent on the type of music you are listening to but for argument sake, let's say 40%.

So....

For a Mono 100W Class AB amplifier, you can figure a sensible minimum transformer rating (assuming sufficient capacitance in the power supply) of:

(100*0.4) / (0.68*0.65) = 90VA

With that said, there are good (technically sound) reasons to use larger than the minimum rating required.
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Old 23rd March 2008, 08:29 AM   #6
dfdye is offline dfdye  United States
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Quote:
Originally posted by AudioFreak
With that said, there are good (technically sound) reasons to use larger than the minimum rating required.
Your explanation makes good sense. The missing part for me was the ~40% duty cycle approximation, but that sounds completely reasonable. Thanks!

As for the reasons to have a larger transformer, of course a larger transformer would run cooler at the same current draw, making the chances of failure over time lower (like cracked enamel on the windings causing shorts and such), and a transformer that was too small might hum, but other than that, I can't really think of good reasons to go much beyond what is needed.

Either way, 0.9 * wattage = VA sounds like a completely reasonable (well justified ) rule of thumb as a minimum for transformer ratings.
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Old 23rd March 2008, 09:13 AM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
the peak output current is supplied by the smoothing caps.
The transformer recharges the caps.

I usually suggest that the VA is between 1*max power and 2*max power. Most are happy with 1.5times.

I have found that using a slightly smaller transformer can be compensated for by increasing the smoothing cap value.
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Old 23rd March 2008, 11:00 AM   #8
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Quote:
Originally posted by dfdye
but other than that, I can't really think of good reasons to go much beyond what is needed.
Regulation and Dynamic Impedance of the power supply will both improve with a larger transformer. There are other reasons also but I just thought I'd list a couple to make the point.
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Old 24th March 2008, 03:01 AM   #9
dfdye is offline dfdye  United States
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Quote:
Originally posted by AudioFreak
Regulation and Dynamic Impedance of the power supply will both improve with a larger transformer. There are other reasons also but I just thought I'd list a couple to make the point.
. . . . proving that I need to read a few more books.

Thanks again to all!

David
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Old 24th March 2008, 10:15 AM   #10
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In addition to his webmaster's voice: first Q is which load ?

For Class AB, the 100W/90VA would be for a true 8 Ohm load.
Depending on the intended load, same calculation for either 4 or 2 Ohm, which boils down to a minimum of ~150VA for 4 Ohm operation.

Some manufacturers used to offer their product with powersupply options.
Good example were the Berendsen power amps, Red and Blue edition, only differed in the toroid and capacitor bank size.
Or The Cyrus PSX.
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