PSU Transformer ratings?

[wildswan]

Hey Guys,
Just opened my 2 x 590w into 4Ohms yamaha power amp to discover that the power transformer is only rated for 300VA!

I know that manufacturers underrate transformers and depend on the filter reserves for transients, however this seems a bit weak?

Which got me thinking that I may be able to use a 180VA toroid to make a light, useful power amp. (300-400w monoblock?)

My question is, how close to the full power output of the amp does the power transformer need to be before performance suffers? Is my amp project likely?

[N-Channel]

Not unless you've figured out how to get more power out of a system than what you put in!

The age-old problem of perpetual motion will have been solved!

Steve

[Eva]

There is nothing wrong there. With conventional speaker loads you are likely to burn the voice coils before the transformer has even time to get warm.

Peak to average power ratio of music signals is typically 5 to 20, and speakers are already sized taking advantage of that.

[wildswan]

lol, that's what I thought - very efficient indeed!

Peak to average power ratio of music signals is typically 5 to 20, and speakers are already sized taking advantage of that.

Sweet! Exactly what I wanted to know! Thanks

Now I have a new project to start

[Eva]

Be prepared for some supply voltage sagging (that's what people tries to mitigate with huge transformers). You should consider zeners, current sources and other tricks in order to keep the bias of the amplifier stable with rail fluctuations.

[wildswan]

Thanks Eva!

[AndrewT]

Hi,

Be prepared for some supply voltage sagging

to put it mildly.
Normally a power amp is sized at about VA rating=1.5times output power. By the time you approach VA=Output power Rail sag is already more noticeable, but at this level extra smoothing capacitance can make this accepatable.
At VA<0.75times output power, as Eva said, rail modulation starts to approach output modulation in level (well slight exaggeration) so you must ensure the voltage amp stages are not compromised.