High Power - Output transformer

[h_andree]

Purely theoretical at this point.

I'm wondering about the following method of
obtaining a higher output. The output of most chips is
limited by the rail voltage. In case of a lm3886 with e.g.
+/- 30V supply you could get 30-40 watts into 8 ohms.

What if you would use a tranformer after the output
and double the output voltage. This means of course
that in order to supply the current, the impedance the
chip sees drops. This could be solved by
putting multiple of these transformers in series.
The advantage could be that small DC offsets would not
matter. In addition maybe matching of components
would not be as critical.

With a bridged design you may quadruppel the power
again and get up to 500W into an 8 ohm speaker.
(Although at the cost of 8 chips).

Has anybody done something similar considered this?
Or is my idea flawed?

Harry Andree

[AGGEMAM]

Originally posted by h_andree
Or is my idea flawed?

Flawed? Nah! You have just invented the perpetual motion machine! Again!

You can't get higher power out by using transformers (or other witchcraft). The end.

[h_andree]

I'm not trying to create power here.

If 2 chips bridged cap put out 100 W bridged at 8 ohms
then 8 chips bridged parallel can put out 400 W at
2 ohms.

You should be able to use a transformer to get the
output to be 400 at 8 ohms.

Now by putting many chips in parallel I
should be able to sustain the higher current that
is required for the higher voltage.

Harry

[bigparsnip]

Why not build an amplfier that is designed for these sorts of output powers, there are plenty out there, and they will be a lot cheeper than trying to find a suitable transformer (in terms of frequency response as well as power ratings), and will give a far better frequency response overall.

It just seams like trying to make a chocolate teapot when you would be far better off going for something a little better suited to the task at hand.

[maylar]

What you're asking for can be done, theoretically. But transformers are big, expensive, and have losses. Finding a 400 watt transformer to go from 2 ohms to 8 will cost you more than a 400 watt amplifier.

[AGGEMAM]

Originally posted by h_andree
If 2 chips bridged cap put out 100 W bridged at 8 ohms
then 8 chips bridged parallel can put out 400 W at
2 ohms.

No, actually if one chip can output 50 watts in 8 ohms, then 8 chips in parallel can output 400 watts in 8 ohms.

You don't need the output transformer if you parallel the chips! Just feed them all with the same input (possibly needing a buffer) and the same supply voltage, and add the outputs together. Voila, done.

[analog_sa]

No, actually if one chip can output 50 watts in 8 ohms, then 8 chips in parallel can output 400 watts in 8 ohms.

The above is correct only as far as the multiplication result goes. Or was it a joke?

The idea of the transformer is not so wild but difficult to implement and expensive. Some of the difficulty comes from the requirement the transformer to have a step-up ratio. Otoh you don't need a lot of primary inductance.

It may be interesting to test this with a mains rated transformer with 1:2 step-up. I seem to recall someone manufacturing similarly speced step-down transformers for OTL tube amps. The response to those was mixed.

[Steven]

In most Public Address amplifiers this is just routine. In Europe PA amplifiers have a standardized voltage output of 100Vrms at full output, in the USA this is 70V. PA loudspeakers are also standardized to these voltages, so you just connect many of these speakers in parallel up to the specified max output power.
The amplifier uses an output transformer to get an isolated 100V output and the loudspeakers use an input transformer to step down to the voltage that matches the actual speaker.
Because most PA amplifiers should also be able to deliver their output when running from a battery (often 24V) instead of mains, the actual supply voltage in the amplifier is very low. While running from mains the supply voltage is often around 28V and the 24V battery is connected to the supply voltage via a diode. If the mains fails, the battery acts as a secondary power supply and takes over automatically. The diode protects the battery from being (over)charged from the mains and avoids that the battery is loaded while the mains is still present.
You can buy 480W amplifiers that run from 24V (supply current 30A). http://www.pacat.co.uk/PDFs/Plena/Bo..._32412.pdf.pdf

Steven

[h_andree]

Does anybody know a source of
high-end transformers for this purpose?

Harry Andree