Determining Power Amplifier Wattage

[avalon123452]

 1000-watt.pdf (127.54 kB - downloaded 64 times.)

This is OCL amplifier described as 1000 W from the website I got. The output stage is using 7 pairs of 200V 12A or 20A MOSFET. With 90V Vcc, 1 MOSFET can conduct  1000W, but why does it need 7 pairs and just described as 1000W? Is it to achieve better power dissipation? How do you determine the power rating of the amplifier without trying it on speaker? Can't you count it from the output stage MOSFET?

[TimFox]

For a proper measurement of power output, you need to connect a good load resistor of the appropriate value to the amplifier output and measure the voltage across it.  1000 W dummy loads are readily available at 50 ohms for RF applications, but harder to find for 8 ohms and audio applications.
I continue to harp upon the proper definition of mean power as the (square of the RMS voltage) divided by the resistance.  This is not the "RMS power", which is a defined quantity but useless.
How would you "count it from the output stage MOSFET"?  Beware of hyped power advertisements, which have been with us since at least 1950 in audio circles.

[avalon123452]

Can you tell me more about these hyped power advertisements? By "count it from the output stage MOSFET" I meant to count it from the MOSFET power rating, Vcc, and how many parallels, but not sure if it is the right way. What I am wondering about is how to count the power in the design stage of the amplifier before testing the hardware. Most of the designs that I encountered on the web always has a much lower rating than the used transistor output.

[gbaddeley]

The output power can be roughly determined from the power supply voltage and load resistance. Allow for powe supply sag, output device saturation, degeneration resistors, wiring and connection losses etc.

Po RMS ~= ( Vcc - Vloss )^2 / ( 2 Rload )

[avalon123452]

So if the output power is 1000 watt then I only 1 pair of final MOSFET. With 90V Vcc, power through MOSFET is 1080W without thinking about any safety factor. Is that correct?

[BradC]

Beware of hyped power advertisements, which have been with us since at least 1950 in audio circles.

Never!

[TheMG]

Since this is a linear class AB amplifier, the SOA (safe operating area) of the transistors is the main limiting factor, and does indeed relate to power dissipation as well.

There also has to be a safety margin, which is not only a good design practice, but also speaker loads are not like a pure resistance, the actual impedance is all over the place and can be above or below the nominal rating depending on frequency.

[Terry Bites]

Dave's mate knows how: https://sound-au.com/project124.htm

[avalon123452]

thank you for everyone reply, it really helps broaden my knowledge

[pqass]

See https://geoffthegreygeek.com/understanding-amplifier-power/ for a simple explanation of all the terms.

To determine in practice:
1. connect dummy load*** instead of speaker,
2. apply 1KHz sine wave to input (with function generator; max. 300mV_rms or 900mV_p-p AC but start at 0V),
3. raise volume to just below clipping on the output (visible on oscope),
4. with a multimeter (or oscope measurement function) measure volts AC on the output,
5. calculate power: P=V²/R_speaker

*** If you're quick with steps 3 and 4 above, you could just use the speaker instead of a dummy load.

Disclaimer: speakers should be appropriately sized for the amplifier.

[TimFox]

This is correct.  You will get a more accurate value if you use a good resistor instead of a speaker, since the speaker is a complex impedance (heavily resistive) that varies with frequency and "8 ohms" is really only a nominal value.

[EPAIII]

All of the above answers are good. But there is one other factor that is important when the output power level of an audio amplifier is specified and that is the quality of the output signal at the specified level.

If you want just raw power and don't care about the quality of the sound, then most audio amplifiers can generate a lot more power than their specs would imply. But if you throw in a decent response curve, a low level of distortion, and a low noise level, then it becomes a lot more difficult to generate the power levels that a simple analysis of the Voltage and current levels that are possible in the output stages. Any high quality amplifier will have a very flat response curve, very low levels of distortion, and very low noise levels. And even mediocre ones will have fairly good specs in these areas. And all of this comes at the sacrifice of at least some of the available power.

In any case, a manufacturer of a high quality audio amplifier would measure the output power under circumstances where those quality factors were within certain, rather low limits. So it is not just the amount of power that the transistors can operate at nor the amount of power that is consumed in operating that final stage but rather the amount of power where the manufacturer is willing to guarantee the stated performance.

If you are comparing the power output of two or more audio amplifiers, then it is important to be sure that these other factors are taken into account. If you are willing to allow the distortion to reach high levels, then I can show you a real cheap amplifier that will deliver more power to the speakers or to a dummy load than any multi-thousand dollar, high quality one. It's like comparing horse droppings to apples.

Of course, with most listeners of popular music all of this is a totally wasted effort. I can remember my brother deliberately mis-tuning his radio to increase the distortion of the audio because he liked it that way. That was before the days when the tuner in most radios was designed to automatically lock on to the center frequency of the station. I wonder what he does now but I won't be asking him.

[TimFox]

Back in the day, amplifier power was often measured at 5% total harmonic distortion (THD), since that was considered to be what you could see on an oscilloscope.  This was typical for single-ended tubes at the output of AM radios.
Things have improved:  there is controversy within the rational end of the audio fraternity about distortion measurements, but THD is easy to measure with an appropriate instrument (to better than 0.01%), and spectral analysis of the distortion is straightforward for steady-state distortion.  There are other considerations for distortion induced by transient phenomena. 
For those only interested in (boomety-boom) raw power, the clipping point discussed above can be exceeded as a sinusoidal input gets transmogrified into a square wave, limited by the amplifier DC power supply.