What does this mean?

sammybocat

Hi,

I have PSW303 sub and was wondering what this spec meant:

High current 100 Watt Continuous (260 Watt Dynamic) power amplifier.

Does that mean it can peak at 260 watts but at normal useage runs at 100 watts?

Thanks!
Jeff

Dr. Spec

The continuous power output is calculated using the RMS value of the voltage peak of the sine wave.

The RMS value is .707 X the peak value for a sine wave function.

A frequency bandwidth, a resistance load rating, and a distortion limit usually accompany this figure. Something like:

100 watts RMS into a 4 ohm load from 10-20,000 Hz with no greater than 1% THD.

Most people say "watts RMS", and that is technically incorrect - watts are watts.

What they really mean is "continuous watts as calculated using the RMS value of the voltage peak of the sine wave". But that doesn't roll off the tongue nicely, so we take the short-cut and piss off Russman in the process.

Peak power is (as the name implies) taken from the peak value of the voltage sine wave. Usually it is over a very short period (typically 40 milliseconds or thereabouts), and there is no bandwidth or THD rating to further quantify the number.

The Peak Power rating should be taken with a grain of salt unless all other specs are included with it. The continuous power rating is the one that matters the most.

The best amps out there have such robust power supplies that the continuous and peak power ratings are almost the same, and dynamic headroom is only typically a dB or so. While this may seem at first to be counterintuitive, it is not.

gatemplin

Good explaination Doc. I was under the impression that peak Watts were more important but now I understand. That also would explain why some amplifier manufacturers only quote the RMS rating.

howie777

"The best amps out there have such robust power supplies that the continuous and peak power ratings are almost the same, and dynamic headroom is only typically a dB or so. While this may seem at first to be counterintuitive, it is not."

"The RMS value is .707 X the peak value for a sine wave function."

Not to be picky but the power supply doesn't make peak and RMS the same. The second statement is true though and if so the power supply could have no effect on bringing the RMS and peak values closer together as RMS is defined as Root Mean Square or .707 of the peak voltage (if memory serves me correct which it never did in school!). A better power supply will allow more current. If you skimp on the power supply usually what you will see is 100 watts into 8 ohms and 150 watts into 4 ohms when a really good power supply will allow 100 watts into 8 ohms and 200 watts into 4 ohms. (half the load = twice the power output). But there are excellent amps out there that cannot deliver the full 2x power when going from 8 to 4 ohms like B&K for example does't but they come really close. Cheaper power supplies are simply done to lower costs as the power supply's Transformer is usually the most expensive component. I calculated what transformer I needed to build a 100 watt x2 into 8 ohms and 200 watt x2 into 4 ohms and for one it was just under $100! But for 30 dollars less I could still get 100 watts into 8 ohms just not the full 200 watts into 4 ohms (maybe 140 or so I guess). So you can save a lot of money if you skimp on the power supply and most likely you will get a great amp anyways.

Other wise the Dr. is correct as usual. Not that I don't trust myself but does that sound correct? I'm just really starting to dive into amplifier design myself so feel free to correct me if I'm wrong.

Howie

sammybocat

Thanks for the clarification Doc!

Jeff

Dr. Spec

You misunderstood what I said (I think).

Here's the crux - the peak value of the voltage sine wave under a continuous condition is not the same as the peak value of the voltage sine wave under a transient condition. I was discussing two different peak values. Continuous peak does not equal transient peak.

I can see why you thought I was trying to violate the laws of mathematics, because you mixed the two peak values and mistook what I was (trying) to say.

Here's what I meant to say, with some bogus numbers.

Robust Amp "A":

Peak Value Of Continuous Voltage Sine Wave = 4.0 (bogus)
RMS Value Of Continuous Voltage Sine Wave = 2.8 (calculated)
Peak Value of Transient Sine Wave = 5.0 (bogus)

Crapola Amp "B":

Peak Value Of Continuous Voltage Sine Wave = 2.0 (bogus)
RMS Value Of Continuous Voltage Sine Wave = 1.4 (calculated)
Peak Value of Transient Sine Wave = 5.0 (bogus)

Robust amp "A", and Crapola amp "B" can both generate a transient voltage peak of 5.0.

But Robust amp "A" can generate a continuous level of power which is 80% of its transient peak power.

Crapola amp "B" can generate a continuous level of power which is only 40% of its transient peak power.

Do you see what I was getting at now?