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kyrie48 1st November 2008 01:47 AM

Power consumption v. rms output
I've noticed on the back of my amps, a power consumption rating, which I don't understand. One says 245 watts power consuption, while the amp is rated at 150wpc min rms @ 8ohms... how do the two specs relate???

Thanks, kyrie

teemuk 1st November 2008 10:59 AM

Quite simple really: The amp consumes 245W to produce the 150W.

Ever heard the term “valve” used in electronics, that is basically a very good analogy of what an amplifier is: A valve controlling how much power gets drawn from the mains and fed to the speaker. Problem is, this “valve” is always inefficient and will waste a certain amount of power in order to operate. The efficiency of your amp is about 61%, which means only 61% of drawn power is fed to the speaker load. Rest 39% is just wasted and appears as the heat that radiates from the components.

east electronics 1st November 2008 12:14 PM

well not really .....
an average class AB amplifier has a performance factor of 50%

meaning that from 100 w of power consumption only 50w it will be transformed to audio .....

this 50% is actually on the safe side it could be easilly more with better tarfos biger caps and so on .....this not the actuall rule about amps the bad side ( worst also exist )

SO !!!!! your amp consumes 245 W ...devided by 2 which is the chanels of your amp makes 122.5 W devided by 50% which is the power factor you have a very good amplifier that produces easilly 2x61W @8R and another 122 @4R if power supply and VI limmiters allow that .....

this is "about" or "more or less" calculations and it could be a bit better ....or a bit worst ....any way ....thats the truth

destroyer X 1st November 2008 12:34 PM

Reading the power consumption we use to discover the RMS maximum total power

So.... around 50 percent (sometimes more) from the consumption results into real audio power (sometimes 65 percent).

And this to AB amplifier only!

But they are the ones we often find.

It is a fast way we have to know the real power...maximum power, added both channels, distorting as a hell and into the lowest impedance the unit can manage to work with.

When we decide 50 percent, it is a nice idea, because other circuits but the power amplifier consumes energy too.... Displays, Leds, pré amplifiers, sometimes motor driven volumes and other features also produces 50 percent, even beeing small value, give you a good evaluation about.

In Brasil we call Electronic Tubes as Valves too.... "Válvulas" is the name used here too... just for your curiosity folks,... if wrong way to least we are near 200 million talking wrongly....not only me! my mind, when i face 250 watts consumption.... having suspection exists an AB amplifier inside the case, i deduce 125 watts of sound.... as it is stereo, will be 62.5 watts each channel...but this into 4 will be around 30 watts into 8 ohms...and distorting..... the amplifier may produce clean 20 watts each channel..... sometimes the minimum allowed impedance is 8 ohms...then around 40 to 50 watts each channel.

Help us.... as fast evaluation...give us "an idea" about how strong the unit can be.

Into the shops, the popular systems offer PMPO (Peak Music Power Output).... this is a crazy thing...they add both channels with 10 percent distortion (this is not sound anymore..this is noise!)_ and multiply by 20 or 30 to produce big numbers...... observing the power consumption we can have a better idea.

BUT...they are writting wrong numbers of consumption there too.

No way to be sure anymore...has to measure...scope, load and to observe the voltage when clipping.



.................................................. .................................................. ...

Sometimes they do not inform the equipment consumption, they install an output mains plug into the back panel, ... an auxiliary outlet to you and they write this outlet power...ahahahaha...big one!...big numbers!

You will find in this crazy land i leave..battery operated portable noisy things with 500 watts power written into the panel...imagine that!..using standard AAA batteries.



AndrewT 1st November 2008 01:04 PM

the rated maximum power consumption has very little to do with the rated maximum output power.
The contradictory posts answering your question are corroboration of this.

It is likely that 245W is an indication of the VA of the transformer used to power the amplifier, if a transformer is used.
This also gives a good indication of the size of fuse to use to feed the equipment. A T2A or T2.5A fuse should allow the unit to operate without nuisance blowing.
Just to confuse the issue further, some manufacturers state the worst case start up power as the maximum whereas others state the worst case continuous operational power consumption.

Finally, the average power delivered during normal music/speech is around 20dB below transient peak levels. The amplifier rarely produces maximum power into conventional speakers unless you, the user, is abusing it.
The transformer is capable of delivering short term overload current well in excess of it's rated maximum. This inherent ability of transformers to deliver two to three times rated current allows manufacturers to justly specify output powers that exceed the maximum power consumption without telling any lies, even after allowing for the efficiency of ClassAB of around 65%.

megajocke 1st November 2008 01:19 PM

The rated power consumption is often measured at 1/3 or 1/8 average power output.

Two 150W channels at full blast will need about 600VA but music has much lower.

245W power consumption sounds reasonable for the active power consumption at 1/3 power output or possibly it could be 245VA apparent power at 1/8 power output.

Nico Ras 1st November 2008 02:01 PM


Originally posted by AndrewT
the rated maximum power consumption has very little to do with the rated maximum output power.....

I totally support your this statement. Commercially you cannot just specify anything.

kyrie48 1st November 2008 04:41 PM

Thank you so kindly everybody... that sure brings light to the subject!!

I popped the cover, and this amp has a 6.3A fuse, a nice LARGE transformer, uses [1 per channel] 80V 10,000uf caps and Sanyo STK4048V power modules. From what I've read, the 4048V is the second largest Darlington module, and produces fairly decent/fast transient power when used with a quality transformer.

Does this sound like a fairly good design/amp?

Thanks again, kyrie

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