How much amplifier power do you realy need?

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48 minutes ago, PeterSt said:

Still the plot is in that quote because it is about an unknown number of frequencies at the same time. No math exists for that.

 

Maybe someone can point to a nice article about how it's done - I am willing to learn.

 

The math exists of course. The complex form of Ohm’s Law is your friend. 

 

There are two different approaches

 

1) measure the output impedance of both your amp & speaker at each frequency and power level from say 20-20 kHz (or wider if you want) and then use the Fourier transform of you input signal — you have the digital file. 

 

2) SPICE

 

You can measure or compute power dissipated by power supply, amplifier and speakers each using complex Ohm’s Law and  the total power is the sum of power from each frequency.

 

 

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13 hours ago, Confused said:

So how were the amps delivering 750 watts if they were not capable of producing current?  For a given voltage watts and amps (current) are directly proportional to each other.  Don't get me wrong, I fully agree that current matters, and that "wpc" ratings are often highly misleading, but @mansr is right, there are simple relationships between current, impedance and power. (and volts for that matter)  If you have lots of current, you will by definition have lots of watts, assuming you have some volts too of course.

 

This highlights an important issue, and with other folks saying “current matters” let’s try to pull this all together with Ohm’s Law. 

 

Typically 750 Watts is specified at 8 Ohms. When the typical speaker doesn’t act like a perfect resistor, the actual impedance is not always 8 Ohms. A passage might present a collection of frequencies that presents a 4 Ohm impedance — to maintain 750 Watts, the current needs to double. The amp might not be able to handle that so “750 Watts” doesn’t act like 750 Watts. Conversely a “better” amp might feed 750 Watts into 1 Ohm. That’s 8 times the current!

 

You could look at the power as a function of impedance instead.

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21 hours ago, One and a half said:

The measurement for damping factor is completed by resistance only, so those with a multimeter can measure the amplifier end and the speaker and work out the damping factor, to a degree. 

 

Some may complete the measurement of damping factor this way but the damping factor is frequency dependent and DF-0Hz is not the same as DF-1kHz ... the damping factor is typically defined as Za/Zs where Za is the actual output impedance of the amplifier and Zs the speaker impedance, so a curve, not a simple value. If the cables matter, either you have a really good amplifier in which case the cables really don't matter or really really bad cables.

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Correction: DF = Zs/Za (Zs - speaker not source), more generally Zload/Zsource

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2 hours ago, Ralf11 said:

a 2 ms rise time is fast if you are really old 

Also depends on the photo.