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Considering Joules in addition to Watts and output impedance in Amplifiers...

Chr1

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I have been interested in reading threads relating to how an amplifiers output impedance can effect the amplifiers FR when driving difficult loads. Also, how it is very important to ensure sufficient power is available for the amps given application.
So, I was consequently curious about how it's ability to provide current in particular, could also affect its real world performance.
Anyway, I came across this on a thread on one of the Steve Hofffman forums and thought it was interesting:

"Joules/ Watt-seconds are more likely more important than Watts alone.
A joule is one watt multiplied by one second (watt-sec), or energy consumption over a period of time. This is what your utility company bills (a kilowatt-hr is 3,600,000 joules). Or, the units of joules is voltage x ampere x time.

The different joule rating of the amps reflects the amount of time required to deliver a given current (based on the speaker impedance only) at a constant voltage. To oversimplify: if volts times amps is constant, the difference is in current delivery time. This is determined mostly by the power supply capacitance.

This time difference is the equivalent of having power in reserve for transient spikes that exceed the steady power rating of the amplifier. The capacitors trade voltage for current and discharge the energy as the speaker calls for a transient spike. If the caps have the joule rating to do it, the spike is transduced and you hear a nice attack on the instruments played.

The power that the speaker draws to produce sound is determined only by the amplifier output voltage and the speaker's impedance. But this is steady power - constantly available at a given impedance. A variation in the speaker impedance causes the current demand to change. A lower by half impedance at a certain frequency will cause the current draw from the amp to double, if voltage remains constant. This extra current has to be available either from the power supply capacitors or transformer. It it's the transformer that's capable, the higher current will flow all day long (the joule rating is infinite since the joules are transferred as long as the amp is playing). But if the extra energy is stored in the capacitors, the joule rating dermines how much and by how long you can have the extra current for. This is basically the headroom that an amp has."

Presumably this means that the ability of an amplifier to provide this increase in current is as important as it rated power in Watts? To reproduce transients accurately is this possibly the more important factor? ie Can an amplifier have a sufficient power rating in Watts, yet not enough output capacitance to accurately reproduce transients?

To use a dodgy vehicle analogy: Kind of like vehicles acceleration is more relevant in the real world than the actual horsepower produced, I guess. The way all of the relevant properties relate needs to be considered (eg mass, time in this case)
ie Mack truck vs Ferrari...

(Ultimately, we are trying to accurately accelerate the speaker/woofer/mass.)

Anyway, again my knowledge of electronics is minimal so apologies for any wrong assumptions here....and thanks in advance for helping me understand these things.
 
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Roland68

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With the unit Ws you have to be careful in which context you use this unit.
In the case of capacitors, this unit indicates the energy content independent of time. This means the stored energy content from real voltage and capacity (not the nominal voltage).
This energy can be released in an hour as well as in 1 / 100,000s.

Actually, this specification of the Ws of the capacitor packs of the output stage, together with the internal resistance or the current delivery capacity and the recharging characteristics of the transformer, would be a good way of evaluating the performance of an output stage.
 

Cbdb2

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That would be the peak (instantaneous) power rating compared to the continuous power rating Some amp manufactures show both. Most have twice the peak power compared to continuous, probably the best compromise. If your peak power is 10 times you'll never use it all and the PS will cost a lot more mostly in PS caps but also transformer and output transistors. A good indicator is the PS caps.
 
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