Amplifier Power Ratings & Other Mysteries Of The Universe Explained - Part 1

Audio amplifier power ratings have long been a hot topic of debate amongst the professional audio community. Particularly controversial are amplifiers that on paper appear to produce more output power than they draw from the mains supply. This has prompted some sceptics to dispute published specifications based on claims that they defy the laws of physics. In this article I will attempt to explain the operation of such "physics defying" amplifiers and hopefully dispel some common myths in the process.

There appear to be three points of misunderstanding causing the majority of the confusion surrounding this topic. Each shall be discussed in turn below:

The Laws Of Physics (and how they relate to audio amplifiers).
Storage Power Supplies (and how they relate to the laws of Physics).
Amplifier Measurement Methods & Standards (and how they relate to the real world).

The Laws Of Physics (and how they relate to audio amplifiers)

The trekkies amongst us would no doubt be familiar with Scotty, the ship's engineer whose pet cry was "you cannot change the laws of physics". Wise words that so far at least have proved to be true, however given that the Star Trek concept was based on a craft which traversed the universe at speeds greater than the speed of light, maybe Scotty is not the best reference. Several basic laws of physics dictate the way the universe operates, and contrary to the belief of some marketing people, these laws also apply to the pro-audio industry.
Vocal critics often dismiss data that suggests an amplifier can produce more output power than it draws from the mains supply. These critics would claim that it contravenes one of the "laws of physics", stating that devices cannot create power and thus output power cannot be greater than input power. However, there is no such law. I suspect they may be incorrectly quoting the law of Conservation of Energy, which in simplified terms states that the total amount of energy in an isolated system remains constant. A consequence of this law is that energy cannot be created nor destroyed. The only thing that can happen with energy in an isolated system is that it can change form.

The key word here is ENERGY. Do not confuse energy (joules) and power (watts). The two are related but not identical. Energy = Power x Time. Thus, a device producing 1 watt of power over one (1) second of time will have produced a total of one joule of energy. Real world devices always have conversion losses, so total output energy is always less than total input energy with the difference usually dissipated as heat.
Audio power amplifiers are actually energy conversion devices. They draw energy from the electrical mains supply and convert it to a form suited to driving loudspeakers. If we ignore conversion losses for the moment, we can develop a simple equation to describe an audio amplifier:

Energy IN (J) = Energy OUT (J)

The above equation can be rewritten in terms of electrical power as follows:
Power IN (W) x Time IN (s) = Power OUT (W) x Time OUT (s)

This is where things start to get interesting. If we assume that Time IN = Time OUT then Power IN MUST = Power OUT. In which case the critics are correct. The amplifier cannot produce more output power than it can draw from the mains supply (input). However, what if Time IN does NOT equal Time OUT? What if Time OUT is much less than Time IN? Then Power OUT would have to be much greater than Power IN to balance the equation.

Not convinced, read on…

Storage Power Supplies (and how they relate to the laws of Physics)

Whilst on the subject of laws, here is another one:

"Technology sufficiently advanced is indistinguishable from magic" Arthur C Clark (1961). Also known as Clarks Third Law.

There are many examples of technology that on the surface appear to contravene the laws of the universe, however most are based on technology, not magic.

In the case of audio amplifiers that appear to generate more output power than input power the 'magic' is in the power supply which draws energy from the mains supply and stores it for use by the output electrics driving the loudspeaker(s). The mains supply (input) can be considered a constant source and thus, in terms of our amplifier energy conversion equation above the input time (Time IN) is very long, approaching infinity. Conversely, the output signal being music or speech is usually very bursty in nature, consisting of many very short duration peaks separated by long periods of much lower amplitude. Thus, the amplifier is able to produce short duration high amplitude bursts of output power, while drawing much less (but longer duration) power from the electrical mains supply.

The storage capacity of the power supply directly determines the amplifier's ability to deliver large bursts of power required to adequately reproduce the dynamics and transience of music replay and is the reason some amplifiers have more 'punch' than others.

Photographic electronic flash guns operate using a similar principle. These devices produce a very intense flash of light many times greater than a domestic light bulb, yet the flash gun operates on a handful of AA batteries. How could the flash possibly produce such high output (power) from such a low capacity power supply? Like audio amplifiers, the answer is all about time. The flash gun produces a high power very short duration pulse, typically 1/1000th of a second duration. The input power supply consists of 4-6 AA alkaline batteries. However these batteries drive a power supply that takes around 30 seconds to charge up a storage capacitor. We can do some rough calculations to qualify the process:

Alkaline batteries have a terminal voltage of 1.5V and are capable of producing around 0.5A. Six cells connected in series would produce a total of 6 x 1.5V = 9V. Electrical power is calculated by multiplying supply volts by load current. Thus the input power produced by this supply is 9V x 0.5A = 4.5 watts.

Assuming the supply takes 30 seconds to charge the storage capacitor, the total stored energy would be 4.5 watts x 30 seconds = 135 Joules.

Assuming 100% efficiency, if the flash tube then discharges this energy in 1/1000th of a second, the effective output power of the flash gun can be calculated as 135 Joules / 0.001 seconds = 135,000 watts. Let's look at those numbers again;

Input power = 4.5 watts, Output power = 135,000 watts.

In other words, the output power (during the flash) is 30,000 times greater than the input power from the power supply (batteries). And it's all done without breaking, bending or injuring any laws of physics. So next time someone tries to tell you it's not possible for an amplifier (or any other device) to produce more output power than input power, you may wish to suggest that they revisit their high school physics text books.

Amplifier Measurement Methods & Standards (and how they relate to the real world)

The last piece of the puzzle is the methods used to measure and rate audio amplifiers. For example, a typical amplifier specification may read something like this:

Output Power, both channels Driven, 4 ohm load = 4,000 watts / channel = 8,000 watts total.

Mains current draw @ 230V = 8A

Electrical input power can be calculated by 230V x 8A = 1840W.

It's easy to assume from this that 1840 watts IN produces 8,000 watts OUT. However, this is not the case as these two ratings are measured using very different methods.

The OUTPUT power specification is an indication of the maximum undistorted signal the amplifier can deliver into stated load impedance. Unfortunately, there is no common standard used by all manufacturers so always pay close attention to the notes section of the data sheet for details of the testing method employed. For example, the figures above were obtained by driving the amplifier with a sine wave signal at various frequencies and increasing the output amplitude until distortion reached 0.35% THD. Different manufacturers may choose different distortion thresholds. The higher the distortion threshold the higher the resultant output power specification. Some manufacturers even use short duration sine wave bursts rather than 'continuous' ratings, but that is a topic for another article. Suffice to say the output power rating is a good indication of the maximum output the amplifier can safely deliver. However, it is not based on typical program material i.e. music or speech.

On the other hand, the mains power consumption specifications are based on a common standard as these specifications are used to determine the conductor size of the attached power cable and associated plugs, sockets, circuit breakers etc. All of these components are covered by electrical safety codes around the world.
IEC 60065 defines the standard test to be used to determine the current (and thus power) drawn from the electrical mains supply. It states that the unit under test should be measured operating under 'normal operating conditions'. Obviously, this is hard to define as it depends on the program material being amplified at the time so the standard defines a test signal to be used to simulate 'normal program material'. IEC 60065 states that 'the apparatus is operated in such a way as to deliver 1/8th of the non-clipped output power to the rated load using a filtered pink noise test signal with bandwidth 22.4Hz – 22.4kHz .

Two completely different specifications obtained using completely different testing methods. Do not assume they directly relate to the same conditions, i.e. in the case of the amplifier above, it does NOT draw 8A from the mains while delivering 8,000 watts of sine wave power.

The Summary

The Laws of Physics, though often misquoted do apply to audio amplifiers.

Audio amplifiers are energy converters.

The heart of any amplifier is its power supply.

Storage power supplies allow amplifiers to deliver greater output power (over shorter duration) than input power.
Amplifier output power and mains consumption (input power) specifications are based on completely different measurement methods. Don't assume they directly relate to the same conditions.

So there you have it. Hopefully this article has shed some light on a very controversial topic. Should you have any questions, comments about this article or suggestions for future articles please feel free to email support@jands.com.au.

Jeff MacKenzie
Manager, Technical Resource Group – Jands