Average amplifier efficiency

For example total energy delivered to speaker in a month to total energy delivered to amplifier in the same time period. Not much unlike a utility bill.


And of course not forgetting the absolute numbers too or money paid for energy. For example a 1 W amplifier in class A (electrons flowing all the time) may sound louder than 1000W of class D (electrons flowing in stop start stop start (digital) mode). How? Class D may sound so inhuman one may not even realize that there is music being played at that particular moment.


Energy is the mathematical integral of power with time being the variable.


A mathematical integral is a sum of infinite terms infinitesimally small. This is why the result is not infinite but finite.


Efficiency is energy taken divided by energy given. Energy taken out is always less as the rest is converted to heat.


Class A amplifiers have a max theoretical efficiency of 50%. This is using a sinewave. A sinewave has 0 dynamic range. And a sinewave may not listen to music at night.


Also efficiency is an instanteneous quantity.


Much more important must be efficiency at real world conditions.


Music has dynamic range.


There is not much point in wasting continues constant 80 Watts in a class A amplifier when one at night listens and does not want to wake up neighbors. At such conditions power needed to loudspeaker may be as low as 100mW.


What matters is average efficiency.


In real terms, will the batteries when listening at different times of day last for 2 weeks or for 2 hours?


One can have an amplifier in class A (electrons flowing all the time), enjoy the surprisingly good sound quality, yet not consume much energy.


For example on the Pleiades BD139 one transistor amplifier, when lower loudness in desired, the volume control is going counterclockwise for loudness reduction but also the bias resistor can be manually changed from say 12KΩ to 17KΩ.


Then the quiescent current becomes just 70mA.


(In fact the amplifier still sounds loud as arrangements were done yesterday to impedance match speaker with transistor, see previous posts).


How do we know a single ended class A amplifier stays in class A even at low quiescent current?


It is very easy. The volume control is turned clockwise. It sounds great anyway. At some point it sounds even greater. Passing that point distortion may be heard. This is possibly going past a 5% second harmonic distortion point.


Of course this is max distortion. The amplifier has distortion tending to 0 near the cross over point, when the music signal passes through 0. And all the values of distortion in between the 2 extremes of 0 and 5%. In fact going towards the 5% or whatever point the sound is more natural to listener's brain. So it more hi fi. Hi Fi is how similar is what the producer has in mind to what the listener's brain perceive. And of course we know most producers at all times have regularly passed the signal through class A amplifiers from mic to even at the mastering stage to get the sound right to listener's brain. See also [Hamm].


There are ways of automatically adjusting bias to avenge signal level, for different times of day. For example on the amplifier book by Clive Sinclair (ZX Spectrum fame?).


References:
Tubes vs Transistors (vs op amps), Is there an audible difference? - Russel O. Hamm - JAES


Transistor Audio Amplifier Manual - Clive Sinclair - Bernard's Radio Manual - page 7 (see PS)


PS: an npn transistor drives the single output pnp transistor. The driver transistor has as collector load resistor the 1KΩ volume potentiometer itself. The wiper is connected directly to the base of the output stage transistor. The driver transistor is connected on the schematic upside down.