Drive your headphones from a power amplifier? Surely not. A power amplifier is designed to drive loudspeakers, and if it can drive a speaker, surely it will blow a pair of headphones in an instant.
But no - here comes the science...
According to DIN specification 45500, a headphone ear piece should be able to handle 100 milliwatts of power. That is 0.1 watts.
If those headphones have an impedance of 600 ohms, then the equation power = V2/R gives us a voltage of 7.5 volts that is required to drive those headphones to 100 milliwatts.
Now take a power amplifier that is capable of supplying 15 watts into a 4 ohm loudspeaker. The voltage it needs to develop to do that is... 7.5 volts!
Since the voltage is exactly what the headphones require, and due to their higher impedance they won't draw any more power than they need, then the headphones are driven to their full capabilities of 100 milliwatts.
And modern headphones can take more power - 200 milliwatts is not unfeasible, which could be delivered by a 30 watt into 4 ohm power amplifier.
Now for the bonus... If the amplifier is capable of supplying 30 watts into a loudspeaker, then if a pair of headphones only takes 200 milliwatts, then the amp can drive as many as 150 sets of headphones simultaneously. One amplifier could power enough sets of headphones for a whole orchestra (and chorus!).
Actually, there is a little more to it than that. If a number of sockets were wired in parallel, then if one pair of headphones short circuited it would bring all the rest down too. It shouldn't damage the amp if it is properly designed. Connecting the headphones through 100 ohm resistors will cure this.
Also, unless all the headphones have the same impedance, then some will be louder than others.
One more thing, the headphones have to be of high impedance for this to work, around 600 ohms. You can check this in their specifications. Don't try it unless you understand.
P.S. You will have a whole lot more fun with multiple headphones if you connect them using XLR connectors rather than jacks. Twenty pairs of headphones wired in parallel and one partially connected jack? Rather you than me.
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