Re: Thoughts about amplifiers at 3/4 volume.
I regret it's all a little more complex than that.
Amplifier sensitivity is the same thing as voltage gain, just stated a different way. It simply means that X volts at the amp input will result in Y volts at the amp output. Power in the loudspeaker is the result of a the voltage at the amplifier output, divided by loudspeaker impedance. i.e. 4 ohm speaker makes twice the power as 8 ohm speaker at same voltage.
The amplifier output voltage, or power, is only limited by two things. the amplifier power supply rail voltages which define the max voltage (power) the amp can possibly make. The other limit involves the amp's sensitivity (for amps like this that make sensitivity adjustable) or more typical amplifiers that use pad/attenuates, to vary the effective amplifier voltage gain or ratio from input to output voltage "and" the max voltage possible at the input.
This may be easier to follow if I use some specific (rounded off for my convenience) numbers... Suppose the amplifier PS allows it to make a maximum of 100V at the output. Working backwards with a sensitivity or voltage gain of 33x, means 3V at the input will drive the output to full power. Now if we change the sensitivity (gain) from 33x to say 16.5x, the amplifier can still make the same output power, but now it needs 6V at the input to reach full power. Since the typical line level devices driving the power can easily make 6V or more, the amp's max power is unchanged, while you will just be be driving it a little harder, perhaps noticeable on the console meters.
We can use adjustable amplifier gain to crudely limit power if we throttle back the gain so much, that the line level gear driving the amp clips or runs out of swing before it can drive the power amp to full output.
It is possible to put a limiter in front of the amp, and dial back this limiter threshold so it limits the drive voltage so it can only put out say 3V, then dial in the sensitivity of the amp so that agrees with your desired output power.
But it seems that it would be far simpler to just use one of the power limiters built into that power amp...
So long story short, sensitivity (gain) only limits output power if the input voltage is also limited or hard clipped such that it is incapable of driving the amp input any harder. Changing the amp gain while you still have extra drive capability in front of it will limit nothing.
I hope this helps
JR
