Headroom has nothing to do with low high is the voltage supply rail?

Well maybe yes.


It has to do with the overload characteristic of the device.


Can a passive device do this? Perhaps a signal transformer?


Or an electron tube diode connecting an input to an output transformer?


Like a digital plugin but analog. It does not amplify the signal but it makes it louder as nasty peaks are trimmed so that the overall signal can be increased without clipping on the power amplifier at reproduction or the A to D at the post production stage of digitization.


On active devices it is the absolute clipping point that is related to the voltage supply rail magnitude. The signal peaks cannot get higher than this.


But it seems headroom is all the range between when the signal starts gently be clipped to where it is fully clipped. Between when signal starts to be distorted until it is clipped.


Electron tube amplifiers do that very gently giving a headroom of many dBs. Not only this but the subtle at the beginning distortion and progressively higher thereafter makes those dBs sound loud subjectively while the size of the signal is limited objectively. They mimic nature itself at high input signals, trumpets, even how our ears behaviour.


The signal is made to sound loud and it is being trimmed or peak limited at the same time.


If an amplifier starts overloading very softly and keeps progressively increasing harmonic production it has a big headroom and will sound loud for at least 2 reasons.


It's peaks are limited so the average signal can be increased.


The increase in harmonics make the brain perceive a loud sound.


3rd reason, there may be compression, ie automatic gain control as on the VF14 prepreamp inside the Neumann U47. As the signal is greatly increased the rectification effect of the electron tube increases the negative bias on the grid. Like a side chain signal of an electron tube compressor.


Like our ears when those tiny bones after our ear tympani change position or mechanical transmission "turn" ratio. The pivot point is changed?


The simplest on the planet class A stages do that gentle overload very well. We can arrange asymmetrical clipping at the input or output according to bias or anode resistor used for example. Asymmetrical distortion is when one side of the waveform is first softly rounded. It is second harmonic distortion which sounds very nice and full as it is really very close to an octave to our brain.


Pleiades V series prepreamps and Pleiades Electra headphone power amplifiers exhibit this effect. Is it possible to do everything in just one stage of amplification?


Reference:


Tubes vs Transistors, is there an audible difference? - Russel O. Hamm - JAES