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Are all amps being built wrong?

The power amplifiers that drive our loudspeakers are mostly built as a low impedance voltage source. They have always been ... but why?

Loudspeakers have a (greatly) varying impedance over the frequency range. A current drive amplifier would eliminate the issues that stem from this varying impedance, and at the same time make discussions about esoteric speaker cables that strive for optimal R, C, L superfluous. Although there still would be these un-measurable ’this (very expensive) cable sounds better’ debates and opinions ... and that’s OK, that’s part of the fun. :)

So ... why are amplifiers not built as a high impedance current source?

This is an interesting read: https://www.current-drive.info/
rudyb
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erik_squires
15,830 posts
 
They have always been ... but why?


Because, for the most part, most speaker drivers are linear to voltage, not current.

Nelson Pass has of course at least one if not more famous papers on how this may not be the right approach with some large drivers. 

If you want to see what a high impedance amplifier, one that is closer to a current source is, then look no further than the Stereophile tube amplifier measurements with a simulated speaker load.  As the amplifier's output impedance rises the frequency response of the amplifier tracks with the impedance curve of the speaker system.

On the VERY opposite end of the spectrum we now even have the top end Technics digital amplifiers who deliberately compensate for the impedance curves and give a near perfect voltage amplifier transfer function.
ditusa's avatar
ditusa
1,736 posts
 
@wyoboy +1
rudyb OP
141 posts
 
@erik_squires "Because, for the most part, most speaker drivers are linear to voltage, not current."

Unless I misunderstood what you tried to say ... I thought linear with current.

The loudspeaker uses a moving cone to create air vibrations. The cone is put in motion via a coil (wire length L) inside a magnetic field (strength B). The force (F) that moves the cone is created by the current (I) that flows through the coil, with the linear equation F=BIL. From this it seems only logical to drive the coil with a current source, making the current independent on the loudspeaker impedance? Apparently there are other parameters and phenomenon in play that made/make amp designers choose for voltage output, which does make the current that flows dependent on speaker and cable impedance.
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