Are all amps being built wrong?

Well, I was talking in engineering and mathematical generalities.

What is ideal is very different than what we would like to hear.  If you ask me what the best amplifier I ever heard was it was probably the CJ Premiere 12.  If only I had the space and money for them. :)

Best,

Erik

I don't think we are saying anything that much different, @atmasphere

Current feedback would raise the apparent output impedance into the range I was discussing, just like voltage feedback lowers the apparent output impedance.

a Bob Carver made amps under the Sunfire brand that had speaker outputs selectable for voltage or current drive. I’m told the current drive was created by the insertion of a series resistor.

He did but it wasn’t absolute. He added a little resistance so it behaved more like a tube amp. This means it was LESS like an ideal voltage source and MORE like a current source, but even then there are limits to how well this can ever be done or how badly it would sound. Tube amps are still mostly voltage sources and deviate only due to the relatively small output impedances. I say relatively because a single multi-way speaker may vary from 3 to 30 Ohms. An amplifier with a 2 ohm output impedance won’t be near an ideal current source. For that you’d need say 300 to 3,000 ohms of output impedance.

A perfect current source amp would perfectly track the impedance curve of the speaker, which would sound awful.  I believe these models use the CJ Premiere 12 (maybe another model, the 8?) so we can use the Steophile measurements as a guide of what to expect.

https://www.stereophile.com/content/conrad-johnson-lp125m-monoblock-power-amplifier-measurements

On the top chart you'll see a frequency response called "Simulated Speaker Load." That squiggly line will show you how much this, more like a current amp, would behave.

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

Nope. Well, linear is probably the wrong term. Proportional is correct. The output of an ideal driver in dB would be directly proportional to the output in db Volts of an amplifier.

That is, if you increase the V by 3 dB, you should measure 3 dB more acoustically regardless of the final speaker impedance (i.e. drivers + crossover).

Of course most speakers are not ideal and compression artifacts occur but this is the goal so long as we are not overdriving the speaker.

This is also why equalizers work. :) If you reduce output by 6 dB at 40 Hz in your EQ then your speaker’s output will be reduced exactly that much. Because everything is proportional to voltage.

PS, db Volts is calculated this way:

db = 20 x log( V original / Voltage now)

So doubling the amplifier voltage = 6 dB louder in both voltage and sound pressure level (SPL) so long as everything is not over driven.

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.