Are all amps being built wrong?

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.

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.

@erik_squires Most of this is not quite correct. If you add a resistor to the output of a solid state amp it will indeed simulate some sort of tube amp that uses feedback. Most transformer coupled tube amps with 15dB of feedback will act as a pretty good voltage source. You might have to play with the taps on the output transformer. Between Voltage source and Current source there is Power Source, which is how a tube amp will behave if it has no feedback or if it has voltage and current feedback of equal amounts.


Most tube amps do behave as voltage sources. You need current feedback to get the amp to behave as a current source. The only amps commercially available that ever did that had variable damping controls and were made in the 1950s. Fisher made a few as did EV. I'm not sure how many others. But the damping control really allowed you to set up the speaker incorrectly since it did cause the amp to misbehave it you had the control set incorrectly- which was usually towards the 'Current source' side of things.

If the amp has a 50 ohm output impedance it can act as a current source. Some speakers can sound amazing if driven by a current source- Nelson Pass did exactly that with some OB speakers he had a RMAF about 10-12 years ago. They played bass far better than you would have expected considering how small the OB speakers were! But for the most part the approach is impractical.

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.

My answer to the original posed question is "not at all, not even close"

I think Eric contributed some good stuff above.


As am amp designer, i think they ought to be voltage sources, with low output impedance.  Now, for the record mine are not zero, because i don't like to employ global control (feedback) loops nor expose those loops to external elements that create unknown stability issues.  But we are talking small fractions of an ohm.

I realize that a higher output impedance may result in a warmer 'bloom", but that's a coloration, whether you like it or not.


A current source would likely result in very distorted sound. Also note that the impedance of a speaker changes on many dimensions - with frequency, with excursion of the electromagnets, with volume, ....so why would we want to make the gain impedance dependent?
G

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