Are all amps being built wrong?

When I was doing some research, I came across a comment in one of the Coda related Audiogon threads that states Coda makes the Sanders Magtech amp; but, Sanders than tweaks it for use on their ESL speakers.

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 mostly incorrect.


The voltage rules haven't been around forever; MacIntosh and EV were proponents of this in the late 1950s but it took until the 1970s before the industry had really switched over from the older power rules. The idea was plug and play with no adjustments on the loudspeaker. You can read more about it at this link:
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
Any time you see adjustments on the back of the speaker like a tweeter level control, the speaker is designed around the older power rules- the control(s) allow the speaker to be adjusted to the voltage response of the amplifier and are not there to adjust to the room.

Of course a current source amp would be even more sensitive to impedance variation than even a power source amp! But there are speakers that worked best this way; most of them are open baffle with very tight suspensions on the drivers.


The problem for many years with the voltage rules has been the to get the amp to be a voltage sources (especially tube amps) you need feedback, and the simple fact has been that most amps made in the last 70 years that have employed it simply do not have enough. As a result the feedback that they do have has caused all of them tube or solid state to be brighter than they should be, since the feedback wasn't enough to allow the amp to clean up the distortion added by the feedback itself.


Since the ear perceives distortion as tonality, and since the distortion added by feedback (while suppressing the innate distortion of the amp otherwise) is entirely higher ordered harmonics, the ear perceives that as brighter and harsher even though the amp might be flatter frequency response.


That is why there are still power source amps around today (SETs being an example). Tube amps that don't use feedback (and so are not as harsh and bright) will act like a power source.


The trick with feedback of course is to have enough, which is usually over about 35dB or else don't use any at all. Its not really been possible to put that kind of feedback in an amplifier until recently. The typical brightness of solid state (which is why tubes are still being made) is caused by insufficient feedback.

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.