Who makes

Double tap. ;-)

Excellent responses by Bruce and Bombaywalla.

Bob R. adds a good point also. During the 1980s and 1990s Bob Carver designed some Carver Corporation and later Sunfire Corporation solid state amplifiers that were intended to emulate tube amplifier characteristics to some approximation. That was accomplished in part by putting a resistor in series with the output, as Bob mentioned.

One of Carver's earlier such attempts, during the 1980s, involved configuring an amplifier such that its "transfer function" (the relation between its output and its input) approximately matched that of a well regarded high end tube amp, at least when driving a specific speaker load. As demonstrated by electronically "subtracting" the output of one amp from the output of the other, with both amps receiving identical inputs, and measuring and listening to the residue. That ended up provoking a good deal of controversy, of course.

I can't recall any other solid state amps that can be said to approximate power paradigm characteristics. I recall reading that Nelson Pass, and perhaps some others, have created some designs that essentially act as current sources, having VERY high output impedances (e.g., 25 ohms). But those would not maintain an approximation of constant power delivery as a function of load impedance either, because their output voltage would vary dramatically as load impedance varies. (Power = voltage times current, oversimplifying slightly).

Regarding the McIntosh solid state amps that use autoformers at their outputs, those have very solid state-like output impedances (e.g., 0.2 ohms or less). The autoformers appear to be used to make life easier on the output devices when faced with low impedance loads, by making those loads appear to the output devices as higher impedances. Thereby reducing distortion, and probably also increasing power capability into low impedances relative to what it would otherwise be.

Best regards,
-- Al

Bifwynne, thanks. I was just going by what I was reading atmasphere.
I think Al hit on one company, Pass Labs, that made the solid state amp act like a current driven amp. And The Carver amp that Bob mentioned looks interesting too.

If it can't be driven with tubes, I am not interested.

And since going with high efficiency speakers, I might also add that if it can't be driven with low power tubes, I'm also out.

Shakey

Ctsooner, I'm not really looking to start another lengthy discussion about tube vs SS amps. I'll leave it this way. It is plausible that a speaker designer, a dealer sales rep and/or a customer could be indifferent to plugging a tube amp or a SS amp into a speaker if either (x) the speaker has extremely benign impedance and phase angle characteristics, or (y) the tube amp has very low output impedance.

As Al and Ralph Karsten (Atmasphere) have said on numerous occasions, a speaker's ultimate acoustic presentation will be affected to the extent an amp has a high'ish output impedance and the speaker was voiced (intentionally or not) to be driven by an amp that has a low output impedance. This is not my opinion -- it Ohm's Law.

It is my experience that when I drive my speakers off the 8 ohm taps of my amp, the speakers sound bright and detailed, maybe too much so at times. In contrast, when driving the speakers off the 4 ohm taps, bass is more extended and tighter, imaging less forward. Admittedly, I am conflicted about which acoustic presentation sounds better to my ears. It just depends.

This is no coincidence. The output impedance of my amp off the 4 ohm taps is about .55 ohms in the lower frequency bands; and about 1.1 ohms off the 8 ohms taps. It is similarly notable that the impedance curve of my speakers is roughly 4 ohms between 70 Hz and 700 Hz, rising sharply and peaking at 21-22 ohms at the 2.2K Hz crossover point, dropping again to about 5 or 6 ohms as frequency increases.

Let's put a sonic face (sic, an oxymoron) on what I just said. John Atkinson's bench test report of my amp states that output voltage regulation is +/- .4 db off the 4 ohm tap and about +/- .8 db off the 8 ohm tap. What this means in plain English is that my speakers should be about 1.6 db louder at the midrange highpoint of about 2.2K Hz as compared to the bass saddle of 70 to 700 Hz -- IF I use the 8 ohm taps. In contrast, the SPL delta off the 4 ohm taps should be about .8 ohms. Those stats cogently explain what I hear.

Oh ... the damping factor of my amp is also affected. Theoretically, about 7 ohms off the 8 ohm taps and about 14 ohms off the 4 ohm taps. Although with my speakers, those stats should be halved because my speaker's impedance is about 4 ohms in the critical parts of the bass frequencies. But those stats explain why I perceive bass to be more extended and tighter off the 4 ohm taps.

This is Ohm's Law, not my opinion. So, unless a dealer's sales rep explains in plain English what I just said above, he's either ignorant or scamming the customer. Of course, he could be honest and say even though a tube amp may be causing certain acoustic colorations, they sound good, in his/her opinion. Then it's on the customer to decide.

Al or Ralph, if you can simplify or clarify what I just said, I'm sure I and the other readers would appreciate your edits.

Regards,

Bruce