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
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