Who makes

Bifwynee/Bruce, good reply. Atleast I appreciate that you have taken the time to get a deeper understanding of the amp-speaker electrical interface. It's a complicated affair there - more complicated than meets the eye once one delves into the details. I'm sure that it's made you a better audio consumer & you can better make the trade-offs when you go out next to purchase audio gear. Like the SYMS clothing company once said in their TV ads "an educated consumer is a better consumer".

To Cdc:
before answering your question, let's take a step back & find out what is really meant by a "voltage paradigm". In a voltage paradigm amplifier, the output power doubles each time the speaker impedance halves (let us assume for the sake of simplicity that the power amplifier output has infinite current source/sink capacity. Such an amplifier is not available practically but it makes the discussion easier). The output power doubles because the output current capacity doubles; not because the output voltage capacity doubles.
For example:
300W into 8 Ohms. P=V^/R implies V = 49V approx.
600W into 4 Ohms. P=V^2/R implies V = 49V again!
1200W into 2 Ohms. P=V^2/R implies V = 49V once again!
So, as you can see, as impedance halves, output power doubles, the output voltage remains the same.
OTOH,
300W into 8 Ohms. P=I^2*R implies I = 6.12Amps
600W into 4 Ohms. P=I^2*R implies I = 12.24Amps. Output current doubled...
1200W into 2 Ohms. P=I^2*R implies I=24.49Amps. Output current doubled yet again!
So, output power is doubling as impedance is halving because output current is doubling.

Now that you understand this, a tube amp cannot double its output current as speaker impedance halves because tubes are output voltage devices & have large output impedance. The output transformer makes things much better to drive a speaker load but the ratio of amp output impedance to speaker impedance is quite high making it hard for the tube amp to output large amounts of current into a low speaker impedance.

Solid-state amps have output impedances that are sub-1Ohm (because the output stage of a s.s. amp is almost always the BJT emitter or a MOSFET source of a JFET source, which has very low output impedance & several semiconductor devices are paralleled to make the net output impedance even lower). So, even if the speaker impedance drops 8 --> 4 --> 2 & even 1 Ohms, the s.s. amp output impedance is still an order of magnitude (ie. 10X) lower than the speaker impedance. By the physics of this, a s.s. amp is able to output progressively more output current into a lower speaker impedance *while maintaining its output voltage* (as shown above).

A "power paradigm" amplifier cannot maintain its output voltage constant as speaker impedance varies over the audio band. This is the key difference between the voltage paradigm & power paradigm.

One would have to make a very high output impedance s.s. amp such that this s.s. amp cannot deliver large output current into a lower speaker impedance. As Bruce has pointed out "your questions are counter-intuitive and contradictory" because the very nature of s.s. amplifier design ensures very low output impedance: paralleling output devices decreases output impedance, negative feedback (local or global) reduces output impedance, increase in temperature reduces output impedance, etc. You have to go the extra mile to make the output impedance high in a s.s amp. IOW, s.s. are naturally voltage paradigm amplifiers.

Now I am stating & asking the forum at large: could McIntosh's philosophy of using output auto-formers be one technique to increase the output impedance of a s.s. amp (I believe that McIntosh makes several s.s. amps w/ autoformers, no?) such that these s.s. amps show some/a lot of the characteristics of tube amplifiers?

Another devislish way to make a s.s. amplifier behave in a power paradigm manner would be to use it with a very low impedance speaker such as using a s.s. amp to drive an Apogee Full Range speaker where the midrange is driven directly by the amp & has a speaker impedance of 0.14 Ohms (I did not make a mistake here!!). Now, the s.s. amp will see a speaker load that is in the same ball-park as its own output impedance & current will be limited (just like a tube amplifier seeing a 4-Ohm speaker load when its own output impedance is in the 4 Ohm region). Such an amplifier's life could be very short-lived if not chosen correctly. Devilish, as I wrote! ;-) Also, how many such speakers exist today? Almost zero. so, this scenario is not realistic.

I don't know of any other s.s. manuf that makes their s.s. amps power paradigm but memory & experience could be failing me.....
It would be great to hear from other far more knowledgeable members. Thanks.

I've been around the industry and an avid audiophile since 71 or so. It's really as simple as listening. I've heard a lot of speakers with tubes that were voiced' with SS. If you speak with a lot of the speaker designers, they'll tell you that they don't even 'voice' their speakers. I know one who told me a couple of weeks ago that the first time he hears his designs can be after they are in production. I know how that sounds, but his speakers are considered some of the best ever made and are one of the biggest names ever in high end audio.

I've heard this guys amps with SS as well as tubes. They sound great with both. Personally I'm a tube guy, but I just got an Ayre and it's the best SS I've heard to date.

I think too many of us get caught up in tech specs etc.. Heck, I have MIT MH 770 CVT speaker cables I"m about to sell off. They were made for tube gear and it's with their highest speaker cables. I used them with Quicksilver pre/silver mono's (selling them off ;) ). They sounded awesome compared to every cable I tried with the amps. Now I'm running an Ayre integrated as I am moving over to a remote control system. The cables sound awesome. I'm changing them to AQ since the Vandy's I"m going to eventually purchase were made with the AQ cables in mind. That said, I have listened to the various cables I'm looking at. Trust in the person who's shop you buy at as they are with these products daily and listen to systems not just components.

Not taking anything away from the question I hope as I find it interesting to say the least. I had this same conversation when I got my Proacs with Quicksilvers as I hated the Krell stuff with them (other than the Krell DAC I got), but I listened to them with both SS as well as AR and Quicksilvers it was tubes all day for me. That said, I know someone who got the same speakers with Levinson gear and the bass was unreal and made me want more. The amps offered different sounds and both would appeal to different listeners.

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