Class A Watts

Going class A is a method of reducing distortion. In a high end amplifier that is supposed to sound like music, this can really help in a lot of cases.

Class A is not the defining criteria though. There are all kinds of design considerations that affect the way an amplifier can sound.

If an amp is running some bias current so that the output transistors are always on but the bias current is (very) low then essentially the amp is biased into class-A. I think that they call these power amps 'sliding class-A power amps' meaning that they have some very low bias current during off-state & the bias increases proportional to the input music signal when it's present.

All transistor amps run some bias current on the output devices at idle. This is not sliding class A, this is AB. Sliding A is where the bias is dynamically changed as the input signal changes.

I was trying to keep the description more simplified, ie. the output tubes are coupled through the transformers so that the speakers see only the transformers.

The transformer transforms the speaker impedance to something that the tubes can handle. Conversely, the tubes are part of the source impedance that the speaker 'sees'.

An implication of this is that a class A tube power amplifier may well be operating in the AB region if the transformer has the wrong impedance on a particular tap, like a 4 ohm load on the 8 ohm tap.

BTW it is not correct to say

SS amps and 4 ohm speakers were made for each other.

The why of it is that all amps, tube or transistor (or class D) make more distortion into 4 ohms as opposed to 8 or 16 ohms. This distortion is audible as increased brightness and loss of detail. Put in a nutshell, if **Sound Quality** is your goal, it is best served by a speaker of 8 ohms or more. If **Sound Pressure** is your goal, there is a weak argument for 4 ohms if you have a transistor amp that can double its power (3db). Put another way, there really isn't an argument for 4 ohms in high end audio, but there might be in sound reinforcement.

Lowrider57, the way the two types of amplifier technologies distort is somewhat different.

With tube amps you might get a bit more of the lower orders (so you would get a richer, fatter sound, that could border on muddy, as those are all terms audiophiles use to describe excessive lower-ordered harmonic distortion), but this is highly dependent on the actual amp design and its output transformer (if it has one).

With SS amps, there seems to be less variation in the distortion types that creep in- it appears to be almost entirely odd orders (the source of brightness/harshness in many amps). It may not be much but because the human ear/brain system uses odd ordered harmonics to sort out how loud a sound is, it is very sensitive to these harmonics.

***IOW, it shows up in the specs and is also audible.***

As an additional note, 4 ohm speakers are very critical of speaker cables so if you want the best results with them the cable can't be skimped. Conversely with 16 ohms the cable is hardly critical at all- so you can run longer lengths of lighter gauge and totally get away with it.

The reason for this is the seemingly very slight DC resistance of the cable is in series with the output impedance of the amplifier, and can affect the damping factor by quite a lot! BTW this particular fact has been known for decades; I first saw it in a nomograph published by RCA in the early 1960s.

Four ohms came in as a means of allowing speaker manufacturers to seem to not loose so much efficiency while at the same time using cheaper drivers that have less precision in the voice coil gaps and the like (this makes the speaker easier/cheaper to build, as much as 10X cheaper, so its a pretty powerful incentive). You get 3 db of Sensitivity back if you have a transistor amp that can double power. The trick is understanding the Sensitivity and Efficiency are not the same thing!!

This is why I say that 4 ohms offers a *sound pressure* advantage, not a sonic advantage.

You can talk to any amplifier manufacturer and they will tell you the same thing- that fact that their amp can double power into 4 ohms is not the same thing as saying that the amp is sounding its best into that impedance.