Why would anyone want class AB amp when class A always sounds better ?

Cost ? Heat? Reliability?
Hello. Interesting debate. I’m in the market for integrated amp to pair with my Goldenear Triton 2+ speakers. In the next couple of weeks I want to demo Pass Labs Int-25, McIntosh Ma-352, Luxman 509/507 590/550 depends what dealer will have in stock and maybe even Arcam Sa30(first 20w in class A)
I don’t care about measurements or numbers,all I want to hear is beautiful music. I rarely go over 85db in my medium size room so I think any of those can do that. A,A/B,G.One of them will win.

@kren0006 wrote:

"As a direct example, some feel that the Luxman 509X class AB at $9.5k sounds better than the Luxman 590axii class A at $9k. Can’t get much more absolute-busting than that. Same company, both integrateds with same features, essentially same price, both current products."

I thought that too (and I own the L-509X), but did you know the L-509X is both A and A/B?  It delivers the first 6 watts as "pure" Class A then transitions to A/B for the rest of its rated output.

I confirmed this with Luxman USA during a discussion of the L-550 (which is also a hybrid A, A/B, not just A)

Quote from Luxman USA email
...The Luxman L-509x is an excellent amplifier. It is considered “top of the line” and most recently introduced Luxman integrated amplifier model. It features circuit and construction details/refinements not present in (earlier introduced) the L-550aXII and L-590aXII.

L-509x is biased such that the class-A operational envelop approaches 6 watts/channel into an 8 ohms resistive load. At lower (or higher impedances) class-A operation transitions to class AB at lower output. (i.e. near 3 watts/4 ohms resistive load)
Just tossing that into the pot for consideration.
Try sitting in a small room with even a low power class A amp here in Florida in the middle of summer.  Happy with class D.  
From Nelson Pass...


Class A operation is integral to the performance in this case, and it is worthwhile to explore why. The primary virtue of class A lies in the smooth characteristics of its operating parameters. The gain transistors are operated in their linear region only, where the distortions are limited to smooth, simple forms, unlike the abrupt distortions created when the transistors in class B output stages switch on and off."


I've built a couple of his DIY Class A projects. They are exceptionally good sounding amps. Why not class A? Cost (especially cost/watt ratio), Heat and nothing else unless you're a greenie. In that case, power consumption.

There is a REAL difference in the functionality of a class A vs anything that has to "turn off" one transistor and "turn on" another one at every wave form. Just considering that it is between difficult to impossible to match those components and operate them outside of their linear reigons. Then you get into negative feedback loops to correct the crossover wave distortion, adding additional complications and additional distortion opportunities. My best example is I've owned a high-end Oddyssey amp that had a billion components in it and costs a ton. My DIY F5 Pass/First Watt amp absolutely blows it out of the water, sound-wise. There are compromises, but I sincerely believe the difference between the functionality of Class A vs class B circuit is obvious and significant.  

Broadly speaking, there are often a lot of other design variables between two SS amps beyond just the bias level of the transistors.  A lot of the high end Class A amps are designed to have very little feedback, which enhances the high linear accuracy of Class A, but often at the expense of slightly higher noise and lower damping factors.  That tends to create a certain sound which I personally feel is a bit closer to the typical tube sound.  On the flip side, it is more common for the Class AB amp designers to use some amount of local and global feedback, which "can" marginally impact the linearity, while reducing noise and increasing the damping factors and tightening the bass.  In my experience, it is very hard to generalize that Class A sounds better than Class AB since there are a bunch of other design factors that have to be considered.  The typical Class A amp may sound different than the typical Class AB amp, but it's not always better...