Why are most High End Amps class A

Kirkus, I'm having a very hard understanding why an all-NPN output stage is very non-linear at the x-over point. Please explain. Thanks.

In a typical all-NPN output stage, the drivers are complementary (NPN/PNP), but the outputs are not (NPN/NPN). In order to make the bias voltages work out, the transistors are connected in such a way so that for the positive pair, there are actually two local feedback loops - one around the driver, and one around the output transistor. But for the negative side, the driver and output are in a single local feedback loop together. This means that in the crossover region, there is an abrupt change in the static gain as the current is transferred from one half of the output stage to the other.

If you ever play around with adjusting the bias on such an output stage (I've actually modded many MC2105s to have adjustable bias), it can be easily seen that as the operating point is shifted, the crossover distortion never goes away, it simply changes shape a bit.

There doesn't seem to be anything primitive about this input stage except that it is (very) low gain since resistors are used as the load to input differential pair.

The following voltage amp is a transresistance amplifier (current in, voltage out) - meaning that the signal doesn't appear as a voltage on the collector of the diff-amp transistor that drives it . . . rather, it's a current. So in the diff-amp, it's the transconductance (set by the transistors' beta and their emitter resistors) that determines its open-loop gain.

And, how do "input stages with current-sources and current-mirrors, and voltage amplifiers with an active load" make the power amp more "linear"?

Using a current source to supply the diff-amp tail current effectively "makes the tail longer", and eliminates the variation in tail current with common-mode voltage. Also, the performance of any diff-amp is VERY dependent on the static balance of current between the two transistors (or tubes) . . . and with resistors setting the current, this critical balance is affected by tolerances in the (carbon!) resistors, and variations in beta and Vbe in the (old!) input transistors themselves, and also the line voltage when using an unregulated supply (as is almost always the case). Using an active current mirror, instead of the resistors, forces the static currents to remain balanced regardless of these variations.

As far as the active load on the voltage amp - the improvement is the same as for any traditional single-ended collector/plate-loaded voltage amp -- that is, by keeping the quescient current constant across the required voltage swing, the amplifier is more linear. And here is one place where increasing the impedance on the collector does indeed increase open-loop gain, which also reduces distortion.

All of this makes the amplifier much more linear in the traditional sense -- its transfer function. And if you ever compare a MC2105 to a MC2255 (an old amp, but which has these refinements) on the test bench - they are worlds apart in the amount of distortion they produce . . . especially in the higher harmonics.

Thanx for the explanations, Kirkus.

So Kirkus,

What AMPS do you like and why if it does not matter if they are class A or class AB? Do you judge an amp mostly subjectively by sound or do you look for a particular construction?

It seems like you are a real expert in this area.

This may be a more general answer than you're looking for, but I like audio components that both sound good and measure well . . . and do so consistently, under a wide variety of conditions, for many years, without requiring much maintainance or repair. I generally feel that measurements are a good indication as to whether the engineering and execution are competent, and good sound is the indication that the basic design (and measurement methodology) is also competent.

To that end (to consistently sound good and measure well), I think there are some things that the best audio products have in common:
1. They are very well suited to the application
2. They make effective use of current technology and financial resources
3. They effectively solve the problems inherent in their basic design choices
4. They have quality and consistency in their execution

There are many topological choices that can fit this criteria well, both tube and transistor . . . just like there are so many different types of amplifiers that both sound good and measure well.

As far as Class A vs. Class B vs. Class AB . . . it's not that I don't think it doesn't matter, it's just that its very difficult to get a true class A design to meet criteria #1 and #2 above. For most solid-state Class AB or Class B amps, the main difficulty comes in meeting #3. For most Class AB or Class B tube amps, the difficulty comes on criteria #3 and #4.

I personally feel that these days, bipolar solid-state amplifiers show the most promise to meet all the criteria similarly well, but regardless of the basic choice of topology, it's a difficult road to get there. If I was designing a solid-state bipolar amplifier to drive a typical high-end domestic passive loudspeaker, I would choose a Class B design. If I was designing a tweeter amplifier for a high-end active loudspeaker, I would choose a class A design. If I was designing a tube amp, I would choose a Class AB design.

Kirkus, you are assuming rational thinking, whereas audiophile is a hobby. A hobby involves high-drive passionate pursuit for the best quality, such that other considerations seem far remote. For audiophile, that means getting the best sound for whatever money one can afford, or dare I say shouldn’t afford. As long as the equipment doesn’t break down often, nobody cares about anything else, except for a divorce if you spend too much money or make too much intrusion on the decorative well-being of your spouse.

My purpose of getting on Audiogon lately is to encourage people to check out Class A amplifiers, the real Class A stuff, if their pocket book can afford its price. Perhaps those in the borderline price range should wait for a while until they can own this good amplifier design. The sound gap between Class A and other classes is so wide that I cannot picture myself returning to Class AB. I am talking about how to retain the strengths of SS amplifiers and gain the values of tube amplifiers without their overkill. Music is a very potent medicine that can heal you in many ways, ways that the medical world could not provide. Yet for years I have such a tough time getting that quality sound at home, as opposed to a live concert with, somehow, less peripheral noises and proper sound projection. I am so much at awe with what true Class A amplifiers can do to my life that I just want people to check them out.