Class A amps

There are two types of solid state class-A
Complimentary push/pull solid state amps that can be class-a/b or class-a if biased to the max so neither of the push pull transistor "ever" switch off and give crossover distortion. If they did you’d have crossover distortion
Or Class-A that uses a constant current source and the bottom instead of complimentary push pull arrangement, and never gives cross over distortion

When a class-a/b amp leaves it’s biased level of class-a and enters class-B then there is cross over distortion.
Only an amp that is biased hard all the way in class-a avoids this.

This "one" of the very best amps push pull solid state that was hard biased into pure class-a, it was the massive and maxed out doing only 25watt @ 8ohm, the Classic Mark Levinson ML2 monoblocks, but they could do almost 200w into 1ohm they were so good at giving current
https://img.usaudiomart.com/uploads/large/2229206-5c605986-mark-levinson-ml2.jpg

Nelson Pass on Class-A and crossover distortion:
In class AB, this effect is "alleviated" by a small bias current, and then is eliminated in class A where the bias current is high.
As he says crossover distortion it's alleviated in A/B with some bias but not eliminated, that only happens in Class-A amps



Cheers George

Just YouTube but you can get the idea. 
https://youtu.be/LYn8Fj7ItnY

Very good demo (2nd half ) even via youtube, this poster says it all

"Class AB sounds like a request but class A sounds like a command"

Cheers George

When a class-a/b amp leaves it’s biased level of class-a and enters class-B then there is cross over distortion.

This statement is false. Crossover distortion is a phenomena that occurs at the zero crossing point of the waveform. There is distortion that happens when the output devices go into cutoff, but that is harmonic distortion, not crossover distortion.

There are two types of solid state class-AComplimentary push/pull solid state amps that can be class-a/b or class-a if biased to the max so neither of the push pull transistor "ever" switch off and give crossover distortion. If they did you’d have crossover distortion
Or Class-A that uses a constant current source and the bottom instead of complimentary push pull arrangement, and never gives cross over distortion

This statement is rubbish, on account of its word salad quality, rendering it without meaning. In particular the second paragraph with the words ’and the bottom’. Constant Current Sources are used in many solid state amplifiers (usually at the input where a differential amplifier is used). Sense cannot be made of this paragraph.

There are two kinds of solid state amps: push-pull and single-ended. Nelson Pass has made a number of single-ended solid state amps but for the most part such amps are quite rare. 99 44/100% of all class A amps are push-pull.

Complimentary push/pull solid state amps that can be class-a/b or class-a if biased to the max

If there is any AB operation at all, the amp is an AB amp and not class A. Class A is a pretty strict definition when transistors are involved: the output devices never go into cutoff at any time right up to full power. When AB is happening, one of the two output devices does go into cutoff. If this occurs at the last watt before the amp clips, its still a class AB amp!

Crossover distortion only occurs at the point where the signal is going from positive to negative or vice versa. Even if the amp is being driven to full power, right around the crossing point the power is nearly 0 Watts. As long as the output devices are biased so that they are conducting around the zero crossing, crossover distortion will not occur. It is *eliminated*.


I’m always open to learning new things. If anyone can find an example of where a class AB amp is making crossover distortion, I’d like to know about it! The thing about crossover distortion is that its really audible. Back when I was putting myself through engineering school, I worked at an Allied Radio Shack (and later simply Radio Shack) service department that serviced a 5 state region. I worked on literally 1000s of amps and receivers. I noticed that certain receivers didn’t sound good at low volume levels (they sounded a bit ’hoarse’ for lack of a better word, sort of like when you have a voice coil rubbing in a loudspeaker). So I tested them on the bench and found that from the factory the output section was under biased. So I would set them up according to the specs in the service manual. I found that the point where the crossover distortion was eliminated **on average** was the bias spec in the service manual, but also that some units needed more bias and some didn’t need as much due to variance in the devices. After being set up properly they sounded fine and with no crossover distortion whatsoever- the correct bias **eliminates** crossover distortion. Nelson Pass or no, lacking crossover distortion is not an advantage of class A operation. Lacking some higher ordered harmonic distortion **is** (and higher ordered harmonics are easily heard as brightness, so this is important).


Again, if anyone can provide a bench test example that disproves what I’m saying here I’d love to see it!

This statement is false.
This statement is rubbish

No it’s the opposite


Take it up with Nelson Pass. The Messiah of solid state class-A amplification (not you)
And why his top of the line amps are low power Class-A!! and always have been.

As he says, "crossover distortion it’s "alleviated" in A/B with some bias but NOT ELIMINATED!!, that only happens in Class-A amps"

Nelson Pass on Class-A and crossover distortion:

In class AB, this effect is "alleviated" by a small bias current, and then is eliminated in class A where the bias current is high.
As he says crossover distortion it’s alleviated in A/B with some bias but not eliminated, that only happens in Class-A amps