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RE: Thank you, exactly.

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In Reply to: RE: Thank you, exactly. posted by 13th Duke of Wymbourne on February 11, 2025 at 13:23:03

I found that amp review on Stereophile (see above). I don't take too much stock in what the reviewer liked and didn't like. You can always find someone to like your product. Having had amps early in my audiophile career that were both Class A SS and what I would call close to Class B (meaning later measurements revealed zero crossing distortion), I can tell you the one with zero crossing distortion sounded nasty and congested and was not very long in my system.

The Naim measurement I found also has zero crossing distortion and I don't know about you, but I find all Naim amps to sound dry (as in no harmonics and poor low level resolution needed for ambience retrieval) and flat. Horrible amps that traded on the selling point of having PRAT...because they had nothing else that sounded like music.

To the best of my understanding, you cannot remove distortion caused by zero crossing with negative feedback. I would need to read the reference I learned that from again to know the exact reasons why. Might have something to do with the fact that it is not an inherent transistor property but related to the stitching together two transistors to make a whole waveform and the glitch created by the imperfect handoff.

Follow Ups:

RE: Thank you, exactly. - 13th Duke of Wymbourne 12:24:48 02/12/25 (4)
Distortion is created if the transfer function is anything other than a straight line (with slope = gain). For a push-pull output stage, if there is a little wiggle in the line around the zero crossing that creates cross-over distortion. The larger the deviation, the higher the distortion and sharper wiggles create higher frequency harmonics. Feedback can only suppress any open loop distortion by the amount of loop gain at the frequency of interest. So, higher frequency harmonics are suppressed less because amplifier loop gain is always rolling off as frequency increases, and this applies to all distortions not just cross-over.
One danger, I think, is that biasing too hard into class B could create a 'dead-zone' at the cross over point where there is a small region with no transconductance and that will give low, or no, loop gain in that region and, hence, no suppression. It would be interesting to study but I don't get much spare time and I'd rather listen!
Conversely, class AB output stage should have twice the slope in the region where both devices are conducting (class A region) and a signal transitioning between A & B regions will be distorted by that slope change and there will also be a halving of loop gain in the class B region. So open loop distortion is created by that transition and the amount of suppression is modated. This is the so-called 'gm doubling' effect that is in the literature. Most recently it was tackled by Jason Stoddard in his blog, he came up with something he calls a 'continuity bias'to get around that problem. But, IIRC, that biasing burns significant power that is still lower than staying class A all the time but much more than a traditional AB.
- If you have a "dead zone" - Steve O 14:50:08 02/12/25 (3)
  Where both finals are in cutoff at idl, it's class "C" not "B" . . . Not good for audio. Almost all audio amps claiming "B" operation are really "AB" with idle bias current set low but high enough to prevent both finals from being cutoff simultaneously under real-world conditions of manufacturing and operation. It could be argued that true class B exists only at a single operating point where "AB" and "C" have an infinite number of points.
  - How do you classify taxonomists? - 13th Duke of Wymbourne 11:18:59 02/13/25 (1)
    :)
    - Hadn't considered that until now - Steve O 15:41:18 02/13/25 (0)
      Interesting how this amplifier class discussion arises every few years and involves some of the same persons each time. The basic amp classes were defined nearly a century ago . . .
  - RE: If you have a "dead zone" - morricab 06:37:17 02/13/25 (0)
    However, that means the performance around zero crossing is very non-linear and will create a lot of distortion that is of a very obnoxious nature.
    
    Look at the stereophile measurements I shared above. You can see the sharp discontinuity they have around the zero crossing. As one can imagine, this is essentially only high order dissonant distortion components. Even very low levels of these harmonics will audibly damage the sound.
    
    As Tre was pointing out, even AB is not putting the bias in a very linear part of the transistors transfer function, which will also result in increased higher order harmonics.

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