Class dismissed

June 4, 2021
 by Paul McGowan

When I was attending school there were no sweeter words than the headline of today’s post. Free at last.

Let me start today’s topic with an apology. Because I disliked school so much my math skills suck. Badly. I tend to get confused around percentages especially when they work in reverse.

Armed with the correct info, let’s review: if a 100-watt amplifier is 50% efficient, it draws 200 watts from the wall and delivers 100 watts to the load. Half of its energy is converted to heat. (I had mistakenly said 50 watts would be converted to heat). Thanks to our ever-helpful eagle-eyed community for correcting me.

According to Wikipedia, the classes of amplifiers are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period. A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse width modulation output is obtained from the stage.

What’s a valuable takeaway from the above droll few sentences is this: a Class B amplifier only draws power from the AC wall socket when a signal is present, where a Class A amplifier is drawing wall power “through all the period of the signal”, including the zero-crossing point where, technically, there is no signal.

Keep that thought in your head as tomorrow we’ll come back to that.

There is a hybrid amplifier we’re all familiar with. This topology shares traits from both Class A and Class B and is appropriately named, Class A/B.

In a Class A/B amp when there is no signal there are still a few watts of power being drawn from the wall. This is because the A part of the Class A/B means the amp is always on—at least a little. This always-on time is called bias, a technique of applying just enough always-on power that we’re not relying upon the application of an audio signal to get things started (eliminating a type of distortion known as crossover notch). The amount of that always-on bias varies from amp design to design. In some amps, like the BHK series, it’s fairly high, which generates a fair amount of constant heat regardless of whether or not a signal is present. In other designs, there’s just enough always-on bias to keep the amp warm to the touch.

The only time the heat sinks of a class A/B amp get good and toasty is when it’s been working out delivering loud music to hungry speakers. That’s the opposite of what happens with a Class A amplifier.

Tomorrow, the strange world of pure Class A.

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29 comments on “Class dismissed”

  1. Well we can’t salute ya
    Can’t find a flag
    If that don’t suite cha
    Then that’s a drag

    School’s out for summer
    School’s out for ever
    School’s been blown to pieces

    No more physics
    No more nerds
    No more chemistry
    Maths or words.

    (Apologies to Vince Furnier)

    1. I think today’s lesson is contemporaneous with Larwood and Bill Voce going after The Don with fast leg theory. We’ve a way to go yet.

        1. I think it was the press that called it Bodyline, it was a fast leg-side theory. It was our sandpaper moment. These days they’d just flick it over deep fine leg. Raining at Lords. Grrhh …

    2. I remember buying that LP when walking back home on the day I passed my driving test. I recall the plastic inner sleeve was over wrapped with a pair of ladies disposable knickers. Don’t know why, perhaps to help fuel those schoolboy fantasies.

      The route also took me past a hi-fi shop where I used to linger, looking longingly at a Garrard 301 turntable with an SME arm mounted on a special plinth with acrylic lid and Quad 33/303 amplification. The owner was very pleasant to me when I dared to enter this emporium to enquire about pricing but there was no way I could afford it at the time. Oh boy, fantasies of a different kind.

      1. Rich,
        For me it was a pair of Ditton 66s when I was 16.
        So I saved like a demon for 5 years & I finally bought
        them when I was 21…& sold them two & a half years ago.
        Goodbye old friends.

      2. And that’s how you cultivate future clients. Remember what it was like to be an archetypal impoverished student and also recognize the potential. Many will remember a small kindness and likely return when their financial situation has improved. While the really good stuff has always been above my means, I was eventually able to contribute to their coffers and buy more modest, but still more than adequately kit.

        1. Yes, that’s absolutely right but the theory didn’t exactly work out. I did go back, by which time the shop had unfortunately closed down, so not good for either of us. Actually I suppose that’s only 99% true. It had closed, though reopened in a new location, but one that was no longer local.

  2. Since I seem to be on a Robert A. Heinlein kick, here’s another bit of received wisdom from “The Notebooks of Lazarus Long” (I think), in his novel Time Enough for Love (1973):

    “You live and you learn; or you don’t live very long.”

    Or words to that effect. Lazarus, born Woodrow Wilson Smith, was the oldest living human at over 2000 years of age at the time of the story, so he must have learned a few things (I am a master of understatement).

    And so to bed.

  3. Jep, I’m in the camp of amps getting colder while being at work…my question would be: is it the same for AB amps during the first few A watts?

  4. Why do I feel that a class A amplifier is more coherent, more seamless throughput it’s frequency range, with much less phase problems than a class D? It’s just the audiophile in me saying yes very low in efficiency but void of problems that could arise in class D. You have to pay on your electric bill to make sure your music sounds best. Is it worth it? You get less watts per pound in class A and manufacturers like to ship more watts in less weight to save on shipping costs. So lets all pretend class D sounds as good or better than class A or class A/B. There’s a reason many audiophiles sacrifice watts for those heavy lower watt class A amplifiers and shell out big bucks for them. And a reason class A manufacturers make them.

    1. Class D got so good that it’s here and there an alternative for Class AB and due to the realizable power output, it can even be superior in this very aspect against a wide range of products.

      But it’s undisputed that it has certainly sonic limitations and disadvantages in other fields than power output against AB and for sure A. Just listen to Darren’s & Duncan’s last podcast about holographic imaging and remember that Darren is probably one of world’s leading Class D innovators.

  5. I wonder who is responsible for amplifier class nomenclature, a sequence that pertains to an audiophile hierarchy and implies a predetermined bias.
    First we have class A, for a start it’s hot. If your ‘A’ you’ve made it, reached the top, a class A student, the best by degrees.
    Next there’s A/B, not bad but ‘B’ has crept in, a backward step. Your good, but a somewhat second choice. Who wants to be second?
    Then there’s poor class D, the dunce of the group. I know C was taken but it would have conferred a greater merit. In hi-fi, the cool new kid on the block. If they really wanted to sell it why not call it class A+.

    Similarly the only batteries I ever buy are triple A’s.
    Well, they’re the best aren’t they 😉

  6. I am sorry, but I find this post very depressing. Math is a wonderful, beautiful, elegant, logical body of knowledge that is the backbone of all of the technological development that we depend on and enjoy in today’s world. I feel that I and all of the other scientist, engineers and mathematicians have failed because we cannot figure out how to get a large portion of people who can read, write and do crossword puzzles to have the same abilities with math.

    Why is it that people who are good in literature or writing are celebrated and people who are good in math, science and engineering are called nerds and geeks?

    1. Everybody’s brain is bio-wired up a bit differently. I’m retired now, but I was one of those “hand waving” geologists. While I’m not math illiterate, there comes a point where the low pass filters kick in with mild roll off at first, then progressively steeper. However, what I was good at is the ability to look at dynamic, interacting physical systems: structural, chemical, and (and to a lesser extent) biological; how they evolved non-linearly over time and could extrapolate forward a bit, with the recognition that there are doubtless additional factors that may not have come into play yet. More in the analog domain. Some people might call this intuition, but it is really the brain working to make some sense of things as they are and then being able to get an idea of what is likely to happen next. Of course we are not infrequently surprised, you don’t know what you don’t know, but then you need to be able to reconfigure your mental model of complex reality. As generalists, we also know the value of input from people with more specialized knowledge and skills: the value of a team. So, yeah, Nerd Pride! It IS fun being weird.

    2. tonyplachy,
      I thought nerds and geeks were cool now, seriously. They’re young(ish), they’re in tech and they make money. That makes them cool to the next upcoming generation.

  7. One of the best teachers of percentages is the stock market. If you lose 25%, you have to then gain 33.33% to make it all back to where you started. If you lose 50% you have to then gain 100% to make it all back. If you lose 90%, you have to then gain 1000% to get back to even. Test question: If you lose everything, what percentage do you have to gain to get back to even? LOL

    1. A re-phrasing of the 3 Laws of Thermodynamics:

      1st: You can’t win, the best you can do is break even.
      2nd: You can only break even at absolute zero.
      3rd: You can never reach absolute zero.

      Read this in the preface of a physical chemistry text book, but I can’t remember which.

    2. Pedants corner but if you lose 90% then you need to gain 900% to get back to even, a huge percentage and unlikely to be achievable. If you lose 90% then you need either better advice or better luck, preferably both. Happy listening 🙂

        1. Here’s the formula:

          X = [ ( 100 / Y ) – 1 ] x 100

          Y = ending % of original value
          X = % gain required to recover loss

          If Y = 75 X = 33.33
          If Y = 50 X = 100
          If Y = 10 X = 900
          If Y = 0 Jump off the roof

          Moral of the story…know when to bail.
          Buy again at the bottom 🙂

  8. In tomorrow’s discussion could you comment in general terms the (subjective) sound quality between the different Amp types and their costs?, Thanks, Wayne

  9. All this takes me back to my radio training times in the ’60’s. Theory drummed into us whilst practically plotting load lines of various valves under different conditions i.e. modulators, audio or RF amplifiers or boring old oscillators. The practical side of it was fun then when we’d got that along came the transistor and surprisingly not a lot changed.

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