Alphabet amps

June 5, 2022
 by Paul McGowan

Power amplifiers come in classes determined by their design. Their classifications are listed by simple letters of the alphabet, but underlying that simplicity is a whole lot of complexity.

For example, amp classes as determined by their bias and turn-on cycle include A, AB, B, and C. The differences in this class of amps is determined by how much constant power they consume and when their output stage turns on and off.

Class H amplifiers are different. Their amp circuitry is rather common but their power supplies are variable: only a little voltage for small audio signals and jumping to big voltage outputs as the music gets louder.

And Class D amps are entirely different in the way they work altogether. These, of course, are Pulse Width Modulated amplifiers that work in fundamentally different ways than an analog amplifier.

Because of the similarity in nomenclature of amp classes (using the alphabet for classifications) it’s easy to think that amp classes are related to each other.

But that wouldn’t be accurate.

A, B, C, D, and F may be simple letters but the science behind them is a lot more complex.

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40 comments on “Alphabet amps”

  1. As most of us know, the majority of commercially available
    home-audio amplifiers today are either class ‘A’, ‘AB’ or ‘D’.
    I’m curious to know what differences there are, if any, in the
    clipping characteristics between an ‘AB’ class amp & a
    ‘D’ class amp…anyone?

  2. I too have a question. How do you treat filament heaters in determining an amps class? I have an old pair of c-j P12 monoblocks. Each one has four 6550 output tubes The circuit is a push pull class AB design. However, filament heaters are used on the cathodes ( rather than run a large current into cathode to start thermionic emission ). Thus as soon as you switch on one of the buggers it draws 200 watts for the four filament heaters. It’s like switching on four 50 watt light bulbs.

  3. Rather than talk about the letters and their lack of relationship. – why not talk about the sound qualities each amp class has? Their strengths and their weaknesses based on their topology. (Not component selection or implementation) Where distortion and or lack of detail is most likely to occur in any given letter class. Keep it in the power amp realm. Just a suggestion. Most everyone posting here already has an opinion or bias. 😀

    1. I have a sunfire signature amp and I think that is class Hish … although I’ve seen it mentioned as class a/b. There is some mention of measuring the input signal to power the rails…

      I’d like to hear the negative/positive of this approach

      1. Timm,

        Like everything audio there’s a plethora of opinions. Advantages / Disadvantages, sound signatures, detail extraction, all from the point of view of the listener. From there are the costs of manufacturing and development on the factory side, then add in profitability , which market or group are you after, and finally factor in the listener points of view and market demand.

        What I am hoping to see is/was not why any class of bias makes product z better than y, but rather a simple but detailed comparison of the different biasing technologies and what might someone gain or lose by picking one over another.

        There are many other factors that effect the sound – The bias scheme becomes a buzz of letters that segregates (distinguishes) and separates. If one takes a look in recommended components, you’ll find almost all the bias configurations are represented in the highest rating class. Yet within that rating class there are huge variations of perceived sound qualities. The straight wire with gain theory seems as elusive as reproduced sound that gets mistaken for live 😀

        I guess to sum things up for me, is that any given amp circuit bias is not necessarily indicative of any given amplifiers sound profile.

    2. I think that’s quite simple. The only advantage of those amps downwards the alphabet is that they have more power available than those upwards have (unless the latter reach quite unaffordable prices). Everything else (except maybe pure bass tightness) is better from those early alphabet amps anyway.

        1. Yes, I’d say it’s like with integrateds. Down the ladder are compromises to a price point. I doubt anyone disagrees (except for the eventual power and bass control advantages in this case).

      1. Joseph, You probably already know that high current brings better bass. The electromotive force ( EMF ) that a coil ( the voice coil ) produces is equal to its inductance ( L ) times the change in current per unit time ( di/dt ). Thus a high current amp ( usually a solid state amp ) versus a high voltage amp ( usually a tube amp ) will produce better base. It is the bass drivers that have to push the most air and thus require a large EMF. Since the bass frequencies are low ( slow ) that means the dt is large, the only way you can get a large di/dt is to have high current ( i ).

        1. Tony, what you say about the bass makes sense. Thanks.

          I have read that high current amps are needed for lower impedance speakers. Many high end speakers like the FR30 are 4-ohm rather than the more common 8-ohm. My 6-ohm Harbeths and 4-ohm Von Schweikerts benefit from high current amps.

            1. My 60-watt per channel BAT tube amp has output terminals for 2-ohm, 4-ohm, 8-ohm and 16-ohm. I have only used the 8-ohm. The owner manual encourages experimenting for best sound. I wonder if driving 8-ohm speakers using the 2-amp speaker terminals would fry the amp or blow out the tubes. I’m not inclined to find out. On the otherhand, driving 6-ohm speakers using the 4-ohm speaker terminals might not be so dangerous. What would typically happen? A lot of clipping?

              1. I think you are probably safest using the eight ohm terminals with six ohm speakers. If you want to try the four ohm terminals be sure to keep the volume set low ( below 1/2 ). If it sounds a lot better than experiment with it. Today 4 ohms is ver common for speaker impedance.

              2. Joseph,
                I think that you may have typo’d when you typed in “the 2-amp speaker terminals”…I think that you meant ‘the 2 ohm speaker terminals’.

                Ok so here’s the rule of thumb:
                You can have a higher impedance load on a particular tap on your amp, ie. a 4 ohm loudspeaker (load) on a 2 ohm tap or an 8 ohm loudspeaker load on a 2 or 4 ohm tap, that’s fine.
                However, going the other way, ie. a 4 ohm loudspeaker (load) on an 8 ohm tap is not recommended as it stresses the output section of your amp.

                1. Many, if not most, amps just have one set of terminals, so you just have to just know that 4-ohm speakers need amps with something like twice the wattage as 8-ohm speakers. I wonder why my tube amp has separate terminals for 2, 4, 8 and 16 ohms. What is going on inside the amp to justify different terminals for different speaker impedances?

                  1. That’s the way tube amps work.
                    It has something to do with the windings
                    in the output transformers…from memory.

                    My ‘M6si500’ can go down to 2 ohms
                    (200 amperes peak-to-peak) so it can drive
                    virtually any pair of loudspeakers…I would
                    imagine that your ‘Pass Labs’ can do the same.

                  2. Most tube amps are transformer coupled at the output. The transformer output winding often has several “taps” so you get the correct winding for the nominal impedance of your speakers. Solid state amp usually are not transformer coupled and can accommodate a wide range of speaker impedance down to 2 ohms or n some cases 1 ohm.

  4. A simplified guide to amplifier classes or, just my opinion.

    A – the best but impractical, for most.
    B – primarily used to make A practical.
    C – companies aren’t exactly falling over themselves to manufacture these for hi-fi applications so largely irrelevant, adds to the confusion.
    D – for people that like new things.
    E, F, G, H and others – see C.
    Valves – for people that like old things.

    As for the alphabet, I’ll stick to soup.

    1. Your listing isn’t complete at all! You have to add the hybrid design as from Devialet (class D and A) or manufacturers mixing tube stages with SS stages. I wonder which target groups you will identify here.

      1. I’m going to claim diplomatic immunity here and say I had those covered with ‘others’ and, I was trying to keep it ‘simplified’ but of course all are welcome to the party 🙂

        1. I apologize, but I couldn’t resist commenting. However I am still convinced that amps don’t “sound”, it is the amp-loudspeaker combo which produces a “specific” sound – and some loudspeaker designs are amp-friendly and other designs require a dedicated amp – as do some Stax headphones. Finally one should better go for active loudspeakers with integrated DSP options! This decision dramatically reduces the efforts and time necessary for finding the optimal sound. 🙂

          1. Most of that matching has to do with whether the speakers drop below 4 ohms or not where some amplifiers don’t like to go. Personally I would like to choose my amplifier and not get what comes with the speakers and for many reasons. But to each their own.

            1. As far as I know things are a bit more complex due to the fact that real world passive loudspeakers show both impedance and (!) phase varying with frequency. John Atkinson of Stereophile publishes nice measurements showing how frequency responses of real world amps vary when connected to real world loudspeakers. A perfect amp should show a flat curve!

              1. Which is why I like passive speakers such as my vintage EPI’s that have low and high frequency two way drivers with natural roll off’s that blend perfectly. The only need is for a single 6db per octave high pass capacitor filter to the tweeter to remove the low frequencies. Pretty flat impedance curve makes the job of the amplifier much easier and it could be heard. There are also good examples of other more complicated passive crossover designs in speakers that have great results. My NHT 2.9 sound superb. I have no need to biamp them but that option is there if I choose to do so. Most speakers have biamp capability if you want to mess around with digital electronic crossovers and biamp your speakers while bipassing the passive crossover within the speaker. It might be a little involving for most who prefer the built in amplifiers and digital processing and there are speakers out there that have that. Problem is you might not be getting the speakers that you want, just the built in amplifiers and active crossovers.

            1. Shouldn’t we all finally have a lot of fun here? (And indeed, you need a huge amount of humor when seriously practicing this hobby.) And sometimes we even get some useful information here for our hobby by shared experiences and know-how! Great! 🙂

    1. On this non-colorized clip the sound quality is “better”. It is amzing that way back then recordings could be as good as they were, and the sound data printed on the film itself, read by the sound strip reader of the projector, which consisted of a light shining through the soundtrack and hittng a photodetector, which based on the strength of the light, sent an electrical signal to the amplifier(s) that fed the speaker(s).

  5. I’m just gonna read the comments and enjoy. Lots to go over in this topic and so far in my experience I’ve had great results with Class A Solid state amplification, which I know is a no brain’er.
    When everything is running at full tilt with in a decent design circuitry /topology it seems more often than not the results are good. Very good. 🙂

    1. I love the sound of class A too. Most class AB amplifiers do run in class A up until around 10 to 20 watts within the level I listen to music most of the time and when I crank it there’s lots of power available. I have a class A or passive preamplifier so I’m basically in class A most of the time.

  6. Too hot for Class A amps in the deep South…A/B has been just Fine, so far!

    Will be participating in a trial period/review of a GaNFET (GaN Systems-gallium nitride FETs) Class D 500W Stereo Amp (Orchard Audio) within the next few weeks…should be very enlightening!

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