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Thread: Class A vs Class AB amplifiers

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    Default Class A vs Class AB amplifiers

    Class A amplifiers have a mystique in audio circles. You can read about it on Wikipedia or elsewhere, but the basic idea is that in a class A amplifier a single transistor handles positive and negative excursions of the signal. A transistor can only handle voltage one polarity from its ground reference, so this means the transistor is biased. It is always working, even to hold zero signal.

    In a class B or class AB amplifier there are two transistors, one for positive and one for negative excursions of the signal. That's more efficient because with no signal they both rest. There is a potential for more distortion because when the signal crosses zero one transistor switches off and the other kicks in. In class AB the transition is softened by arranging it so that both transistors are active in the crossover region. Depending on what you read, this reduces distortion to a negligible level, or is a travesty and no substitute for a class A amplifier.

    Now, my question. A class A power amplifier will burn your house down, but it strikes me that a class A headphone amplifier might be quite practical. Are there reasonably priced class A headphone amplifiers, and does it make a substantial difference, or is it just audiophile superstition?

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    There’s the budget Bravo Ocean Class-A headphone amp. The advantage of a Class-A design is that because both the input and output stages are constantly on at full power, they exhibit the most linear response with the lowest distortion of the various designs. I’ve had mine since February and not a day goes by that I don’t enjoy it. Even with the stock Chinese valve, it sounds excellent, but you can also roll other valves, and the 12AU7/ECC82 is easy to find and replace because it’s used in a great number of guitar amps. It gets hot but that’s normal for a class-A amp and will not burn down your house. There are also a number of helpful reviews on Amazon, and if you like it you will understand and appreciate the value of a valve amp and its advantages in its musicality, including a soundstage that you don’t have to spend a fortune to get. I use it with my audiophile Grado GS-1000s. The amp has clarity, punch, and power. It can drive virtually any set of headphones.

    Last edited by Larkenfield; Jun-20-2019 at 09:51.
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    I don’t have any headphones and so have no direct experience but I doubt that a class A headphone amp will make any difference from just plugging your headphone into the CD player jack. Headphones are so easy to drive and need hardly any power, it’s hard to get it wrong. Delivering 10mW into 600 ohms is easy. Having said that the make of amp which friends of mine recommend is Lehmann, but as I say, I don’t have direct experience.

    More generally I don’t think that you should jump to the conclusion that just because it’s running class A it will sound great, and just because it’s running A/B it’s not going to sound so good. The design and component quality matters.

    I’m sure I don’t need to say this to you but my advice with amps is never to buy without trying first in your own home, not in the shop, even if you pay a premium for so doing. A demonstration in the shop or a brief dem in your home is not sufficient to make an informed judgement.
    Last edited by Mandryka; Jun-20-2019 at 10:21.

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    The classes of amps are just indications of how they are designed. It doesn't necessarily indicate quality, it's more of an indication of suitability for a particular purpose. Everything in designing audio equipment is a trade off. There's always more than one way to skin a cat. You can end up in the same place several different ways. How well it works depends on whether the compromises you make aren't a big deal in your particular application.

    But the home audio world is different now than it used to be. In the past, amps were small and not terribly powerful and speakers were big and very efficient. The opposite is true today. Modern solid state amps are much more powerful, and speakers are smaller but less efficient. One goes hand in hand with the other. It's easier to design smaller inefficient speakers to higher sound quality standards though. That's why the market has gone the way it has. The amp really doesn't matter. The amount of distortion in speakers is MUCH more of an issue than the inaudible distortion in modern solid state amps.

    Thankfully, technology and manufacturing standards have gotten to the point where super clean, super powerful amps are inexpensive to manufacture. Some folks may want to explore esoteric designs from the past for the novelty, but there really isn't any reason to do that because of sound quality any more. Tiny amps work fine with efficient headphones, but it wouldn't be practical to push a 7.1 speaker system. And after all that effort, it wouldn't sound any different.

    I think it's human nature to feel like something isn't good unless you've jumped through hoops or suffered for it. But we live in an age where things that people used to struggle to achieve are available on our doorstep in two days from Amazon. However, if your interest is in the equipment more than the music, you'll be drawn to the esoteric rather than the commonly available. In the end, it probably doesn't matter one way or the other. But if doing things the hard way floats your boat, then do it.
    Last edited by bigshot; Jun-20-2019 at 18:04.
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    Quote Originally Posted by bigshot View Post
    The classes of amps are just indications of how they are designed. It doesn't necessarily indicate quality, it's more of an indication of suitability for a particular purpose. Everything in designing audio equipment is a trade off. There's always more than one way to skin a cat. You can end up in the same place several different ways. How well it works depends on whether the compromises you make aren't a big deal in your particular application.

    But the home audio world is different now than it used to be. In the past, amps were small and not terribly powerful and speakers were big and very efficient. The opposite is true today. Modern solid state amps are much more powerful, and speakers are smaller but less efficient. One goes hand in hand with the other. It's easier to design smaller inefficient speakers to higher sound quality standards though. That's why the market has gone the way it has. The amp really doesn't matter. The amount of distortion in speakers is MUCH more of an issue than the inaudible distortion in modern solid state amps.

    Thankfully, technology and manufacturing standards have gotten to the point where super clean, super powerful amps are inexpensive to manufacture. Some folks may want to explore esoteric designs from the past for the novelty, but there really isn't any reason to do that because of sound quality any more. Tiny amps work fine with efficient headphones, but it wouldn't be practical to push a 7.1 speaker system. And after all that effort, it wouldn't sound any different.

    I think it's human nature to feel like something isn't good unless you've jumped through hoops or suffered for it. But we live in an age where things that people used to struggle to achieve are available on our doorstep in two days from Amazon. However, if your interest is in the equipment more than the music, you'll be drawn to the esoteric rather than the commonly available. In the end, it probably doesn't matter one way or the other. But if doing things the hard way floats your boat, then do it.
    So this boils down to "a well designed class A/B amps is just as good as class A."
    Last edited by Baron Scarpia; Jun-20-2019 at 18:45.

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    Quote Originally Posted by Mandryka View Post
    I don’t have any headphones and so have no direct experience but I doubt that a class A headphone amp will make any difference from just plugging your headphone into the CD player jack. Headphones are so easy to drive and need hardly any power, it’s hard to get it wrong. Delivering 10mW into 600 ohms is easy. Having said that the make of amp which friends of mine recommend is Lehmann, but as I say, I don’t have direct experience.

    More generally I don’t think that you should jump to the conclusion that just because it’s running class A it will sound great, and just because it’s running A/B it’s not going to sound so good. The design and component quality matters.

    I’m sure I don’t need to say this to you but my advice with amps is never to buy without trying first in your own home, not in the shop, even if you pay a premium for so doing. A demonstration in the shop or a brief dem in your home is not sufficient to make an informed judgement.
    True enough that the power requirements are trivial. I don't use my CD player output directly for two reasons. Reason 1 is I like a little cross-feed and I have an old headphone amp with analog cross-feed that I like. Reason 2 is I like the chance to introduce a little equalization. I currently have a peculiar arrangement, the line-level output of my Marantz CD/SACD player goes to my NAD 1180 pre-amp (which has tone controls I like, I typically set the treble control down and bass control up) The output, instead of going to a power amp, goes to the headphone amp, which adds cross-feed and drives the headphones. The headphone amp is set-and-forget and I can use the tone and volume controls on the pre-amp to adjust to my liking for each selection. If I were sensible I'd implement the equalization and cross-feed in software. I'm currently using a Mac with Swinsian to listen 99% of the time and I'm sure there is equalization, but I don't know of a way to introduce cross-feed through software.
    Last edited by Baron Scarpia; Jun-20-2019 at 20:13.

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    Quote Originally Posted by Baron Scarpia View Post
    So this boils down to "a well designed class A/B amps is just as good as class A."
    They sound just as good, assuming your impedance and power requirements are met.
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    Quote Originally Posted by bigshot View Post
    They sound just as good, assuming your impedance and power requirements are met.
    And assuming there's no crossover distortion, which means it must be perfectly calibrated. If the components age from use, this will show. It has nothing to do with design.
    Last edited by millionrainbows; Jun-21-2019 at 13:32.
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    Quote Originally Posted by bigshot View Post
    They sound just as good, assuming your impedance and power requirements are met.
    As long as the class A/B amp is well designed then it can sound excellent. I’m listening to a class A/B amp now as I type and it’s perfectly satisfactory.

    One thing I believe is that, basically, all Class A/B amps which use lots of negative feedback to reduce distortion are a bit grey, because the harmonics aren’t presented homogeneously; and they tend make the most prominent sounds drown out the details which hold the spacial cues. This is certainly the case with the perfectly satisfactory class A/B I’m listening to now. This is why I much prefer pure class A. It’s as if there’s an upper limit to what you can achieve with class A/B.

    Basically, I really think that if you haven’t heard a good class A, you probably don’t know how good SS hifi can be! Valves are another kettle of fish.
    Last edited by Mandryka; Jun-21-2019 at 14:25.

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    Missing harmonics the way you are describing them would show up in measurements. It would be easy to prove with a null test.

    However your point about prominent fundamentals drowning out harmonics is true. It's called auditory masking and it's just how our ears work. Auditory masking of harmonics in a cymbal crash is totally natural. Even in person you experience it. But masking can have an effect on recorded music. If there is a spike in a particular frequency, it can block the frequency an octave above. The solution to that is a flat frequency response so there are no spikes blocking frequency bands.

    Audiophiles always talk about detail, but the problem rarely has anything to do with harmonics or jitter or distortion levels. It's response imbalances. When you put a transducer in a room, you get response peaks and dips. You can't help it. But over and over, I see audiophiles addressing their "opaque sound", "veils" and "congestion" in certain frequency bands by either buying equipment with incredibly low distortion levels or slathering on euphonic distortion with tubes to cover the problems up with a candy coating. They never seem to address the real problem directly by arranging their furniture in a way that doesn't create odd reflections, applying room treatment wherever they can, and cleaning up the rest with EQ. If they did that, even budget priced systems would sound better than their badly implemented high end ones.

    Sound is frequency and amplitude expressed in time. We have no problem with time with digital audio. This isn't the days of LPs with wow and flutter. And distortion levels even in inexpensive amps and players is far below the threshold of audibility. That leaves frequency response as the single best way to improve the sound of home audio. Why don't people pay any attention to that? The irony is that home theater folks know more about putting together a sound system than audiophiles do.
    Last edited by bigshot; Jun-21-2019 at 18:05.
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    Quote Originally Posted by Mandryka View Post
    As long as the class A/B amp is well designed then it can sound excellent. I’m listening to a class A/B amp now as I type and it’s perfectly satisfactory.

    One thing I believe is that, basically, all Class A/B amps which use lots of negative feedback to reduce distortion are a bit grey, because the harmonics aren’t presented homogeneously; and they tend make the most prominent sounds drown out the details which hold the spacial cues. This is certainly the case with the perfectly satisfactory class A/B I’m listening to now. This is why I much prefer pure class A. It’s as if there’s an upper limit to what you can achieve with class A/B.

    Basically, I really think that if you haven’t heard a good class A, you probably don’t know how good SS hifi can be! Valves are another kettle of fish.
    It is hard for me to imagine how an amplifier with distortion that is barely measurable can fail to present harmonics homogeneously. The amplifier simply tracks electrical potential (voltage) as a function of time. It doesn't know what is a fundamental and what is a harmonic and there is no way for an amplifier to treat them differently. What an amplifier can do is mask signal with noise (easily measurable) fail to reproduce all frequencies with equal gain (frequency response, easily measurable) produce spurious harmonics (harmonic distortion, easily measurable) or produce spurious frequency components by mixing different frequencies (inter modular distortion, easily measurable) or have inadequate impulse response (easily measurable). If an amplifier literally produces an output which is identical to the input, there is no mechanism by which harmonics can be presented inhomogeneously.
    Last edited by Baron Scarpia; Jun-21-2019 at 19:04.

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    Quote Originally Posted by Baron Scarpia View Post
    If an amplifier literally produces an output which is identical to the input, there is no mechanism by which harmonics can be presented inhomogeneously.
    Yes, absolutely.

    I encourage you to listen to a good Class A amp, a Krell or a Luxman or an Accuphase.
    Last edited by Mandryka; Jun-21-2019 at 21:10.

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    Quote Originally Posted by Baron Scarpia View Post
    It is hard for me to imagine how an amplifier with distortion that is barely measurable can fail to present harmonics homogeneously. The amplifier simply tracks electrical potential (voltage) as a function of time. It doesn't know what is a fundamental and what is a harmonic and there is no way for an amplifier to treat them differently. What an amplifier can do is mask signal with noise (easily measurable) fail to reproduce all frequencies with equal gain (frequency response, easily measurable) produce spurious harmonics (harmonic distortion, easily measurable) or produce spurious frequency components by mixing different frequencies (inter modular distortion, easily measurable) or have inadequate impulse response (easily measurable). If an amplifier literally produces an output which is identical to the input, there is no mechanism by which harmonics can be presented inhomogeneously.

    There are no measurements which will tell you whether an amp sounds good or not, bad measurements mean that it will probably sound bad but there are amps with excellent measurements which sound grey, flat, lifeless, sterile and if you raise the volume, it assaults you with a wall of sound. And there are amps which, on paper, are nothing special and which are a joy to listen to for hours and hours.

    What makes a good amp is a mystery, but you know one when you hear one.
    Last edited by Mandryka; Jun-21-2019 at 21:46.

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    Quote Originally Posted by Mandryka View Post
    there are amps with excellent measurements which sound grey, flat, lifeless, sterile and if you raise the volume, it assaults you with a wall of sound.
    Can you please list a few examples of makes and models of amps that fit this description? I'd like to look into it.
    Last edited by bigshot; Jun-21-2019 at 22:46.
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    Quote Originally Posted by Mandryka View Post
    There are no measurements which will tell you whether an amp sounds good or not, bad measurements mean that it will probably sound bad but there are amps with excellent measurements which sound grey, flat, lifeless, sterile and if you raise the volume, it assaults you with a wall of sound. And there are amps which, on paper, are nothing special and which are a joy to listen to for hours and hours.

    What makes a good amp is a mystery, but you know one when you hear one.
    I think you may have a point if you are discussing power amps with loudspeakers. I could imagine an amp that has perfect specs in a bench test driving an 8 ohm resistor, but manifests pathological behavior driving a badly behaved reactive load of a loudspeaker. I had a set of speakers I drove with two different amps and I could convince myself there was a subtle difference in bass (one more punch, the other a bit more transparent). Maybe the responded differently to the impedance characteristics of the speakers. But it was at the "barely perceptible" level and I can easily believe it was my imagination. But interaction with the speaker load should manifest itself in tests driving the actual load. If we are talking about a pre-amp that simply drives a high impedance input stage I can't imagine an amp with perfect specs that sounds "grey."

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