Crossover distortion and NFB

Discussion in 'Electronic Design' started by ChrisGibboGibson, Oct 24, 2004.

  1. A few years ago I posted a similar question and got...

    A) Told to stop trolling.

    B) Told to stop being a cock

    Or

    C) Told to go back to school

    I'm still no wiser. I did all three and I'm still no wiser.

    On audio, take the output of an opamp and feed it into dual emitter followers
    with no overall NFB.

    The distortion looks horrendous on a scope and sounds it. Take the NFB from
    outside the emitter followers and it looks great on a scope, sounds better, but
    the distortion as measured by a THD meter is the same. The harmonics have moved
    (much higher in frequency) but the total %age is the same.

    Even if you only count the distortion products within the audio band (which
    gets lower with each passing year for me) the overall THD is only slightly
    lower. Yet the difference in sound quality is dramatic.

    Now I know about logarithmic ears etc. But why was I told to stop trolling?

    I'm only posting this because everyone else is talking about Dubbya or Kerry
    which I suppose means that, technically, it *is* trolling but surely it's more
    interesting. Especially for some of us outside USA who don't give a flying fu*k
    who you get lumbered with. Whoever it is, our kissass PM will be stuck right up
    him anyway.

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #1
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  2. ChrisGibboGibson

    Joerg Guest

    Hi Chris,

    >On audio, take the output of an opamp and feed it into dual emitter followers
    >with no overall NFB.
    >
    >The distortion looks horrendous on a scope and sounds it. Take the NFB from
    >outside the emitter followers and it looks great on a scope, sounds better, but
    >the distortion as measured by a THD meter is the same. The harmonics have moved
    >(much higher in frequency) but the total %age is the same.
    >
    >Even if you only count the distortion products within the audio band (which
    >gets lower with each passing year for me) the overall THD is only slightly
    >lower. Yet the difference in sound quality is dramatic.
    >
    >

    If by emitter followers you mean a push-pull stage with an npn/pnp pair
    but no or nearly no quiescent current this might be explained by the
    fact that this power stage can't swing it fast enough despite the
    feedback. It goes through a brief phase where its input voltage changes
    but the output voltage doesn't since the driving amp is not infinitely
    fast, neither are power transistors. Usually that is the range to reach
    the respective Vbe. With feedback it tries hard to "regulate out" this
    gap which is why you hear an improvement. Leftover distortion products
    move higher in frequency because the drive amp speed is kind of ok to
    transit through that non-linear range reasonably fast for a few hundred
    Hertz of audio but maybe not for 5KHz. This is why all linear audio amps
    need a certain amount of quiescent current. Class D doesn't, of course,
    but that's a whole other topic.

    As to trolling, I have no clue why somone would have said that. AFAIK
    trolls are characters in Scandinavian fairy tales and I don't know why
    this word got such a bad rap.

    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Oct 24, 2004
    #2
    1. Advertising

  3. Joerg wrote:

    >Hi Chris,
    >
    >>On audio, take the output of an opamp and feed it into dual emitter

    >followers
    >>with no overall NFB.
    >>
    >>The distortion looks horrendous on a scope and sounds it. Take the NFB from
    >>outside the emitter followers and it looks great on a scope, sounds better,

    >but
    >>the distortion as measured by a THD meter is the same. The harmonics have

    >moved
    >>(much higher in frequency) but the total %age is the same.
    >>
    >>Even if you only count the distortion products within the audio band (which
    >>gets lower with each passing year for me) the overall THD is only slightly
    >>lower. Yet the difference in sound quality is dramatic.
    >>
    >>

    >If by emitter followers you mean a push-pull stage with an npn/pnp pair
    >but no or nearly no quiescent current
    >


    Yes that's exactly what I mean. No bias whatsoever.

    > this might be explained by the
    >fact that this power stage can't swing it fast enough despite the
    >feedback. It goes through a brief phase where its input voltage changes
    >but the output voltage doesn't since the driving amp is not infinitely
    >fast,
    >


    Correct

    > neither are power transistors. Usually that is the range to reach
    >the respective Vbe. With feedback it tries hard to "regulate out" this
    >gap which is why you hear an improvement. Leftover distortion products
    >move higher in frequency because the drive amp speed is kind of ok to
    >transit through that non-linear range reasonably fast for a few hundred
    >Hertz of audio but maybe not for 5KHz. This is why all linear audio amps
    >need a certain amount of quiescent current. Class D doesn't, of course,
    >but that's a whole other topic.
    >
    >As to trolling, I have no clue why somone would have said that. AFAIK
    >trolls are characters in Scandinavian fairy tales and I don't know why
    >this word got such a bad rap.
    >


    I assume you are scandinavian?

    Apparently it's old chestnut in HiFi circles.

    THD remains the same but it sounds better. But the same ones who argue that the
    NFB outside the emitter pair is better because the frequency of the distortion
    products have moved are the same ones who argue that tube s can't be better
    than transistors for the same reason. Like I said, it's an old HiFi chestnut.

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #3
  4. "ChrisGibboGibson" <> wrote in message
    news:...
    > Even if you only count the distortion products within the audio band
    > (which
    > gets lower with each passing year for me) the overall THD is only slightly
    > lower. Yet the difference in sound quality is dramatic.


    OK, you've got a scope and a distortion analyzer. So, what do the residuals
    look (and sound) like? If your question is answerable, that's where the
    answer will lie.

    Personally, I confess to some skepticism about the assertion that the THD is
    only slightly lower; I've not tried that precise experiment myself, but it
    seems counterintuitive. Before I gave it too much credence I'd want to make
    sure that the THD meter was working properly and there wasn't some other
    oddness going on. For instance, if your sine wave source was distorted to
    begin with (in some visually innocuous way, like slightly flattened peaks)
    that could provide enough THD to mask a large change in xover distortion,
    without being particularly audible. Looking at the residuals would quickly
    tell you the difference.

    Note that crossover distortion, although it is sonically ugly, only lasts a
    short while and happens at close to zero volts, and therefore does not have
    much energy. Thus it doesn't actually contribute that much THD to begin
    with, in percentage terms. This is just one reason why THD, as a raw
    number, is not very informative about audio quality.
     
    Walter Harley, Oct 24, 2004
    #4
  5. "Walter Harley" wrote:

    >
    >"ChrisGibboGibson" <> wrote in message
    >news:...
    >> Even if you only count the distortion products within the audio band
    >> (which
    >> gets lower with each passing year for me) the overall THD is only slightly
    >> lower. Yet the difference in sound quality is dramatic.

    >
    >OK, you've got a scope and a distortion analyzer. So, what do the residuals
    >look (and sound) like?
    >


    Far better

    > If your question is answerable, that's where the
    >answer will lie.
    >
    >Personally, I confess to some skepticism about the assertion that the THD is
    >only slightly lower;
    >


    Exactly my feelings. But measurements don't bear out my gut feelings. And other
    posters proved mathematically (with maths that is way beyond my abilities) that
    NFB cannot reduce the overall crossover distortion, it can only move it
    elsewhere.

    > I've not tried that precise experiment myself, but it
    >seems counterintuitive.
    >


    It does indeed

    > Before I gave it too much credence I'd want to make
    >sure that the THD meter was working properly
    >


    I did, many times

    > and there wasn't some other
    >oddness going on. For instance, if your sine wave source was distorted to
    >begin with (in some visually innocuous way, like slightly flattened peaks)
    >that could provide enough THD to mask a large change in xover distortion,
    >


    Indeed. We are talking the 6% THD caused by crossover as compared with real
    equipment with a THD figure of 0.01%

    >without being particularly audible. Looking at the residuals would quickly
    >tell you the difference.
    >
    >Note that crossover distortion, although it is sonically ugly, only lasts a
    >short while
    >


    But it's during the silent period, and ears are logarithmic. But how quickly?

    > and happens at close to zero volts, and therefore does not have
    >much energy. Thus it doesn't actually contribute that much THD to begin
    >with, in percentage terms. This is just one reason why THD, as a raw
    >number, is not very informative about audio quality.
    >


    Indeed. Which expains why I was puzzled when I was accused of being a troll
    when I first posed this question.

    For example why is 0.1% THD as a result of clipping inaudible to most people
    yet 0.1% as a result of crossover distortion is blatanly obvious to anyone with
    even the clothest of ears? They are still odd harmonics.

    Having said that. The one who accused me of being a troll has since been shown
    to be a cock.

    A sinewave with 5% THD as a result of clipping actually doesn't sound so bad.
    Some people can't even hear it. But add THD as a result of crossover and it's
    blantantly obvious and sounds awful.

    I was genuinely asking a real question.

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #5
  6. ChrisGibboGibson

    Joerg Guest

    Hi Chris,

    >Yes that's exactly what I mean. No bias whatsoever.
    >
    >

    It would be next to impossible to obtain reasonable audio quality with
    that. Maybe ok for a noisy pub but not at home if you want to listen to
    Mozart or Tchaikovsky.

    >I assume you are scandinavian?
    >
    >

    No, but I liked some of their fairy tales when I was a kid. I don't like
    lutefisk though.

    >THD remains the same but it sounds better. But the same ones who argue that the
    >NFB outside the emitter pair is better because the frequency of the distortion
    >products have moved are the same ones who argue that tube s can't be better
    >than transistors for the same reason. Like I said, it's an old HiFi chestnut.
    >
    >

    FB across the whole chebang is better because it evens out all
    deficiencies or at least the ones it can, for all stages.

    Tubes? Well, they just sound great. Then there is that glow, a little
    hum, a crackle now and then. Anyway, feedback also works great with
    tubes. For some reason it wasn't done a lot in their days.

    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Oct 24, 2004
    #6
  7. (John S. Dyson) wrote:

    [snip]

    >My guess is that the term 'trolling' comes from a fishing technique
    >where (AFAIR), the boat is moving slowly and the hook/bait is moving
    >by attempting to tempt fish to bite. The 'trolling' metaphor is
    >probably meant like throwing out some 'bait' for the fish to bite.
    >
    >The 'bait' in this case is likely some kind of outrageous or
    >controversial comment or claim. When someone responds to the
    >'baited' claim, then that is equivalent to the fish taking the bait.
    >
    >How the slow movement of the boat, with the fishing pole hanging
    >over the edge (loaded with hook and bait), trying to entice a fish
    >to bite is associated with the term 'trolling' is something that
    >is apparently not directly related to the mythical creature. The
    >term that describes the slow movement probably comes from the
    >Germanic word 'trollen.'
    >


    All that makes perfect sense.

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #7
  8. ChrisGibboGibson

    Joerg Guest

    Hi Chris,

    >Exactly my feelings. But measurements don't bear out my gut feelings. And other
    >posters proved mathematically (with maths that is way beyond my abilities) that
    >NFB cannot reduce the overall crossover distortion, it can only move it
    >elsewhere.
    >
    >

    Theoretically it could move it way up in the frequency range. Problem
    is, most audio power transistors aren't fast enough to do that and
    neither is the usual amp in front of them.

    >For example why is 0.1% THD as a result of clipping inaudible to most people
    >yet 0.1% as a result of crossover distortion is blatanly obvious to anyone with
    >even the clothest of ears? They are still odd harmonics.
    >
    >

    Just a hunch: Distortion in music and other material is less noticed
    when it happens at high amplitude segments but crossover distortion hits
    the "pianissimo" segments and is much more noticeable. Clipping often is
    desired, especially for some early hard rock music. There are some great
    tricks here, such as up converting to RF, clipping there, then down
    converting back to audio. I've got one of those clippers, made by Datong
    in the UK. It is meant for radio communications but also sounds great
    with an E-guitar.

    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Oct 24, 2004
    #8
  9. Joerg wrote:

    [snip]

    >Tubes? Well, they just sound great. Then there is that glow, a little
    >hum, a crackle now and then. Anyway, feedback also works great with
    >tubes. For some reason it wasn't done a lot in their days.
    >


    No doubt whatsoever. But it isn't because of their fidelity. It's because of
    their distortion.

    I have an old radio (circa 1930) that I keep because it fits in with the
    surroundings (I live in a narrowboat with a vintage style stern and like to
    keep some of the tradition) but the sound is beautiful. But hardly HiFi.

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #9
  10. ChrisGibboGibson

    Joerg Guest

    Hi John,

    >My guess is that the term 'trolling' comes from a fishing technique
    >where (AFAIR), the boat is moving slowly and the hook/bait is moving
    >by attempting to tempt fish to bite. The 'trolling' metaphor is
    >probably meant like throwing out some 'bait' for the fish to bite.
    >
    >

    That's it I guess. Even some small outboard motors in marine stores are
    listed as trolling motors. A few are electric to keep the noise level down.

    >... The term that describes the slow movement probably comes from the Germanic word 'trollen.'
    >
    >

    In German it's a bit of a slang word, not in a bad sense. When someone
    gets tired late in the evening and says 'ich trolle mich' that means 'I
    am turning in for the night'.

    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Oct 24, 2004
    #10
  11. In article <c9Ded.17655$>,
    Joerg <> writes:
    >
    > As to trolling, I have no clue why somone would have said that. AFAIK
    > trolls are characters in Scandinavian fairy tales and I don't know why
    > this word got such a bad rap.
    >

    My guess is that the term 'trolling' comes from a fishing technique
    where (AFAIR), the boat is moving slowly and the hook/bait is moving
    by attempting to tempt fish to bite. The 'trolling' metaphor is
    probably meant like throwing out some 'bait' for the fish to bite.

    The 'bait' in this case is likely some kind of outrageous or
    controversial comment or claim. When someone responds to the
    'baited' claim, then that is equivalent to the fish taking the bait.

    How the slow movement of the boat, with the fishing pole hanging
    over the edge (loaded with hook and bait), trying to entice a fish
    to bite is associated with the term 'trolling' is something that
    is apparently not directly related to the mythical creature. The
    term that describes the slow movement probably comes from the
    Germanic word 'trollen.'

    John
     
    John S. Dyson, Oct 24, 2004
    #11
  12. ChrisGibboGibson

    Rich Grise Guest

    On Sun, 24 Oct 2004 01:00:56 +0000, Joerg wrote:

    > Hi Chris,
    >
    >>On audio, take the output of an opamp and feed it into dual emitter
    >>followers with no overall NFB.
    >>
    >>The distortion looks horrendous on a scope and sounds it. Take the NFB
    >>from outside the emitter followers and it looks great on a scope, sounds
    >>better, but the distortion as measured by a THD meter is the same. The
    >>harmonics have moved (much higher in frequency) but the total %age is the
    >>same.
    >>
    >>Even if you only count the distortion products within the audio band
    >>(which gets lower with each passing year for me) the overall THD is only
    >>slightly lower. Yet the difference in sound quality is dramatic.
    >>
    >>

    > If by emitter followers you mean a push-pull stage with an npn/pnp pair
    > but no or nearly no quiescent current this might be explained by the fact
    > that this power stage can't swing it fast enough despite the feedback. It
    > goes through a brief phase where its input voltage changes but the output
    > voltage doesn't since the driving amp is not infinitely fast, neither are
    > power transistors. Usually that is the range to reach the respective Vbe.


    It's the deadband, where the input is in the range -vbe1 < vin < +vbe2.

    > With feedback it tries hard to "regulate out" this gap which is why you
    > hear an improvement. Leftover distortion products move higher in frequency
    > because the drive amp speed is kind of ok to transit through that
    > non-linear range reasonably fast for a few hundred Hertz of audio but
    > maybe not for 5KHz. This is why all linear audio amps need a certain
    > amount of quiescent current. Class D doesn't, of course, but that's a
    > whole other topic.
    >
    > As to trolling, I have no clue why somone would have said that. AFAIK
    > trolls are characters in Scandinavian fairy tales and I don't know why
    > this word got such a bad rap.


    It's because they eat goats.

    >
    > Regards, Joerg


    Cheers!
    Rich
     
    Rich Grise, Oct 24, 2004
    #12
  13. Rich Grise wrote:

    [snip]

    >It's because they eat goats
    >


    And was it the beer that made me laugh or was it Rich?

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #13
  14. ChrisGibboGibson

    Rich Grise Guest

    On Sun, 24 Oct 2004 02:36:11 +0000, ChrisGibboGibson wrote:

    > (John S. Dyson) wrote:
    >
    > [snip]
    >
    >>My guess is that the term 'trolling' comes from a fishing technique where
    >>(AFAIR), the boat is moving slowly and the hook/bait is moving by
    >>attempting to tempt fish to bite. The 'trolling' metaphor is probably
    >>meant like throwing out some 'bait' for the fish to bite.
    >>
    >>The 'bait' in this case is likely some kind of outrageous or
    >>controversial comment or claim. When someone responds to the 'baited'
    >>claim, then that is equivalent to the fish taking the bait.
    >>
    >>How the slow movement of the boat, with the fishing pole hanging over the
    >>edge (loaded with hook and bait), trying to entice a fish to bite is
    >>associated with the term 'trolling' is something that is apparently not
    >>directly related to the mythical creature. The term that describes the
    >>slow movement probably comes from the Germanic word 'trollen.'
    >>
    >>

    > All that makes perfect sense.
    >

    And plus, not only that, "trollen" sounds a lot like "trollin'", so
    it's only natural to turn it into a verb.

    But I do think the noun form is intended to have unpleasant connotations,
    as in a monster who lives under a bridge and terrorizes passersby, and
    whose intent is to eat the unsuspecting.

    Hm. I think I've just described "Homeland Security." ;-)

    Cheers!
    Rich
     
    Rich Grise, Oct 24, 2004
    #14
  15. ChrisGibboGibson

    Tam/WB2TT Guest

    "ChrisGibboGibson" <> wrote in message
    news:...
    > "Walter Harley" wrote:
    >
    >>
    >>"ChrisGibboGibson" <> wrote in message
    >>news:...
    >>> Even if you only count the distortion products within the audio band
    >>> (which
    >>> gets lower with each passing year for me) the overall THD is only
    >>> slightly
    >>> lower. Yet the difference in sound quality is dramatic.

    >>
    >>OK, you've got a scope and a distortion analyzer. So, what do the
    >>residuals
    >>look (and sound) like?
    >>

    >
    > Far better
    >
    >> If your question is answerable, that's where the
    >>answer will lie.
    >>
    >>Personally, I confess to some skepticism about the assertion that the THD
    >>is
    >>only slightly lower;
    >>

    >
    > Exactly my feelings. But measurements don't bear out my gut feelings. And
    > other
    > posters proved mathematically (with maths that is way beyond my abilities)
    > that
    > NFB cannot reduce the overall crossover distortion, it can only move it
    > elsewhere.
    >
    >> I've not tried that precise experiment myself, but it
    >>seems counterintuitive.
    >>

    >
    > It does indeed
    >
    >> Before I gave it too much credence I'd want to make
    >>sure that the THD meter was working properly
    >>

    >
    > I did, many times
    >
    >> and there wasn't some other
    >>oddness going on. For instance, if your sine wave source was distorted to
    >>begin with (in some visually innocuous way, like slightly flattened peaks)
    >>that could provide enough THD to mask a large change in xover distortion,
    >>

    >
    > Indeed. We are talking the 6% THD caused by crossover as compared with
    > real
    > equipment with a THD figure of 0.01%
    >
    >>without being particularly audible. Looking at the residuals would
    >>quickly
    >>tell you the difference.
    >>
    >>Note that crossover distortion, although it is sonically ugly, only lasts
    >>a
    >>short while
    >>

    >
    > But it's during the silent period, and ears are logarithmic. But how
    > quickly?
    >
    >> and happens at close to zero volts, and therefore does not have
    >>much energy. Thus it doesn't actually contribute that much THD to begin
    >>with, in percentage terms. This is just one reason why THD, as a raw
    >>number, is not very informative about audio quality.
    >>

    >
    > Indeed. Which expains why I was puzzled when I was accused of being a
    > troll
    > when I first posed this question.
    >
    > For example why is 0.1% THD as a result of clipping inaudible to most
    > people
    > yet 0.1% as a result of crossover distortion is blatanly obvious to anyone
    > with
    > even the clothest of ears? They are still odd harmonics.
    >
    > Having said that. The one who accused me of being a troll has since been
    > shown
    > to be a cock.
    >
    > A sinewave with 5% THD as a result of clipping actually doesn't sound so
    > bad.
    > Some people can't even hear it. But add THD as a result of crossover and
    > it's
    > blantantly obvious and sounds awful.
    >
    > I was genuinely asking a real question.
    >
    > Gibbo


    There is basically something wrong here. There are probably 100 million
    amplifiers out there that work the way you describe that don't have high
    measurable THD. If you built the amp yourself, the op amp probably has
    insufficient slew rate, or you have no diode equalizer for the Vbe drops. If
    you bought it, it is probably broken.

    Tam
     
    Tam/WB2TT, Oct 24, 2004
    #15
  16. "Tam/WB2TT" wrote:

    [snip]

    >There is basically something wrong here. >


    Obviously

    >There are probably 100 million
    >amplifiers out there that work the way you describe
    >


    No I doubt it. They all use biasing to get rid of 99.9% of the THD as a result
    of crossover.

    > that don't have high
    >measurable THD. If you built the amp yourself, the op amp probably has
    >insufficient slew rate, or you have no diode equalizer for the Vbe drops. If
    >you bought it, it is probably broken.
    >


    I think you misunderstood the question.

    The question is why that particular amp with no biasing to the output pair,
    sounds almost acceptable with NFB but totally unbearable without it. It
    shouldn't really be much different. THD figures are almost identical both ways.

    Even spectral analysis bears this out, just that the harmonics are in a
    different place (but still. audible)

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #16
  17. Rich Grise wrote:

    Summat....

    You're close!

    Gibbo
     
    ChrisGibboGibson, Oct 24, 2004
    #17
  18. ChrisGibboGibson

    Rich Grise Guest

    On Sun, 24 Oct 2004 01:53:02 +0000, ChrisGibboGibson wrote:
    > "Walter Harley" wrote:
    >>"ChrisGibboGibson" <> wrote in message
    >>> Even if you only count the distortion products within the audio band
    >>> (which
    >>> gets lower with each passing year for me) the overall THD is only
    >>> slightly lower. Yet the difference in sound quality is dramatic.

    >>OK, you've got a scope and a distortion analyzer. So, what do the
    >>residuals look (and sound) like?

    > Far better
    >> If your question is answerable, that's where the
    >>answer will lie.
    >>Personally, I confess to some skepticism about the assertion that the THD
    >>is only slightly lower;

    > Exactly my feelings. But measurements don't bear out my gut feelings. And
    > other posters proved mathematically (with maths that is way beyond my
    > abilities) that NFB cannot reduce the overall crossover distortion, it can
    > only move it elsewhere.


    I was gonna try to follow this thread a little, to be able to think my
    imput here might make some sense.

    And bottom line, when I think about in the context that I _think_
    ChrisGibboGibson is alluding to, is that the objective numbers and the
    subjective sound don't necessarily correlate, although I'm kinda
    tending to a discontinuous or lumpy inverse proportional-type thing.

    I'm doing a gedankenexperiment here, where I'm imagining what a clipped
    sine wave might sound like vs. what a crossover-distorted sine wave might
    sound like, and the crossover-distorted one "sounds" worse. ;-)

    It's kinda like, "Boooooowwwwwrrrrrrrrrzzzzz" vs. "Boorraaaaaaeeeeeeiii"

    Try it some time!

    Cheers!
    Rich
     
    Rich Grise, Oct 24, 2004
    #18
  19. ChrisGibboGibson

    Ken Smith Guest

    In article <bpEed.17820$>,
    Joerg <> wrote:
    [...]
    >Tubes? Well, they just sound great. Then there is that glow, a little
    >hum, a crackle now and then. Anyway, feedback also works great with
    >tubes. For some reason it wasn't done a lot in their days.


    When you have to pay an hours wages for a gain of 10, you are a lot less
    willing to give it up than when it costs you less than 1 seconds wages.
    Tubes cost a lot to make even back then.


    --
    --
    forging knowledge
     
    Ken Smith, Oct 24, 2004
    #19
  20. ChrisGibboGibson

    Mac Guest

    On Sun, 24 Oct 2004 01:00:56 +0000, Joerg wrote:

    [snip]

    > This is why all linear audio amps
    > need a certain amount of quiescent current. Class D doesn't, of course,
    > but that's a whole other topic.


    Have you ever read Douglas Self's book, _Audio Power Amplifier Design
    Handbook_?

    In it he argues that if the output transistors are not conducting half the
    time for a sine wave, then it shouldn't be called "class B." He reserves
    the term "class B" for when there is an essentially seamless transition
    from the upper transistor to the lower one, but no period where they
    both conduct. So if the bases of the output transistors are connected
    together, it is NOT class B, according to Self. This makes some sense.

    But the interesting thing is that he measures the THD and residuals for
    class B (his definition) and Class AB open loop output stages, and
    the class B stage has lower THD than class AB. This makes a certain amount
    of sense, too, because when both output transistors with their finite
    betas are on, the driving stage (op-amp or other) experiences lower load
    impedance than when one of the transistors is off. This non-linearity is
    bound to have repercussions. But if you can get the bias just exactly
    right, you would never have both transistors on, and you wouldn't have a
    dead zone, either.

    Anyway, he concludes that class AB is not a good idea. Either go fully
    class A, or just go class B (again, with his definition of class B).


    [snip]
    >
    > Regards, Joerg
    >
    > http://www.analogconsultants.com


    --Mac
     
    Mac, Oct 24, 2004
    #20
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