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Bob Lee or other amp guru: Class A, A/B, H?

Discussion in 'Amps and Cabs [BG]' started by mgood, Feb 14, 2002.

  1. mgood


    Sep 29, 2001
    Levelland, Texas
    Could someone explain the difference between Class A, Class A/B, and Class H amplifiers? I have a vague understanding of what each is used for, but don't remember what the difference actually is.

    Or, could you give me a link to a site that might explain this? Preferably in a way that could be understood by someone that does NOT have a degree in electrical engineering, but does have an ok, basic knowledge of electronics.
  2. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    taken from:

  3. mgood


    Sep 29, 2001
    Levelland, Texas
    I think that's just what I was looking for.
  4. VicDamone


    Jun 25, 2000
    Nice job JMX.
  5. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Classes G and H are really just extensions of AB or AB+B.

    Class G uses multiple (usually two) sets or tiers of output transistors on a channel, each with its own positive and negative rail voltages, and the amp channel uses the set with the lowest rail voltage necessary to reproduce the signal, automatically switching up to the higher voltage when necessary.

    Class H uses multiple rail voltages but with the same output transistors. Normally the circuitry uses the lowest rails, but switches up to the higher rail(s) when the signal amplitude requires it. The advantage is that the circuit uses far fewer transistors to achieve the same thing than in Class G.

    Most Class H designs (including QSC) use fast switching between the rail voltages. (Using diode-ORing as in the original 1977 Hitachi method is slow and can produce audible glitch artifacts in the audio.) These designs can be made with two, three, or even up to four steps.

    Other Class H designs, such as Crest (and I think Carver pioneered this in pro audio), use somewhat linear modulation of the rails between the lower rail voltages and the higher. This scheme is colloquially called a "wobbler." For example, the lower rail voltages might be ±30 volts, and the higher ones ±60 volts. Most of the time, the signal is well below the lower rail voltages, so the output transistors run on ±30V. If there's a peak in the audio signal and it exceeds 30 volts, as the signal approaches that point, one or more transistors (usually FETs or IGBTs) start pulling the rail voltage upward, so that it follows maybe a volt or so above the signal, and it can do that up until it hits the higher rail voltage.

    IMHO, there aren't really any advantages of "step" or "wobbler" over each other; it all depends on how well-implemented the design is.

    Please note that in the text above I'm using the usual North American designations for Classes G and H. Japan and the UK reverse them, so what we call Class H here would be called Class G in London or Tokyo.

    I don't know what other countries use, but I've heard that in Germany, BH holds the two channels together or something? ;)

    A good reference for this sort of stuff is the book High Performance Audio Power Amplifiers by Ben Duncan.
  6. mgood


    Sep 29, 2001
    Levelland, Texas
    Thank you.
  7. PICK


    Jan 27, 2002
    Sydney, Australia
    What class does an Ampeg SVT-2 pro fit in to?? If anyone knows!!

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