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Class D and A heads

Discussion in 'Amps and Cabs [BG]' started by TjMetalhead, Jan 2, 2014.

  1. I've played quite a bit of solid state amp heads and I've noticed they have classes Ex. Hartke has class A heads and Ampeg has class D heads...what does that mean exactly?
  2. It is the output section typology chosen. Class A is usually a lower power design, Class A/B is most likely what the Hartke is, higher power from less devices. Class B does not lend itself well to audio and class C is usually Radio Frequency amplification. Class G and H also exist for audio and all of these directly respond to the input signal. Class D is PWM (Pulse Width Modulation) and is the most efficient but handles the input signal in a different way to control output power.

    This may helps some I just skimmed the info, but still required some basic electronics knowledge to fully grasp. http://forums.afterdawn.com/thread_view.cfm/79383
  3. BbbyBld


    Oct 13, 2005
    Meridian, MS
    Class A is often used as a marketing buzz word. In the case of Hartke, it refers to the preamp section of the head. There are class A power amps, but they are usually under 20 watts because they are very inefficient. Class D will always refer to the power amplifier.

    One cycle of an audio signal starts at 0 degrees and ends at 360 degrees. In the case of your low E string, the signal goes through roughly 40 cycles in 1 second, so the frequency is said to be 40 Hertz or 40Hz. Anyway, amplifier class partly refers to how many degrees in the cycle a transistor or tube is on, or in other words conducting electricity. If the transistor or tube is conducting for the full 360 degrees, that's class A. Other classes of operation are for increasing efficiency. Efficiency is increased by using multiple devices that take turns conducting instead of conducting all the time, or by using a single device that periodically "kicks" a circuit that resonates which in turn drives the load. That's a pretty rough description.
  4. pfox14


    Dec 22, 2013
    So, if I understand this correctly, a Class D amp could have tubes or be solid state? Is that right?
  5. It would be impractical and self defeating to build a class D tube output amp but in theory you could.
  6. aphexafx

    aphexafx A mind is a terrible thing.

    Dec 10, 2013
    Denver, Colorado
    Designer - Big Machine Electronics
    Yup, the whole point of a class D design is to keep the FETs either fully or fully off, which is where they are the most efficient; this is the source of the very high efficiency of class D amplifiers. In any other state the FETs will be wasting energy as heat. This is why Class D amplifiers are lighter and smaller...they don't require the large heat sinks that the others require. This is also the source of the term "switching" because the FETs are either open or closed just like a switch.

    BUT to do this you have to translate the signal amplitude domain into the time domain and this can't be done without sampling the signal at some specific rate. Sampling means quantization and distortion of the original signal. In a Class D audio amplifier this sampling happens at a rate much higher than the frequencies that we can hear and a power filter after the FETs converts the amplified time domain signal back into the amplitude domain...it's HIGHLY distorted going through the FETs, but MOSTLY reconstructed at audible frequencies after the output filter. Note that it's analog all the way through, even though it's sampled over time. That doesn't make it digital.

    I agree with B-string in that Class D takes advantage of FET characteristics and would be nowhere near as efficient using tubes.
  7. CL400Peavey

    CL400Peavey Supporting Member

    Nov 7, 2011
    Grand Rapids Michigan
    Disagree. Peavey built the VB-3. It is a 37 pound, 300 watt tube amp. What doesnt make sense about that? :eyebrow:
  8. How do they handle the output? Torroid transformer?
  9. dincz


    Sep 25, 2010
    Czech Republic
    Makes sense but it's certainly not class D
  10. wcriley


    Apr 5, 2010
    Western PA
    I highly doubt the VB-3 is class D. The light weight probably comes from use of a switch mode power supply.
  11. Definitely not class D. It's a class AB power stage with a switched mode power supply and using a toroidal OT.

    Tubes can't switch on and off fast enough to be sensibly used in PWM amplifiers. Neither can BJTs.
  12. That's it. So they are able to eliminate one of the two large transformers.
  13. Exactly. Pretty good solution if you ask me.
  14. dincz


    Sep 25, 2010
    Czech Republic
    Straying a bit off topic but tubes can operate up in the gigahertz range so speed is not the issue.
  15. Switching speed is the issue. It's not the frequency the device can operate at once turned on, it's the time it takes the device to actually switch on and off. FETs can do this way faster than tubes or BJTs and is precisely why you never see anything but FETs in computing equipment.
  16. teemuk


    Mar 1, 2011
    The switching speed is NOT the issue, assuming you ditch the conventional audio tubes which have no place in the application to begin with.

    Efficiency an size, however, are issues. For instance, tube filaments alone can easily negate any benefit you'd gain from increased efficiency of the amp stage, tubes will always have more voltage drop across them in saturation state than good switching MOSFETs and tubes will also require somekind of impedance matching and you can't use a conventional audio transformer. They are also bulky and heavy compared to solid-state solutions. The same restrictions of physical size will also begin to show great mportance once we step up to design WAY above the audio bandwidth. These circuits really benefit from being physically small. And so on.

    Class D amps have been built with tubes but overall their performance is easily beaten by MOSFETs. Even ancient MagAmps likely provide a better platform to build a switching mode amp than vacuum tubes. Asking why not build class-D amps with tubes is akin to asking why not build cars from wood. Leave the old technology and solutions (tubes/wood) to old applications (class A-C amps/horse drawn carriages) and use modern stuff for the modern applications. But if you really want to do it the hard and inefficient way then yep, you can make a class-D amp with tubes.
  17. BbbyBld


    Oct 13, 2005
    Meridian, MS
    To the OP...this is what happens when you ask questions like you did :D.

    You are correct. The VB-3 power amp is not class D. The SMPS is the main weight saver. There are several class A preamp stages leading into a class AB power amp stage.

    The output transformer design helps save weight also in the VB-3, but it's not toroidal. It's a traditional EI core, except the steel composition used in the core and class of wire used for the windings allow for a smaller and lighter transformer.

    You hit the nail on the head.

    There's some truth here too. BJTs can be difficult to turn off at high speed when they are operating at high collector current. One of the main attractions of FETs when used in digital circuits is ease of interfacing.
  18. Marketing leads to false assumptions and are to blame for the confusion "New lightweight Class D" is seen often. Yes there are weight savings from class D designs but far greater weight savings from SMPS replacing line frequency power supplies.
  19. wcriley


    Apr 5, 2010
    Western PA
    Marketing is also responsible for a single tube in the preamp becoming a "tube amp".
  20. SactoBass

    SactoBass There are some who call me.......Sactobass Supporting Member

    Jul 8, 2009
    Sacramento CA
    Google is your friend......