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Power Amps: Diff between Class D & H?

Discussion in 'Amps and Cabs [BG]' started by Rezdog, Apr 1, 2004.


  1. Rezdog

    Rezdog Supporting Member

    Feb 17, 2004
    T.Rez, Canada
    Greetings from the North.
    I was wondering if some one could explain the differences in power amps that use Class D or Class H and what are the pros and cons of each?
    Happy Times,
    Rezdog
     
  2. Well, here goes.

    Class D amps are "digital". They use a series of pulses to generate the music, which is then put through a lowpass filter to make it sound like music again. The bonus is efficiency. While a regular class AB amp might be 30%-50% efficient, a Class D amp will be 80-90% efficient - that is, how much of the wall juice actually turns into music. So you get a much lighter amp because you don't need all those big honking parts in your power supply. They are usually couple with a digital power supply too. Kind of the same idea, they chop the rectified AC from the wall into short pulses and then run it through a transformer and filter it and voila' - you have DC to run your amp.

    Class H is still analog, while your are running your amp at lowere power it uses a smaller power supply, and then when you drive it harder it switches in a higher voltage power supply to give you the extra headroom that you need. One of the drawbacks is that there is issues when the switching happens. Also, since we bassists like it loud most of the time, it really wouldn't save you any juice, because you would probably be running at the high voltage level anyway. Plus you still have to have all the hardware in there to give you the higher voltage.

    They're both more efficient, it's just that the class D is better at it, and now they have perfected in to the point where it can be used in audio. IIRC, EA, Clarus, WW, proly ampeg portabass, etc.etc. use class D and digital power supplies to get the weight down.

    Where's Bob from QSC when you need him?
     
  3. From TalkBass TechTalk (see my signature):

    5.8. Amplifier topologies

    Due to the nature of loudspeakers, amplifiers have to be designed so that a loudspeaker can move in (negative) and out (positive) the cabinet. In practise, this means separating the amplifier in two halves: one for each half of this movement. A problem occurs when the two halves have to take over from each other. At near-zero current, both tubes and transistors are non-linear. It means they will not reproduce the signal well (aka "crossover distortion"). This problem returns every half cycle of the waveform, as a waveform crosses zero twice each cycle.

    Another problem occurs when the output devices of the amplifier are not fully "on". Because a music signal is of a constantly changing amplitude, this is practically always the case. The connected load of the amplifier receives part of the output voltage, while the output section gets the remainder of the supply voltage. This remainder is converted to waste heat. When an amplifier is working somewhat below its maximum power, more heat than output power is produced, even if the amplifier is theoretically ideal.

    There are several ways to address these problems. They're called "classes". Not every amplifier class is suited for audio (there are more purposes for amplifiers). Only those who are, are listed below.

    · Class A: Maximum current flows through the output stage at all times. This way the near-zero current is avoided, and thereby crossover distortion eliminated. An unavoidable side-effect is, when no signal is present, power consumption is at maximum, and the amplifier will run hot when no sound is produced. Better still, the amplifier will cool down when operating at moderate to high output power.

    · Class B: The opposite of class A. No current flows through the output stage when in rest. Stand-by power consumption is nearly non-existent, but crossover distortion is eminent, be it acceptible for some applications (like speech or sirens).

    · Class AB: The best of both worlds. A small stand-by current keeps the crossover distortion at a low level, and when silent, power consumption is only a fraction of the maximum power. Nearly all conventional power amps are class AB.

    · Class D: As mentioned above, heat is produced when an amplifier output device is not fully "on". Class D amplifiers use digital technology to rapidly and constantly switch the output devices on and off, effectively avoiding the "in-between" state. By filtering (averaging) the switching frequency out of the output, the intended amplified signal appears on the output. This class is a.k.a. switching amplifiers. It won't be before long when every amplifier uses class D topology ('cept for them good-ol' tube amps, but then again, ya never know). When combined with a switching power supply, instead of a conventional heavy mains transformer, weight, mains power and cooling requirements can be drastically reduced.

    · Class G: This topology uses two sets of output transistors and two supply voltages. One set controls low-to-medium power signals, keeping power consumption and heat at a moderate level. When high power is needed (during signal peaks), the second transistor set takes over and provides the higher voltages, fed by the higer supply voltage. As soon as the peak is over, the first set gets back to work.

    · Class H: Much like class G, this system uses two stages. Only now the supply voltage is temporarily increased (switched) to deal with the peaks. The advantage is: you only need one set of (expensive) output transistors, and the switching can be done by much cheaper electronic switches.
     
  4. Arranger

    Arranger

    Mar 9, 2003
    Pennsylvania
  5. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Most--nearly all--are actually analog. Only a few use digital modulation.

    Joris has one of the best explanations I've seen anywhere.
     
  6. Rezdog

    Rezdog Supporting Member

    Feb 17, 2004
    T.Rez, Canada
    Greetings,
    Of Class AB,D,&H is one better than the other for the purposes of amping a bass guitar signal thru the speakers of a bass cab? Thanks to all that have responded.
    Rezdog
     
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Actually, Class-D is STILL analog.

    While the waveform is switched, and so looks digital to the eye, the controlled parameter is pulse length. The pulse length is an analog quantity, so it must be said that it is still analog in its fundamental nature.

    The analog amplitude information has been transformed to time duration information, but the time duration is still non-quantized. By that I mean that it does not have any inherent step size, it can take any arbitrary value between two limits.

    A truly digital signal device has only a given number of possible signal values available.
    A signal at any given time can only be at one of those countable values, it cannot have just any arbitrary value.

    Ok, maybe not the most important piece of info, but its interesting even so.
     
  8. Just exactly what does determine the duration length of the pulse. Is it determined by a device that can generate an infinate set of values ( analog ) or is it a digital device that works with a given set of integers?
     
  9. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    The vast majority of Class D amps use analog modulators to generate the pulse stream. Only in a very few is the pulse stream source actually a digital modulator--that is, the pulse stream would have to be the direct result of numeric processing of audio data.
     
  10. Rezdog

    Rezdog Supporting Member

    Feb 17, 2004
    T.Rez, Canada
    Greetings,
    I'm using an SVP Pro pre with Carvin DCM 1000 power. I'm thinking about getting some more head room via the purchase of a power amp with more wattage. Would their be any less benefit to my sound if I went to a Class H or AB power amp?
    Rezdog
     
  11. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Which class is best for bass?

    Which is longer--a piece of string or a piece of rope? ;)

    It all depends on not on the amp class but its implementation. Classes G and H are just more efficient versions of Class AB, so at power levels where they become economically viable, they would be preferable to Class AB unless they just happen to be poor implementations.

    Class D is generally better suited for bass frequencies than for highs.
     
  12. Years ago I was in a band that used one of the Peavey Decca amps on the horns of a 4 way JBL system. I always thought it sounded a little dull and my impression was confirmed when it went down one day and we had to replace it with an old Phase 400. The added sparkle and clarity was instant. Was the Decca Class D as we are discussing here?
     
  13. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    I think it was, but I'll have to check to make sure. I'm not greatly familiar with Peavey amps.
     
  14. From what I've read in this post, class G and H are really modified Class a/b so they should sound the same. Where they differ is in weight over a certain wattage output. For example the QSC PLX 1202-1602 are class A/B but the 2402-3002-3402 are Class G. They all weigh about the same.
     
  15. I've posted about this before and I'm still a little un-clear as well. I have the newest version of the Deca line of amps, the DPC 1400X. It is listed in its product literature as Class D. Bob Lee has said that this amp is not a Class D amp. Is that because of this?-

    I would also be interested in knowing what brands/models actually use digital modulation and if those particular designs would still be considered Class D amplifiers. Maybe my thinking "D" was for "digital" is to blame? This is an informative thread BTW... Thanks!
     
  16. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    AFAIK the DECA amplifiers are "phase modulated".

    That means that there are two pulsed signals, and the output pulse length is determined by the overlap between them. One is a reference, and the other has its phase modulated by the input analog signal so that it overlaps more as the signal gets larger.

    I believe the Crown "K" amplifiers are also phase modulated.

    The resulting output has essentially the same appearance as that from a PWM or "class D" amplifier.

    A "textbook" PWM Class-D amplifier compares the analog input signal to a reference signal which is some form of sawtooth or triangle wave.

    When the signal is higher than the reference, the output is switched "high" and when the signal is lower than the reference, it is switched low.

    There are a number of variations, clocked sampling, natural sampling, etc, etc. These generally relate to the way the reference is generated, if it is signal-dependent or independent, etc.



    The degree of "dullness of sound" is a design issue, and no longer directly related to the technology used. Used to be that "class-D" tended to be dull, but that has changed.

    FWIW, most PA amplifier products of "the brand mentioned previously" have sounded dull to me.
     
  17. Bob Lee (QSC)

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

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    That's what is called a "natural" modulator. There are a number of other types also. Each possesses strengths and weaknesses, depending on their design and implementation. Some have a tendency to produce distortion, birdies, and other artifacts. If it were easy, there would be a lot of really good class D amps on the market.

    Professor John Vanderkooy of the University of Waterloo in Ontario has done a lot of research into PWM modulators for class D amplification. If you can find some of his published work, you'll get good information.
     
  18. Rezdog

    Rezdog Supporting Member

    Feb 17, 2004
    T.Rez, Canada
    Greetings,
    Thanks for the tip about Professor Vanderkooy at Waterloo.
    Happy Times,
    Rezdog