Tell me about Class D Guitar Amplifiers

Discussion in 'Amps and Cabs' started by jeffhef, Aug 25, 2005.

  1. jeffhef

    jeffhef Member

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    I've been playing with my Crate PowerBlock now for around a month and it's gotten me interested in Class D amplifiers. I was talking to a friend today who told me Class D is used a lot by Jazz players.

    Anyone know about Class D amps?

    jeffhef
     
  2. riverastoasters

    riverastoasters Member

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    Yeah. Class D is an interesting departure way away from the usual guitar amp approach. It is basically a switching amplifier that uses the fact that MOSFET devices don't lose much power when they are fully in the conducting or non-conducting state. So the signal from the guitar modulates pulses which are amplified by the MOSFETs and then that signal is filtered to remove the noise by-products of the switching. The switching has to be fast to make this filtration work really well, and so good MOSFETs which can switch fast enable good sounding Class D amplifiers.

    Here is an more careful explanation of this sort of amp: http://users.ece.gatech.edu/~mleach/ece4435/f01/ClassD2.pdf

    Class D amplifiers are usually used in audio for car components or for pro audio amps that have high power.

    I didn't know that the Crate Powerblock was one, but that isn't surprising given it's relatively high power and small size and weight. The fact that players like this thing suggests that guitar amps can be more adventurous in technology choice than they have been traditionally.
     
  3. John Phillips

    John Phillips Member

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    Class D is inferior to Class C which is inferior to Class B which is inferior to Class AB which is of course inferior to Class A.

    :)


    No, really it's a very different type of output section from the others - it's essentially a sort of switch-mode operation which gives very high efficiency compared to traditional Class A or AB (B and C are not suitable for audio). This means that the amps end up very small and light for their output power... you'll know that from your Crate.

    I don't know why jazz players in particular should prefer it, other than that most of them use small, light amps - maybe because unlike rock guitarists they tend to have to carry their own gear... I can't think of any tonal attributes of it that would make it more suitable for jazz.

    Randall Aiken might know :).
     
  4. Jazz guitarists would in some cases prefer a class D amplifier because it is easier to build a light amplifier with lots of output, which translates to headroom, with switching technology than with more traditional class A and AB designs. Essentially, jazz players want, for the most part, clean reproduction with lots of headroom which is something that class D amps can easily excel at. Lots of jazz players really hate carrying Fender Twins around because they are so God-damned heavy and a class D amp of similar output can weigh quite a bit less.

    Cheers,

    Alan Tomlinson
     
  5. reaiken

    reaiken Member

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    Class D amplifiers are ultra-efficent, so you get lots of power for very low heat dissipation and power consumption. They are also usually very lightweight, because you don't need large heatsinks, and you don't need large output transformers like you do with tubes. Also, they typically use off-line switchmode power supplies so you don't need a heavy power transformer. All this translates to lightweight, low-cost amps at high power.

    The real nice thing is that there is no crossover distortion with a class D amplifier, no matter what the power level, so they can reproduce pristine signals at low or high power levels. They are a bit difficult to wrap a current feedback loop around, because of the phase shift in the output filter, which can cause oscillations sometimes no matter how you try to compensate it. Most manufacturers use an additional first-order RC filter in the feedback loop for voltage feedback, only, and run the output filter open-loop, so you get a very low output impedance, which makes the amp sound "solid-state". Higher switching frequencies are easing the filter requirements, and making this less of a problem, so we should see current feedback designs appearing, which will sound much more "tubey" (maybe they already are - does anyone know if the Crate is a current-feedback amp?).

    As for jazz, those old jazz farts, er...jazz cats are too frail to carry a manly tube amp around, and it woudn't look right for them to play a Marshall - they'd get kicked out of the jazz purists club. I'm glad I'm still a young kid who hasn't outgrown AC/DC and Van Halen to start playing jazz. :D

    Randall Aiken
     
  6. jeffhef

    jeffhef Member

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    I'm intrigued. The thing is the Crate has surprised me. I've been a tube guy for a long time. I have used some modeling stuff but usually for practice. Right now I use a V-amp because it has the Aux in and lets me easily run songs into it to play along with. When you gotta learn 2 or 3 new songs in a given week this setup makes it easy and I can practice late at night without disturbing the family.

    But I've been using this little Crate PowerBlock and I'm really liking it. Doing a little further reading I found out it's a Class D. I don't know the particulars on the Crate design and I haven't seen an in depth review on it yet. I'll be doing more research with all this great info.

    It's kinda funny...Crate is the company that caused me to go tube years ago. I bought one of their first amps. The one that actually looked like a CRATE. It was ugly and Solid State. I actually got rid of my Ampeg VT22 and got one of those things. Mainly because the Ampeg was soooo heavy. So I went to the other extreme. Anyway...it was a major mistake and I went back to tube and have been there ever since.

    Now...Crate comes out with this little thing and I can't seem to find a reason to not continue to play it. I don't know that I would want to play it without mic'ing but in my situation - mic'ed with a good monitoring system - this thing's doing the trick.

    Ok...so class D is solid state ONLY...right?

    Good stuff guys. Thanks for all the info.

    jeffhef
     
  7. riverastoasters

    riverastoasters Member

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    I don't know of any that aren't, but there is no reason you _couldn't_ make a Class D amp with tubes. All the original digital switching circuits were first done with tubes.

    There is a good reason why you _wouldn't_ make a Class D amplifier though. Same reason your computer today isn't made of tubes any more.
     
  8. "As for jazz, those old jazz farts, er...jazz cats are too frail to carry a manly tube amp around, and it woudn't look right for them to play a Marshall - they'd get kicked out of the jazz purists club. I'm glad I'm still a young kid who hasn't outgrown AC/DC and Van Halen to start playing jazz."

    I'm glad I'm young enough and not brain-damaged enough to be unable to appreciate jazz and not want to play any dumb-ass, wanker rock through some miserable piece o' **** Marshall because of an inability to play more than three chords. :D

    Many happy returns Mr. Aiken.

    Incidentally, some class D amps like this one for example use relatively large transformers. Class D is a rapidly evolving technology, some amplifiers that use it sound extremely good and some don't.

    Cheers,

    Alan Tomlinson
     
  9. riverastoasters

    riverastoasters Member

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    The big transformer there appears to be the power supply, not an output transformer.
     
  10. reaiken

    reaiken Member

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    Yes, if you don't use a switchmode power supply, you have to use a power transformer capable of supply the entire output power of the amp. In this case they used a toroidal power transformer.

    Randall Aiken
     
  11. reaiken

    reaiken Member

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    Ouch! I see my little attempt at humor must have backfired. :)

    Randall Aiken
     
  12. spikeRI

    spikeRI Supporting Member

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    Word;)
     
  13. Terry Hayes

    Terry Hayes Member

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    And I'm glad that I learned to truly appreciate any kind of music. After playing out for over 25 years, I am still amazed that I can earn a few dollars doing something I love so much - whether that is playing polkas, classical, rock, jazz, country, oldies...whatever:D

    Terry
     
  14. TNJ

    TNJ Gold Supporting Member

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    I also have the Crate Power Block, and will be toting that little 4.75 lb beast to all my gigs...as a backup if I blow a tube.
    It sounds great with my cabs, and has a effects loop, an XLR line out to the P.A., can be run in mono or stereo, and an RCA input for that all important CD player during set break.
    :D
    Nice little rig,
    S>
    j
     
  15. Mea culpa with respect to output transformers, obviously I failed to see the word output. Apologies to Mr. Aiken. With respect to his attempt at humor, I was amused. I just thought a bit of turnabout might be nice. I play jazz and listen to just about everything. I just find the rock fixation a bit boring when I see it in almost every post.

    With respect to class D amps, I heard some studio monitors (ADAMS) that were powered with switching amps and I must say they sounded very good indeed, and I've heard more than a few studio monitors in my life.

    Cheers to Mr. Aiken and the rest,

    Alan Tomlinson
     
  16. jeffhef

    jeffhef Member

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    I'm curious as to if there have been breakthroughs lately on Class D where guitar amps are concerned. Randall brings up some good info I'd like to study on further. I mean...I hate to beat this to death...but I'm REALLY impressed with this little Crate. I took it by a friends music store this afternoon to let him check it out and now he's buying one as well.

    I'd like to see someone do a really deep technical review of this amp. I know TQ probly isn't interested since it's not tube and 5k.

    Anyway...

    jeffhef
     
  17. reaiken

    reaiken Member

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    Yeah, me too. Class D is the coolest topology (no efficiency puns intended), second only to tubes in relative coolness. MOSFET manufacturers are coming out with faster devices every day, so we should soon be able to switch high powers at fast enough rates to make the output filter inductors and caps tiny. Inductor manufacturers are making very high current flat-wire and planar inductors for switching power supplies that can handle huge currents. If we can get good current feedback topolgies to go along with this, we can have very lightweight good sounding high-power amps. These things would be ideal for a wet/dry or reamp rig.

    Randall Aiken
     
  18. reaiken

    reaiken Member

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    I just wanted to make sure you didn't think I was serious! Actually, it was a poke at me being at old fart myself - I stay young by listening to AC/DC. Wait a minute...they are old farts, too....pretty soon they will be played on the oldies stations. I think I'm having a midlife crisis!!

    Randall Aiken
     
  19. reaiken

    reaiken Member

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    I don't think you can get a low enough "on" resistance with tubes. They still drop quite a large voltage at full saturation, and you'd probably have to drive the grids positive by quite a bit to get them even in the ballpark, which would requite lots of grid-drive power.

    Didn't the venerable Norman Crowhurst originally propose class D as a theoretical possiblity back in the tube days? I seem to recall reading that somewhere.

    One downside to class D is that the output filter needs to be matched to the load impedance. If you have a filter designed to be terminated into 8 ohms, and run it into 16 ohms instead, you get a different transient and frequency response. I suspect this will be less and less important as the switching frequency increases.

    Randall Aiken
     
  20. jeffhef

    jeffhef Member

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    Randall,

    Sounds like you're more than interested. Anything planned for the future along these lines? Just thought I'd ask... ;)

    Seriously...excellent thoughts.

    Just curious...I've always heard good things about the JC120 but I've never played one. So I guess I'm coming around to the realization that something other than a tube can be acceptable.

    Wait...did I say that?

    jeffhef
     

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