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Amplifiers "Class" System

Discussion in 'AV Receivers & Amplifiers' started by 50/50, Feb 5, 2003.

  1. 50/50

    50/50
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    Could someone please explain the "Class system" of amplifiers. Ive heard that some subwoofers have class A amps, but what does this accually mean ?
     
  2. alexs2

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    I'm sure this will be explained far more eloquently by other contributors,but the basics are as follows:

    Class A amps have output stages that run at a very high standing current,and basically conduct full power all the time,irrespective of the signal applied....they are thus very inefficient,and dissipate large amounts of heat(I know,as I have a number of Krell Class A amps).
    Since the output stage works in this fashion,there is no crossover distortion introduced as in a Class A/B stage where one half of the amplifier handles the negative signal,and another half the positive.
    Typically they sound superb,but I dont know of any subs using Class A technology.

    Class A/B amps are otherwise called push/pull circuits,where one half of the output stage handles the negative,and the other half the positive end of the signal.
    They can be much more efficient as a result,and can be made to sound very good indeed if properly designed.

    Class D amps are so called switching amps where the incoming signal is used to switch the output current on and off effectively....the efficiency is typically very high,and these ARE used widely in subs due to space,efficiency and heat generation concerns.
    There are very few Class D hifi amps....these are quite well suited to use in subs,but generally not elsewhere.

    This link puts it better than I have!
    http://www.walrus.co.uk/tech/classa.htm

    Hope that helps!
     
  3. john-boro

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    sorry to hijack, but I have a technics Su-C04 that says class A on the front. Could this be correct, or do they just mean to say it is very good as opposed to actual "class a" spec?
    :confused:
    john
     
  4. Flimber

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    Beware of technobabble, Technics' "Class AA" system is not "Class A" and certainly not a whole "A" better anyway ! Far from it.

    Mike.
     
  5. john-boro

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    just had a look, it says, "New class A, sycnhro bias".
    Any ideas?
    cheers
    john
     
  6. Flimber

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    Search usenet via Google. Recall a thread detailing the ins and outs of various pseudo-class-a amps.

    HTH,

    Mike.
     
  7. EvilMudge

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    In short.

    Class AA - not entirely sure but I think this is two joined output stages both running in Class A, but with lower standing currents to increase power output without requiring a heftier design. Won't sound as good as a proper Class A amp, because the outputs won't be perfectly synced.

    Class AB - again two output devices, with one running in Class A but with a quiescent current much lower than a pure Class A stage - so that in the event of high power demands, the second device takes some of the load - reducing quality but preventing the outputs from suffering thermal death.

    Class D&T are radically different to Class A&B amplifiers. Class D uses a fixed switching frequency, usually around 200KHz, but Class T's is variable, between 1.2MHz and 200KHz depending upon load. Class D suffers from frequency dependant distortion whereas the distortion in a class T design is practically 0 up to the point of clipping.
     
  8. alexs2

    alexs2
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    Not sure about your definition of a Class AB output stage there,Mudge...think the link I quoted may be useful...it's a short working definition,but clearer than a lot of the manufacturer's nonsense.

    Below is a quote from that link....

    "Class A operation describes the situation where neither transistor ever switches off - they both pass a constant high current, on which the audio signal is superimposed. This is the exact opposite of Class B, and it is very inefficient. Because such a high constant current is flowing, the output transistors get very hot, so need a huge heatsink arrangement, and the power supply needed to sustain this current needs to be oversized and expensive. However, as there is no longer any crossover region between the two devices, there is no longer any crossover distortion, and you can get a better sound (sometimes). A few other, more obscure, distortion mechanisms are also eliminated at the same time, leading to the generally good name that Class A amplifiers have acquired.

    The majority of amplifiers (95%) have Class AB output stages, a sort of mixture of the two. The operation is mostly Class B, except at the crossover region, where a modest standing current (quiescent current) is allowed to flow, thereby almost eliminating crossover distortion, whilst retaining the efficiency benefits of pure Class B. These amplifiers effectively run in Class A up to a certain power level (normally a Watt or two), sliding seamlessly into Class B at higher levels, up to their maximum output."
     
  9. EvilMudge

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    That one's a bit easier to follow, but Class AB amps operate mainly in Class A, using one output transistor(or valve) until the voltage changes are too large, at which point the second output device handles the large swings into the negative region.
    As you might have noticed the big pure Class A Krells can get very very hot even when not playing, because of the large standing current necessary for Class A operation.
    The trick with Class AB amps is to get the bias high enough into class A that the second output device is hardly ever used unless the volume is turned up to such a level that other components in the reproduction chain mask the drop in quality. Because of this only a very cheap AB amplifier should operate mostly as a class B device.
     
  10. alexs2

    alexs2
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    Very much so,Paradigm....I have a few of the things sitting round my living room,and in this weather they function very nicely as room heaters....not so good in summer though!

    They dissipate the same heat levels whether playing or idling,and certainly the older ones(as mine are) require fan cooling to avoid meltdown.

    The newer models(some of which don't have the same bass grunt as the older ones)employ a sliding bias system,whereby the input signal is used to track the output stage bias up and down,thereby reducing heat output,and increasing efficiency.
    A similar system is used by Levinson on their newer Class A amps.

    Class A/B amps use one set of output devices to handle the negative side of the signal,and one for the positive side,and bias the devices into Class A at the crossover point,it's not a matter of one device doing all the work until a certain output level is reached....hence the other name for this type of circuit being "push-pull".
     
  11. EvilMudge

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    Alex,
    I'm kind of agreeing with you, but my point is that a modern high grade Class AB amplifier spends the majority of it's volume range operating as a pure Class A amp. It's not that the second output device ever turns off, only that since the potential change only goes outside the limits of the first device at very high volumes or when there is a massive increase in signal amplitude, the second device is hardly ever used. Previous generations of AB amps were effectively class B amps with a slight concession to sound quality but this is no longer the case except at the low end.
    What I'm trying to say is that modern AB amps are so biased into A operation that the B part is hardly a factor.
     
  12. Charlie Whitehouse

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    EvilMudge,

    I don't think this is true or all modern AB amps would have the same heat dissipation problems as the Krell class A amps. There is no comparison! It is my understanding that most AB amps only produce at most a few watts in pure class A.

    Alexs2,

    I have heard comments before about the S series having less bass slam than the earlier pure class A Krells. As an owner of an S series amp myself, I can say that there are some of us that prefer the sound of the S series. The earlier models can sound a bit overblown in the bass department. The S series is a bit sweeter in the mid range and treble and has obvious advantages in the power consumption and heat dissipation departments, including not having to rely on fan cooling which can be audible in a quiet room. Depending on your musical preference, mine is pure classical, the S series can be preferable. Also, across the S series range the amount of bass slam increases. The 100-S is a bit short, and the 300-S can sound a bit 'slow' I have found. The 200-S that I have is a nice compromise for my purposes. I wish I had the funds to replace all my amps with FPB monoblocs but sadly this will have to wait a bit! All IMHO of course. ;)
     
  13. alexs2

    alexs2
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    Charlie...having compared the KMAs Vs the KSA100S,definitely the latter has less bass slam,and I also agree that some of the early models can sound a bit overblown,but usually only in the first half hour after power-up,when the thing still isnt up to full operating temperature....I previously had a KSA100 at one point which really was prone to that problem....sounded awful until it was fully heated up.

    As far as music goes,I like Jazz and Rock where the KMAs grunt and control is superb.....and if I had enough cash lying about for a brace of FPB's,like you,I'd do the same....or maybe a set of Levinson 20.5s.....still the best for outright control that I've ever heard....one day maybe!
     
  14. EvilMudge

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    Charlie & Alex,

    From what I've been told, the standing current in a modern high power class AB amp only has to be about 1/3rd to half that of a pure class A design, and will produce similar results throughout most of it's operating range to a purely class A design.
    By comparison older class AB amps have very minimal quiescent currents. So yes, there is a trade off between thermal efficiency and sound quality, but when trying to match the quality of a big class A, the biased class AB is apparently the way to go.

    I'm currently investigating the ESLab Class T amplifiers, since apparently they have the most amazing neutral sound, yet with total control over the entire audio range.
     
  15. alexs2

    alexs2
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    Mudge.....good class A/B designs like the Brystons for instance will certainly rival Class A designs,but I dont think they're there yet....none of the Brystons I've heard equal the bass regions of a Krell,or a big Levinson for that matter...but yes,they are more efficient.

    As to the ESLab amps...I have heard good reviews,but no persoanl experience...would certainly help my electricity bill though!
     
  16. sinister_stu

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    I'm agreeing with alex here. A class AB amplifier has one section for the positive and one for the negative parts of the signal. It is possible to bias this so the transistor(s) being used for the positive part of the signal are used more often, which I think is what evilmudge says they do in modern class AB amplifiers. So if they operate in class A mode most of the time, then you would expect the power consumption to be much closer to a standard class A than you suggest. That is not the case, so what is it that causes this to be lower?
     
  17. alexs2

    alexs2
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    There will be little point in biassing one half of the output stage more heavily into Class A than the other,as more than likely it may result in distortion.
    As I said previously,a Class A/B amp has 2 sets of output devices,one for the negative,and one for the positive side of the signal.
    A pure Class A amp has it's output devices biassed to allow all of the incoming signal to be output by the same set of devices...this requires a very high standing current or bias,with consequent heat output and cooling problems.Have a look at that link...it explains it reasonably simply...(for my understanding of physics it has to be!).
    Class A/B amps can be biassed quite heavily into Class A,but usually this is not the case....feel the case of most amps...even when driven hard they dont get more than warm.
    My Krells will be too hot to touch after more than 5 minutes,and the heat sinks are shrouded internally for safety.
     
  18. Charlie Whitehouse

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    I will second what alexs2 says about heat. My 200-S heatsinks which are NOT shrouded internally but are on the outside edges of the beast, even with its sustained plateau biasing system, average about 50 degrees C in normal running. This is seriously hot - quite uncomfortable to keep your hands on. Pushed really hard it is theoretically possible to get them up to around 85 degrees C before the biasing system shuts off its two upper bias levels reducing the amp to a more normal class AB. Having said that I have NEVER got it so hot to reach this stage yet, but the heat genuinely pours out. Taken with the CRT projector, it can make the room quite stifling during the summer, though more tolerable in winter.

    I can barely imagine what something like the old MDA300's are like in such circumstances and I'm grateful I don't have a problem of that magnitude... :eek:
     
  19. EvilMudge

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    :hiya:
    My point is that much of the power consumption of a high power pure class A design comes from the very high standing current.
    High bias class ABs have a lot lower standing currents, so the power consumption is therefore much lower.
    AFAIK the Brystons are high biased ABs - though obviously their performance is not quite in the same league as the big Krells.
     
  20. dunkyboy

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    I suppose ATC uses similar amplification design to the Brystons in that area..? As they put it, their amps feature "Low distortion Class A operation up to 2/3rds maximum power output". I've always wondered what that really meant, and I'm still a bit confused by what people are saying. Does that mean it won't sound as good if I crank the volume because it's running in Class B rather than Class A?

    BTW, I can tell you that the bastards run hot - the big ol' heat sinks on the backs of my 250-watt Active 10s get nearly too hot to touch, and face plate of the the amp (also 250-watt) on the Concept 2 subwoofer (which doesn't have as large heat sinks) *does* get too hot to touch!

    Cheers,

    Dunc
     
  21. EvilMudge

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    Definitely sounds like the behaviour of a high biased class AB to me.

    On a related note the Class Ts are looking more interesting, but only since I won't be using electrostatics or ribbons on the end of them - apparently they will quite happily kill themselves if you ask them too. Well except for the ES Lab monobloc model, which will put out 1KW into 8Ohms. That's some serious power! Admittedly doesn't double into 4Ohms - possibly because the amp doesn't respond purely ohmically.

    Going to have to get myself over to Horsham and have a demo.
     
  22. alexs2

    alexs2
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    Point taken,Mudge....the Brystons are indeed fairly heavily biassed Class A/B designs,and do sound very good.

    On the Class T designs though,if you should get to hear what the Es Labs amps sound like I'd be interested to hear....all the reviews have been good.....I can understand that they may not be able to sustain increasing outputs into severe loads,but not even some of the newer Krells were rated down to 1ohm as the older ones are.....the KSA100 and KMA series were( and in many instances still are) the only things that would reliably drive the Apogee Scintilla in its original form,this being a 1ohm load with a heavy capacitive element also.

    Into that,the KMA100 would deliver 800W.....but it does have around 180 000uF of supply with a 2kW transformer...as you can guess,Charlie W and I do like the old things!:clap:
     
  23. EvilMudge

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    I've read up a bit more, and the lower power version of the ESLab stereo model actually drops in output between 4Ohms and 2Ohms. The voltage behaviour is quite strange. Then again it should be, as the design of these units has very little in common with any conventional amplifiers. They can't be clipped for one thing, apparently the unit either throttles back the gain or shuts down cleanly. They'll happily drive a 1Ohm load as far as I can tell, just not for very long or very loud. Then again, if you can get 1KW into 8Ohms, you're probably not too worried about it still only doing 1KW into 2Ohms eh?
    P.S. I've heard all sorts of scary tales about the mighty Scintillas, sadly I'm not old enough to have experienced them.:(
     

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