Amp draw, Class A vs AB, oh my?

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#63
Audio design is subject to the laws of engineering,everything is a compromise and a trade-off.The laws of Physics cannot be circumvented,for ULTIMATE sound quality,a price must be paid,in this case,power consumption.John Linsley-Hood reached the conclusion that the problem was loss of loop gain at cross-over,leading to the unpleasant "Transistor Sound" which bedevilled early solid state amps.J. L-H.covers the topic very succinctly in the section on PA development.This can only be overcome by either Class A operation,or a high Iq for Class AB.
 
#64
I think the mistake Andrew has made is that V RMS = 2X V Peak. 25% efficiency is perhaps a little pessimistic
To drive 10Watts into 8 Ohms requires a fixed current of 1.25A. and V RMS of 9V.

V RMS 9V,=12.6V Peak. X2=25.2, X1.25 = 31.5 Watts (min.)to be dissipated.This does not allow for any headroom at all.
 
#66
Built a pair of Hiragas for use as monoblocks,very disappointing sound,found it rather "woolly".Lovely circuit on paper,the symmetry is most appealing.Now using a pair of GEM 25Ws designed by Graham Maynard.Very happy with the results.They drive my Rogers Pro9-TLs most satisfactorily.
 
#70
Given that Class B is not much more efficient (at full output) than Class A someone may wonder why we use Class B. The reason is that Class B maintains good efficiency for smaller signals too, whereas Class A gets worse for smaller signals.

For zero signal Class A still consumes the same power, but Class B consumes nothing.

Reality is not quite that simple, but the general idea still holds.
 
#72
Having been away from here for a few years I need to be careful not to dish up old hat; I would presume that the 'class-action' matter would have been discussed earlier.

Thus apology if so - but I must say that neither simulation nor practical measurement by me bears out the point of "pure class-A". In fact, it seems too have become a cliché left over from early years when poorly designed transistor class-B forever introduced the myth that class-A is/must be better.

(By the way, can I presume that we are actually not talking of theoretical class-B here at any stage? That would make a large difference in high-order harmonic distortion content. It should only be class-A vs. class-AB.)

Having said that, one should find that with the right quiescent bias current, there is no difference in low-level distortion between the classes. (One is talking about low level because that is where the class difference really comes to the fore.) Simply put: With class-A there will be no cross-over distortion; with class-AB there need be no cross-over distortion. I must admit that I have not yet delved into the well over 100W power amplifiers, but the basics should be the same. When testing from a very low standing current, one can observe higher order components sink into the noise floor as one advances the bias current .... and increase again when too high a standing current leads to a switch-over action going from class-A to class-AB. (In that sense it is wrong to say the longer in class-A the better.)
 
#73
Alan Frobisher said:
John Linsley-Hood reached the conclusion that the problem was loss of loop gain at cross-over,leading to the unpleasant "Transistor Sound" which bedevilled early solid state amps.J. L-H.covers the topic very succinctly in the section on PA development.This can only be overcome by either Class A operation,or a high Iq for Class AB.
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If that is what JLH concluded then he was wrong. The solution is either Class A or the correct Iq for Class AB. High Iq merely moves the problem to larger signals, where it may be worse (although less noticeable with some music).

The correct Iq depends critically on the output stage design, including the value of the 'emitter' resistors. CFP typically requires around 10mA. Darlington more like 200mA.
 
#74
triplej said:
is anyone aware of dynamic / active bias systems where all output devices conduct for 360 degrees of the input cycle yet have low idle current? I'm aware of non switching / square law amps and current dumping (sort of) but to me they are not pure class A.
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Malcolm Hill's sliding bias (see Ben Duncan's book). But why are you not using the best of both worlds like the Technics SE-A1 topology.
 
#78
Hi guys ,very interesting and informative debate. I suppose that the exchange of ideas, suggestions and discussion is what moves the audio quality to what we all want to achieve, the ultimate amplification system.I will assemble The Hiraga 20 w class A and will make my own judgement. Let's hope that i will not be disappointed.
 
#79
Good luck with the construction.Some tips: Ensure transistor gains of opposite pairs are as close as possible(I bought 50 of each type <12 of o/ps>and measured the gain of each).Keep wires between driver pcb and o/p transistors as short as possible,and take great care to ensure that you have a single "star" earth point.
Keep us posted on results.
 
#80
Good luck with the Hiraga. the only thing that might disappoint is the lack of headroom with a 20W amp. After a lot of consideration the guy who built my amp erred on the expensive side and went for 100W / Ch of pure Class A. Now that gets expensive and weighs a mighty 50Kg. There is a thread on here somewhere about "how much power do you need". It's worth a read, BUT, at most listening volumes even my mighty Aleph 4 is only outputting about 5.8W. It's a difficult quantity to measure as the musical content has a lot to do with transient power as well as average power.

With Class A the PSU is very important. Don't try to save money with cheap components here. Even the Hiraga benefits from a 200W PSU.
 
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