Class A or AB ?????

Nothing wrong with a good helping of negative feebback :) - ho ho ho - let battle commence!
 
I hear cans of worms being opened in the distance :JPS:

Radford, Tektronix and Hewlett Packard all made suitable low distortion generators. Some of the ones by the later two were as part of a distortion analyser which can both generate the low distortion signal and then measure the THD of the amp.
If money is no object then check out Audio Precision......
Or you could always build your own. Look at papers by Linear Technology and Analog Devices for a start.
The cheapest and easiest way (but not as good) is to use your PC and suitable freeware to generate a sine wave digitally. Obviously there are sound cards and then there are sound cards :D
Have fun.
 
nando said:
digital to measure analogue,!!!!????? how?
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Simply use the PC to generate a sinewave.... it would of course be made up of slight steps due to the digital nature of the process but these are then smoothed out of existence to leave a low distortion sine wave, in the same way that a CD player can reproduce a low distortion sinewave.
If 24 bit 96KHz is used then the results would be mainly limited by the distortion of the sound card or DAC output amplifiers etc. Around 0.005 % should be possible.
One of the main drawbacks is that you will sometimes get general background crap from the PC and its PSU.
The PC can even be used to measure distortion and can easily give a spectrum analysis on screen showing all the distortion products against frequency as well as a Total Harmonic Distortion reading.
 
I already use the PC method, and while it is good, it's not good enough to show the distortion of my ADC. I only end up measuring the DAC I connect as the signal gen. That's why I thought a good analog signal gen might be nice. I'm not really that worried though, since I figure if the ADC can show the difference in DACs connected to it, the distortion of the ADC is low enough.
 
Certainly an analogue low distortion oscillator is the best and if measurements significantly below 0.01% are required then its the way to go.
Figures of some 0.00001% are achievable from analogue. Possibly even less in fact....
It can be virtually impossible to measure the distortion of the very best ones.
My own THD meter sits at 0.0014% with no signal input, this being the noise floor. If I connect my low distortion oscillator directly to it then it stays at this reading! Obviously the oscillators distortion must be well below 0.0014%
If I use this oscillator to put a signal straight into my external DAC connected to my lap top (at 24-96) I get a reading around 0.004 - 0.006% from the computer, so this is obviously the measurement limit of the digital setup I have. It's nice to be able to plot and print a graph of the results though!
 
I have recently re-read the Ivor Humphreys review of my Quad 77 Integrated Amp.

http://www.gramophone.net/Issue/Page/February 1995/126/763221

I have thought for years it was a 70w RMS Class AB amp. Wrong! It will deliver at least 84w, with distortion less than 0.005% up to 70w, and it is Class B, not Class AB.

Now I wonder what that means - is it possible to sail so close to the wind - is it not more likely it is just a little but Class AB?

But the distortion figures are excellent, as it the sound.

With modern semicondcutors in a well designed circuit do we need to get fussed about lots of forward bias?


Humphreys:

It is a very capable unit, rated at 84 watts per channel into 8 ohms (115W into 4 ohms) with a high peak current capability of 11 amps per channel. All Quad power amplifiers since the famous 405, introduced in 1975, have employed a feed-forward power amplifier circuit for which the name Current Dumping was coined. In this, a small Class A stage supplies the finesse and a rugged output transistor pair the muscle. It is a fascinating, if in some circles controversial circuit which won for the company The Queen's Award for Technological Achievement.
For the 77, Quad's young Danish circuit designer, Jan Ertner, has abandoned Current Dumping in favour of a new, outwardly more conventional arrangement which takes full advantage of modern semiconductor components. In essence there are two main types of power amplifier output configuration: Class A and Class B. Very high quality results can be obtained with the former but it is grossly inefficient since the output transistors conduct fully at all times regardless of the signal; energy not required for the loudspeakers is dissipated as heat. In Class B the work is shared, with the positive-going half of the waveform handled by one transistor (or a group or transistors) and the negative-going by the other. It is efficient but the output devices need to be very closely matched and made very linear to avoid socalled crossover distortion (which becomes relatively more intrusive with reducing signal level). The usual compromise is to 'bias' the circuit a little towards Class A, thus smoothing out the crossover glitch, hence the commonly used description Class AB. Ertner has exploited the latest, closely matched, high speed, high gain output devices in his very symmetrical circuit, and with careful control of the bias point stability has been able to achieve optimum results using Class B—distortion is negligible at less than 0.005% at any level up to 70 watts. The efficiency is therefore very good, which is why it has been possible to build this powerful amplifier into such a remarkably compact cabinet
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Labarum. That would be because your amp is a current dumping amplifier. It's a technology unique to Quad and cannot really be compared directly with a conventional amp in terms of forward bias etc..... It has non!
Here is a link to Peter Walkers original article on the subject from 1975

http://quad405.com/currentdumping.pdf

Hope this explains it :)
 
No. I have a 405 - that is a current dumper. The Quad 77 amps, except the 707, are not - they are Class B.

Read the review.
 
Arkless Repairs said:
Class A is certainly the ultimate both in measured and subjective performance.
As the previous poster said though, there are VERY few genuinely class A amps about. There are plenty which claim to be class A and several that many enthusiasts assume to be class A 'cos they get quite hot.
They become fairly impractical above 25 Watts or so. The Krell KSA50, mentioned previously, is indeed one of the most powerful genuinely class A amps around and that needed fan cooling to be as small and light as it isn't :D
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And what about the Krell Full Power Balanced X-series?
 
I stand corrected sir! Quads first non current dumper since 1975 eh.... That'll teach me to jump to conclusions too quick :(

As you say, there must be some bias current there even if small. Amps using the CFP type output stage tend not to need much bias so it could well be using them.

Not that familiar with the full circuit details for those particular Krells I'm afraid... I doubt they are true class A though. I know Krell have used some sliding bias systems on some of their amps.....

If there is any amplifier out there that can make a genuine 100W RMS per channel in true non cheating class A then I would be interested to see it! There must be one or two (not including home made water cooled jobbies that I have heard of) out there?!
 
Labarum said:
and it is Class B, not Class AB.

Now I wonder what that means - is it possible to sail so close to the wind - is it not more likely it is just a little but Class AB?
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If you stick to the textbook definitions of classes then you can say that 99% of the push-pull transistor amplifiers out there are class AB.

But some have only a tiny idle current, putting them just a hair above class B.

And some others have a massive idle current, making them class A for almost all purposes into some nominal impedance.

But when faced with a lower impedance these happily start switching, e.g. the original Krells.


I am only aware of one such amplifier that remained in class A with declining load impedance, the Levinson ML2.
 
I read this about Krell:

The new X Series Amplifiers use the Krell Sustained Plateau Bias III microprocessor control system that maintains Class A operation, regardless of music or movie demands. Class A bias is the most accurate method used to amplify musical signals, but it has its engineering challenges. Class A operation means that all transistors in the amplifier draw current all the time. The large current consumption gives us one of the benefits of Class A operation, namely a distortion that is low and kind to our hearing.
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or

Sustained Plateau Bias III
A patented microprocessor control system that maintains Class A bias operation regardless of musical demand. Class A bias is the most accurate method used to amplify musical signals.
The new Sustained Plateau Bias III software maximizes an ampli- fier's efficiency both in power consumption and heat generation, and is featured in the Full Power Balanced X Series amplifiers.
Click to expand...

I don't know if this is though really relevant for class A.
 
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Another form of "sliding bias" then... probably better than most but must still cause some changes to the amplifiers transfer function and sound as it changes the bias level.
 
Sliding bias has been with us for many years.

I recall as a youngster we had a Technics SUV6 using 'New Class A' and the lovely old Pioneer SA9800 amplifier I have here uses similar circuits, dating from 1979.
The Technics amps from that period had big yellow illuminated logos proudly proclaiming the technology - just in case you forgot :)
 
I reckon many of the Jap amps from that period were very unfairly slagged off by the British comics of the time. They seem to be getting classic status now though!
 
Arkless Repairs said:
I reckon many of the Jap amps from that period were very unfairly slagged off by the British comics of the time. They seem to be getting classic status now though!
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Yes and rightly so in many cases.

Accuphase and Luxman seem to have survived largely unchanged since the early days, with the new kit retaining full feature line-up and looks of the early stuff.

I always liked old Trio amplifiers. In fact I still have a very early KA1200G here and that was my first real amp as a kid. Still works fine.
There is also a nice old Teleton GA202 waiting for me to collect from the post depot in the morning. That was my second real amplifier after the trio and I must have been 10 or 11 when that was in use.
I understand that Teleton were a division of Mitsubishi and that the circuit is more than a little similar to that in a Nait :)
 

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