PURE CLASS A - fact or fiction.

[jammy395]

Hi folks the class A debates rumble on - Thought I would start this thread to try and keep it in one place.
Have copied Harlequin's post from AOS thread over here to kick off.

Re: AoS
Unread postby Harlequin » Wed Oct 16, 2013 10:32 am

Yet more Carping On from The Fat Controller quoting the 'Imaginatively Named Amps' Alleged 50w Pure Class A performance ! I note that in the one and only Professional review of this amplifier in HiFi choice way back in 98 Jason Kennedy is careful to write " Anthony ***claims*** pretty impressive specs , un-clipped class A output *** Is said to be*** 28 Watts RMS , while bandwidth *** Allegedly*** stretches from 20Hz to 30KHz .!
One again No creditable independent verification any ware alse for Marco's tedious boast , other than what is spoon fed to him from AM of course. I wonder what real world result would transpire from a full suit of penetrative bench testing from the likes of John Atkinson or HiFi Critic !!!

I had a Sugden 21 yonks ago which if memory serves was very good - They say its pure class A. (Possibly)

[jammy395]

What makes an amp Pure Class A - And just how difficult is it to get right.

[Yomanze]

From: http://www.aikenamps.com/ClassA.htm

"For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance."

[Daniel Quinn]

From: http://www.aikenamps.com/ClassA.htm

"For audio amplification, a class A amplifier can be either single-ended or push-pull. Now, you might be thinking, how can a push-pull amplifier be class A? Doesn't one side amplify half the waveform and the other side amplify the other half? Isn't this why we use a phase splitter? These are common misconceptions. You can, indeed have a true class A amplifier that operates in push-pull mode. Amplifier class has absolutely nothing to do with output stage topology. If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions. As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount. The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding. As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier. Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance."

Alas , I'm not able to think any of this :D

edit - given that only yesterday a deputy district judge said I had a particularly abrasive attitude , I should clarify the fault lies with my stupidity not the description .

[Andrew]

I struggle with it too, especially after hearing that amps like Krell aren't class A. I think it was some bloke who used to visit here who said that. I think he was called Dr something or other. Hasn't posted lately, so he must be off enjoying himself somewhere or other

If and when he comes back, I'm sure he will clear up any misconceptions about Class A

[jammy395]

Im sure he has done a thousand time before Andrew - But it is difficult to grasp if you aint clued up on yer design stuff.

I would just like to see those that sprout this and that is Pure class A when it clearly isn't - Brought to justice....

Well by brought to justice - I mean, shown the error of their self deludedness (Is that a word - I would use it in scrabble)...!!!

[jammy395]

Dr something or other. Hasn't posted lately, so he must be off enjoying himself somewhere or other (Andrew)

Probably poped out for some solder and a XXL tin of doping paste for his Ref Cubix - Or Nadia's dragged him to Asda...............

[Daniel Quinn]

Isnt he in Spain ?

[jammy395]

Spain !! - For doping paste

[Dr Bunsen Honeydew]

There is only one company in the UK producing Class A solid state amps and that is Sugden. But some people have argued even these are not true class A. The only inarguably true class A amp is single ended.

Wont post much until next Tuesday - in Spain.