Yeah, Pure Class A. That’s the best. Isn’t it?

well there are class AB amps better than pure calss a and vice versa. in the end it is not the topology used but the implementation of design which matters the most.

YOu can google it out. here are some links to give you an idea

http://en.wikipedia.org/wiki/Electronic_amplifier
http://www.duncanamps.com/technical/ampclasses.html

http://www.recherche.enac.fr/~puechmor/e_aclass.html

Thanks Arj,

In the mean time I got this:

http://www.mesaboogie.com/US/Smith/ClassA.htm

there are 2 important factors (There could be more but this is all i could think of !)

1. Electrical efficiency ie how much input power is required for a specific ouput.

In class As efficiency is in the 10-20% range ie with a 25W input you only get 25W output. and this output and input is the same irrespective of the load. hence if the speaker requirement falls to 2w, 23w come out as heat !

AB is biased as class A upto a region but above that it gets to calss AB hence efficiency is in the 50-80 range depending on the bias.


hence for speakers needing huge power requirements, pure calss A will not fly.

2.Sound Quality.
there are 2 parts here
a) Switching distortion (OR Crossover distortion) since the power supply has to play catch up with requirement in Class AB, there is a little distortion introduced. while Class A has none of this since the putput is constant and at maximum anyway. this is why Class A has a smoothness associated with it
b) THD. class A of the single ended veriety has a much larger distortion but being even order (2nd , 4th...)it is quite pleasing to the ear and does not soound harsh. Class AB is much more accurate with better specs and hence gives better measurements.


of course there is a lot more to this. if not done well a Class A can sound really bad and well done class ABs can eb really musical.

..mine is a laymans view, Siva, Amp_nut, Deaf, Bombaywalla would be able to add much more to this. (BTW Our friend Screaming Gigi is missing since long..any idea where he is ? ).

IMHO, RIP and all others added to be safe.

Why Pure Class A? Can anybody provide me a real explanation?

I will try to keep my 2 cents short

First of all, Class A Amplifiers are nothing esoteric.


ALL ( ok, ok, Almost all, just to be technically correct ) amplifying stages in Pre amps and Power Amps ( except the Output stage ) are run in Class A bias. ( Bias = Electrical Setting of the device )

Class A bias simply means that the amplifying device ( a Valve or transistor ) is made to carry Atleast the Maximum anticipated current to provide the full required output.

For most amplifying stages, this current varies from 2 mA to 20 mA. ( 1 mA = 1 milli Ampere or a thousandth of an ampere ). Maintaining standing currents of this magnitude is typically no sweat.

For Output stages... particularly those driving Loudspeakers, currents of a few ( and sometimes SEVERAL ) amperes is required. This results in a HUGE amount of heat generated.

In a Class A biased device, the device is carrying atleast the full current, whether the signal is there or not. Hence a signal .. even a large signal ( but not larger than the Max output ) will not perturb the device or its operation, as the signal varies.

That, IMHO is the advantage of Class A.

P.S: In case what I have said makes sense, then the next step … Push Pull can be Class A or Class B or Class AB.

Single Ended Operation ( SET : Single Ended Triode ) Can be Class A ONLY. ( Class C too, but that is another story…. thankfully, NOT applicable to audio)

PPS: Class A can be Balanced or Unbalanced.

G_S_Madhav schrieb:

Thanks Arj, In the mean time I got this: http://www.mesaboogie.com/US/Smith/ClassA.htm

here is another one for you to read:

http://sound.westhost.com/class-a.htm

This is the one that started it all
http://www.tcaas.btinternet.co.uk/index-1.htm

Hi Ani,

Thanks for that link. I did not know that this article existed on the net. I have the Original, article and the WW Reprint Book on Audio.

I had built this amplifier, using a BS-157, SL-100 and 3055 transistors. Worked perfectly and sounded divine to my ear ( I was then in Engineering College ). The 10 Watts was restrictive ....

Incidentally, the late JL Hood, in the article referred to the Williamson as his reference Class A Ckt ... :-)

I learned almost all I know on Audio design from JLH and Wireless World.

I am told that JLH did vist Bombay more than once. Sadly for me, I never met him.

HI Ampnutji,

Glad that I could bring memories of good ole days. I had lost my WW book on Audio I had to leave home and stay in hostel durig my college days.

That 10w was very good, the OVAL philips fullrange speaker used to sound nice of course the source was EP's.

I have downloaded the site using httrack for reference.

Regards
Anil