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-   -   Class A with AB Output-stage (http://www.diyaudio.com/forums/solid-state/23418-class-ab-output-stage.html)

ManUtd 29th November 2003 05:21 PM

Class A with AB Output-stage
 
Any site where i can find amplifier project that starts all
in Class A mode but with Output stage in Class AB.
I have trafos and Caps from a SAE mkIII amp and also
a 2x650VA trafo with 2x27,5V AC (Very Strong & heavy indeed!!)

Regards,

ManUtd

sreten 29th November 2003 05:42 PM

Nearly all power amplifier designs are class A except for
the output stage which is nearly always class aB.

So take your choice of most of the designs out there.

Unless I'm missing something ? :) /sreten.

sam9 29th November 2003 06:30 PM

Previous post was what I was going to say except to add that you might want to check in to the question of Clas AB vs. pure Class B for the output stage. AB sounds like a good idea, but there is body of opinion that has an essential flaw that offsets the benefits with the result that pure B output stages workout a bit better.

sreten 29th November 2003 06:54 PM

Hi sam9,
 
Notice that I used a small a for class aB.

In my book :

class B has no bias and exhibits bad crossover distortion.

class aB has optimum bias for minimum distortion.

class AB means running part of the output deliberately in class A,
above the requirements for aB - I don't really want to go into
the distortion consequences here as it wasn't part of the question,
and also depends on how much is A and how much is B.

We are really talking about what is pure class B and how
class B differs from class C.

I tend to go with historical acceptance, I believe its Doug Self
that claimed unbiased class B was really Class C, but this ignores
the fact that classical class C in radio circuits only conducts for
half ! the cycle, so I only agree with him in the pedantic sense.

In the historical sense class B usually refers to bipolar motor
controllers with no bias, i.e. very low static dissipation.

Whatever, I say class aB, you say pure class B, same thing.

class A enhanced class B wasn't part of the question (yet!),
so let's leave it there ?

:) /sreten.

Richard C 29th November 2003 07:00 PM

Quote:

Originally posted by sam9
AB sounds like a good idea, but there is body of opinion that has an essential flaw that offsets the benefits with the result that pure B output stages workout a bit better.
Class-b can produce lower total distortion than a/b but since a/b pushes the crossover region higher up the curve it produces less distortion at those critical low levels than class-b. Also for very low distortion in class-b the bias setting is fairly critical requiring tight control of Iq, over-biased a/b is more forgiving in this respect:)

john curl 29th November 2003 09:51 PM

Heavy class AB is best in my opinion, and I design the majority of my power amps that way. There 'can' be a problem with the crossover from class A to class B. I first found this out 35 years ago, when I used too large of value emitter resistors in my first personal amp design. Heck, I thought that 1 ohm with a .5A standing current was OK, but NOOOOO! Actually I should have used a .05 ohm resistor or so, for best transition. This is because an emitter follower, when voltage driven, prefers a small emitter resistor that equates to about .015-.025V when the amp is idling. The problem with this solution, is that thermal runaway of the output stage is possible, UNLESS you do your bias thermal compensation right. Still, it is possible to get an almost perfect transition from class A to class AB if you do it right.

sreten 29th November 2003 10:24 PM

Hi JC,
 
I'm quite happy to argue the various merits of AB topologies
but given the original question I think its fairly pointless here.

:) /sreten.

P.S. I perfectly understand JC's post, but its not simple to
explain, and IMO not really relevant to the the original post.

millwood 30th November 2003 12:31 AM

Quote:

Originally posted by john curl
This is because an emitter follower, when voltage driven, prefers a small emitter resistor that equates to about .015-.025V when the amp is idling.

I have read about that value (15-25mv) here and somewhere else (in an Iq control IC from Linear I think but 30mv is also used). I never understood why that's case tho.

Any rationale for it, John or anybody else?

sam9 30th November 2003 03:45 AM

Anyway getting back to the original question, here are two kits described as class AB

http://info.hobbyengineering.com/specs/DIY-k48.pdf

or

http://www.aksaonline.com

Actually, I'm not sure there are all that many expressly Class-AB diy projects published although there are exables of Class A or Class B amps biased into AB (deliberately or otherwise). AB seems a lot more common anong tube amp builders.

Steven 30th November 2003 09:29 PM

Quote:

Originally posted by millwood



I have read about that value (15-25mv) here and somewhere else (in an Iq control IC from Linear I think but 30mv is also used). I never understood why that's case tho.

Any rationale for it, John or anybody else?

Have a look at http://www.diyaudio.com/forums/showt...900#post247900

Steven


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