[linux-audio-user] Re: Re: Digital Fidelity (fons adriaensen)

Phil Mendelsohn phil at rephil.org
Thu Mar 2 11:25:49 EST 2006

Fons said:

> On Thu, Mar 02, 2006 at 03:04:29PM +0900, res0u2uc at verizon.net wrote:
>> Hmm. Well, my knowledge here isn't great, but I think having
>> a capacitor at the output of a Class A amplifier would
>> prevent it from operating with a DC offset.
> No. The essential point of (balanced) Class A is that both
> halves of the output stage (the one giving the + and
> the one giving the - voltage) are both active all the
> time and never 'cut off'.

Sorry -- I have to split a hair here.  You're describing Class AB.

Class A means that you are never out of the linear region of conductance
for the semiconductor.  This means that there are no "halves" to an output

> Anyway the DC component required for Class A would burn
> your speakers in a fraction of a second.

Only if you run from 0 to Vmax with a positive DC bias.  You can run a
class A device between positive and negative rails so that the output has
no DC offset.

There's no free lunch, though -- you want to consider a lot of practical
things that outweigh the "pure" approach -- boy, your devices had better
be thermally stable, class A is pretty wimpy for power dissipation, what
happens to your DC offset when your output device fails, and a lot more
that have little to do with Linux, so I'll stop there...

Upshot:  It's easier to make class A preamps than power amps.

("Man!  I can't believe you got me monologuing again!")
--The Incredibles

Phil M

Dept. of Mathematics, 342 Machray Hall
U. of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
Office:  446 Machray Hall, 204-474-6470
http://www.rephil.org/   phil at rephil dot org

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