[linux-audio-user] Re: Digital Fidelity

fons adriaensen fons.adriaensen at skynet.be
Thu Mar 2 06:53:50 EST 2006

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'. This doesn't imply there is
any DC in the output signal, since this is usuaully the
difference between the two halves, and so the DC cancels

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

> You're right about most gear using capacitors. Most
> transistor amps are Class B or AB which output AC.  Older
> tube amps, however are usually Class A to reduce the number
> of tubes, and like my dad's old Heathkit, have a big
> transformer on the output.

Transformers in tube amps are mainly there to do impedance
matching, not to block the DC. There won't be any DC going
into the transformers primary in a decently designed amp
anyway - it would saturate the transformer and make it
appear as a near short circuit to AC.


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