Recently there have been one or two posts wondering why the headphone
jack of amplifier X produced audible white noise at a constant volume
all the time. People were told that the amp was defective, but I don't
think this is correct.

I believe I know the answer. Some amplifiers of recent vintage (such as
my AMC) have been skimping on parts, and a new design implementation
that is apparently on the rise is to drive the headphone jack with the
speaker power, instead of a separate mini power amp. The result is that
the amplifier's *effective* signal-noise ratio (assuming you don't want
to listen at 130 dB) is ruined when you strap on a pair of headphones,
because it is literally like strapping the speakers to your ears. We
all know that if you put your ear next to the tweeter while the amp is
on, you hear a hissing noise of perhaps 30 dB volume.

I bought a headphone amplifier to eliminate this problem. I couldn't
stand the noise using my amplifier's headphone jack.

-Sean