On Fri, 03 Oct 2003 16:09:47 +0100, Jim Lesurf
wrote:

In article , Stewart Pinkerton
wrote:
On Thu, 02 Oct 2003 19:56:27 +0100, Laurence Payne
wrote:


Any really good amplifier will be able to handle full power
more-or-less continuously - certainly for several minutes - without a
problem.

And wouldn't object to being asked to deliver considerably more if
momentarily demanded? You seem to be arguing against transient
headroom :-)

Anyone who knows anything about amps *does* argue against 'transient
headroom', which is just another word for a cheap power supply.

In that case, I'm afraid I don't know much about amplifiers. :-)

Oh, come now, that can't be true! :-)

My own view/experience is that it can be quite useful in the context of
reproducing musical signals to arrange for the amp/PSU to have a short-term
power ability that is well above the sustained power ability. The reasons
for this in my view are;

1) Most music tends to include relatively brief musical transient events
which reach short-term levels well above the mean level.

Fair comment, *but* 'relatively brief' is generally (a lot) more than
20 milliseconds, so that it's still necessary for the power supply to
be fully recharged by the mains.

2) Allowing the rail voltages to fall under sustained demand helps reduce
the power dissipation in the output devices compared with what you get if
you try to maintain the same rail levels as existed for short-term high
power bursts. This keeps the device tempeatures down, etc, which can be
useful.

That's true, but also leads to bias changes in many amps, meaning that
the sound can have a different character after loud passages. Note
that I'm referring to pretty poor amps here - but then you've pretty
much confined the field to such amps, with your pre-qualification of
extremely saggy power supplies. I take it that we're not dealing with
Class G multi-rail amps here?

The main disadvantages are in my view:

1) The sustained mean power levels measured in a review tend to be
under-reading the actual ability (e.g. the 700 I designed only rated at
200Wpcs 'rms' sic both channels sinewave driven, but could actually
deliver around 300Wpc mean per channel for a tenth of a second or so. (This
is not the peak 'instananeous' transient value, which I'd agree is
misleading.) Thus in terms of actual use the 200Wpc perhaps didn't really
indicate how much the amp could provide with music.

2) You have to ensure the amp does not produce distortion or become upset
in some way when the rails fluctuate. This seems sensible, anyway, to me,
as it avoids things like distortion crosstalk between channels.

I'm interested in the logistics of an amp with such a disparity
between its 'toneburst' and continuous ratings. Was this a PSU
problem, or just thermal dissipation in the output stage?

I should say that for myself, I much prefer a 'stiff' power supply,
with all that means in terms of coping with tough loudspeaker loads.
This may be overkill for many systems, but it gives me confidence that
thew sound of the amp won't change under any conditions of load or
musical dynamics.
--

Stewart Pinkerton | Music is Art - Audio is Engineering