In article , Rob
wrote:
Trevor Wilson wrote:


**The magic words in this statement a "...are properly designed and
manufactured.."


I think the point here is that virtually all amplifiers from major
manufacturers are properly designed and manufactured - that's what I'm
led to believe.

It us hard to be sure due to lack of info, but in many cases you are
probably correct. However two amplifiers might deliver indistinguishable
results under some conditions of use, but give different results in other
conditions of use. The most obvious example being a change in speakers and
room, leading to much higher power levels being required.

**Sure. Measuring speakers into actual loudspeakers can reveal
differences which may be quite audible.


That sounds fair enough - is there a particular measurement that stands
out?

For me the obvious ones are the output impedance of the amp, and its
ability to deliver high peak or steady currents with minimal
buzz/distortion. Some reviews give the 4 (and 2) Ohm power levels as a
guide for this, but I would tend to prefer the medium-term current that can
be delivered. e.g. something like 'can deliver X amps RMS for 100 ms' with
the conditions specified.

In some cases you might need to check the stability of the amp, but I'd
hope this would rarely be a concern these days.

Alas, obtaining a full and useful set of results is time-consuming, and
requires both the test gear and the reviewer knowing what the results mean.
It also means they might have to explain their usefulness rather than
simply giving their 'wine tasting' opinions. It also means editors not
worrying that readers would be put off by being expected to read and
understand so as to be able to decide for themselves. Simpler just to
present a 'golden eared opinion'... Even if it has no actual relevance for
the reader. :-/


1) Valve amps tend to clip (Voltage limit) rather gracefully. Thus
they can be driven further into clipping, without obvious 'nastiness'
and an increase in average SPL. 2) Valve amps tend to possess a more
benign current limit characteristic, due to the abscence of current
limit systems, which are normally used in (BJT) transistor amps. 3)
Valve amps, by virtue of their higher Voltages, tend to have quite
large energy storage systems (the equation is: 1/2CV^2), though the
actual capacitance seems to be quite small.

In addition, a valve design may store energy in a series inductor in the
PSU.

However some transistor amps may have quite large amounts of energy and
charge stored in the PSU caps - or may simply use a stabilised supply or be
able to use an extended portion of the mains waveform. Whatever, it just
needs to be 'more than enough' to supply the required audio power. 8-]

Slainte,

Jim

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