USB DONE RIGHT: Two magic boxes that let computer audio ...

I’ve just read a review of two ultraluxurious
sedans, from Rolls-
Royce and Bentley. At one time
the two car brands were made by
the same company, though today they
belong to BMW and VW respectively.
Back then, the literature included no
information on engine power, or even
the number of cylinders (an urban legend
of the day claimed that the hood was
welded shut, and the company would
fly in an engineer if your car ever broke
down). You needed to know only that the
power was “sufficient to exceed any speed
limit in the world.” Today, Bentley says
its Mulsanne touring car’s V8 engine
develops 512 PS (505 bhp/377 kW), while
the “massive torque” (1020 Nm/712 lb ft)
is delivered at 1750 rpm. Got that?
I’m a little surprised that no amplifier
manufacturer has ever been as reticent
to advertise available power. Of course
if your amplifier will put out a kilowatt
you’d be silly not to feature the fact
prominently in your ads, but what about
those very expensive amplifiers with
available power of maybe 10 watts? Or
even less? Has no one ever been tempted
to say merely that an amplifier has “sufficient
power to drive any speaker made
today beyond concert levels?”
True, that could be dangerous,
because out there somewhere is, no
doubt, a speaker that requires energy
from a small substation to produce
more than a pitiful squeak. Still, just
as the power race in the auto world
overshadowed more significant technical
virtues, so the question of amplifier
power has often taken on an exaggerated
importance.
This is all the more evident today,
when efficient speakers are so much
in vogue, making smaller amplifiers
an interesting choice. Though it’s not
impossible to design a high-powered
amp that sounds transparent, the challenge
grows with every additional
amplifying device you add. Besides,
does anyone remember when a 25 watt
amplifier was a muscle amp?
But do those power ratings matter,
and do they even mean what they appear
82 ULTRA HIGH FIDELITY Magazine
State of the Art
by Gerard Rejskind
to mean? Let me return to the automobile
analogy again.
Consider an engine promising 450
horsepower. By any standard short of
Formula One that’s a lot of power, or
is it? Put that engine into a two-seater
sports car and, sure enough, you’ve got
yourself a mean machine. Put the same
engine into a giant SUV, like a Cadillac
Escalade or a Lincoln Navigator, and you
won’t be getting more than leisurely performance.
That’s because those horses
have a lot of steel and plastic to haul.
Put the motor into one of those pickup
trucks with the double rear wheels,
meant to pull a small yacht, and you’ll
find yourself struggling to make it up a
steep incline.
And so it is with amplifiers. Pick up
antique AR2 acoustic suspension speakers,
and your muscle amp may seem
barely adequate. Listen to uncompressed
recordings of large musical ensembles,
and suddenly you’ll be glad you set your
amplifier choice to “overkill.” Then
again, with a different speaker, with
other music choices, you may be content
with a tiny amount of power.
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And, as with cars, raw power doesn’t
tell the whole story. Power is voltage
multiplied by current, but some “powerful”
amplifiers can’t deliver adequate
current into real-life loudspeakers.
Similarly, some small but “powerful” car
engines have low torque, or can deliver
adequate torque only at high engine
speeds. It’s the torque that gets you to
cruising speed, and it’s the current that
drives the loudspeaker.
But there’s one more problem with
power ratings. Watts are a linear measure,
but loudness is logarithmic.
Double the power, from 50 watts to
100 watts, say, and the increase is only
3 dB (supposedly the smallest audible
increment, though we know that’s not
true). Some listening tests seem to indicate
that most listeners will judge a 10 dB
increase to be subjectively a doubling of
loudness, but in fact it’s a tenfold increase.
That’s right…increasing output from
10 watts to 100 watts sounds subjectively
like a mere doubling of loudness.
Some years ago there was an attempt
to develop a logarithmic scale for amplifiers,
and perhaps we should return to
that. Let us call the new unit the dBl
(“l” for loudness), and let us arbitrarily
set 0 dBl to correspond to an output of
1 watt. Then 10 watts would be 10 dBl,
and 100 watts would be 20 dBl. A monster
amplifier with an output of a kilowatt
would be rated at 30 dBl. The advantage
of such a rating would be that the numbers
would correspond, subjectively, to
what you could expect to hear.
I don’t expect to see the dBl adopted
by the audio industry, if only because
of the damage it would do to bragging
rights. Anyway, even a logarithmic
rating wouldn’t tell the whole story.
Many amplifiers appear to deliver twice
the lower into a 4 ohm load as they do
into an 8 ohm load, and some amplifiers
can sustain more current draw than
others. The interaction of amplifier
and loudspeaker shape the listening
experience.
But don’t be overimpressed with
those power ratings. Perhaps Rolls
Royce had it right all along.