PassLabs Rushmore Brochure

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Pass Rushmore

Background

In the 1950’s most audiophiles only had
maybe 10 watts of tube amplifier available, but
they achieved realistic concert performances
with loudspeakers whose design emphasized
powerful and efficient motor assemblies and
diaphragm materials chosen for musicality, low
mass, and large radiating surfaces. Enclosures
were designed with the same thought that goes
into musical instruments, with a live harmonic
characteristic and an appreciation for fine
wood craftsmanship. Many of these products
endure as classics, and are still highly prized by
audiophiles as treasures from a golden age. They
were as much a result of refined taste and trial
and error as they were science and engineering.

For years we have asked ourselves why these
old designs are so good, and why modern
high-end audio does not show all the audible
improvements to be expected of 50 years of
technological advances.

In the 60’s and 70’s, high power amplifiers
became available, and loudspeaker design took
a left turn onto the road it follows today. With
mighty solid state amplifiers at the designer’s
disposal, efficiency was no longer an issue, and
the design goals revolved around raising the

power handling ratings and sacrificing efficiency
in order to deliver low bass frequencies in small
enclosures. Never mind that the bass was
boomy and that the music sounded like it was
pushed through a sock; it fit on a shelf, and it
used up the higher power of the amplifier the
industry was eager to sell.

Many of the differences between then and
now are obvious. With efficiency as a priority,
classic high-end loudspeakers had sensitivities
in the range of 100 dB/watt. The old designs
used expensive magnetic circuits and tightly
toleranced motor assemblies to achieve high
force from a small amount of electrical current,
and they coupled these to lightweight paper
cones whose sonic signature was the result of
much trial and error – more art than science and
engineering.

Today, most speakers are about 87 to 92 dB/
watt, which is about 1/10 the acoustic output of
the old classics. This is the difference between
10 and 100 watts of amplifier for the same
level. The cones are heavy and the magnets
are working into wide voice coil gaps. Why is
this? It costs a lot less to do it this way, and
also loudspeaker enclosures can be made less

conspicuous while retaining some low frequency
response. Much of loudspeaker science operates
on the presumption of the cone material as a
rigid piston, which plays well into the use of
heavy, thick materials in order to achieve the
character of a piston. The high mass of the
cone results in slow attack and decay response
to impulses from the amplifier, but this has been
considered an acceptable trade off. Of course,
there really is no such thing as a loudspeaker that
acts as a true piston.

The old designers knew they were never going
to get a really rigid neutral piston, so they
researched cone materials that were light, well
damped, and whose deviations from the ideal
were at least musical. This philosophy was
in keeping with the approach to the old tube
amps as well; they didn’t measure that great,
but their faults were at least musical and fairly
inoffensive. The old designers measured and
listened carefully, and were persistent. Most of
them had taste, and they knew what they wanted
when they heard it.

These light diaphragms and efficient motors
have a very dynamic quality. From silence they
spring to life in response to musical transients.
Well done, they articulate infinitesimal details
and have a warm, spacious, easy character. The
paradox is, of course, that modern designs in
many ways are not as sonically pleasing as the
old classics. For all the power available, they
have traded off dynamic range, transient attack
and decay, and articulation. They have sold their
musical souls, and they sound uneasy about it.

The old speakers came in big enclosures, made
of spruce, maple and other acoustically live
woods designed to get the last bit of bottom
end performance from a big lightweight paper
cone. Large bass-reflex boxes and horns filled
audiophile listening rooms. The wood in the
enclosures was flexible and had a sonic signature
of its own. Like the material used in the paper
cones, it was chosen for sonic harmony with the
drivers, and was the object of craftsmanship in
construction and finish.

Today? Monkey caskets: Medium density
fiberboard, or worse, particle board rules the
marketplace. It’s cheap, easy to machine, and
is supposed to be acoustically dead. Actually,
it pretty much is…… dead, lifeless and
uninteresting.

As speakers have gotten less efficient, amplifiers
have gotten bigger and more powerful. In
an evolution similar to speakers, amplifiers
achieved better specifications through the use
of more complex circuitry and greater amounts
of feedback. The old simple ways of building
good amplifiers gave way to a specifications race,
and similar to the loudspeaker paradox, we find
ourselves with complex circuits achieving lots
of feedback in order to correct for the poorer
linearity of more complex circuits.

The big difference between then and now is that
much of the industry relies more on science and
engineering than persistence and good taste in
the development of products. Even the most
ardent subjectivist designer has a rack of test
equipment, and he keeps at least one eye on it
all the time.

The Pass Rushmore loudspeaker design
originated with speculation as to how
loudspeakers would have evolved if in the 60’s
designers had stayed with high efficiency drivers,