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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,
industries. We used the prototype XVR1
crossover network to mate these to each other
in test systems in order to evaluate each driver
objectively and subjectively, separating the wheat
from the chaff. Over the course of two years of
testing, we isolated a few candidate components
from hundreds. No horn driver made the cut,
nor did any dome. The bass to upper midrange
drivers that met our requirements ended up
all paper cone type drivers, and only ribbon
tweeters satisfied us at the top two octaves.
We spent a year with the permutations of
systems which could be built with these drivers
in combination with each other, tweaking,
listening, and measuring, until we settled on the
best combination of parts and the crossover
and amplifier characteristics most ideal to each
component.
The result is a four-way system, with a 15 inch
deep bass driver, a 10 inch mid bass driver, and a
6 inch midrange, and a ribbon tweeter. The bass
driver is 97 dB/watt efficient, and the remaining
drivers are at least 98 dB/watt. The cone drivers
are all high quality professional drivers rated at
high wattage levels which by coincidence work
very well together. As a group, they were the
best we could find.
relatively low amplifier power and live sounding
enclosures.
So what are we doing here that’s different?
Some parts of this design are similar to other
commercial offerings, but no product embodies
such a comprehensive integration of design
and technique with this level of taste and
performance.
Very Sensitive Drivers
When we began this project over three years
ago, we decided high efficiency drivers at least
96 dB efficient were the only ones likely to meet
our criteria for dynamic range, inner detail, and
transient response. This observation comes
through strictly personal experience with a
large number of successful classic designs.
We purchased or borrowed pairs of every
component that met this specification that
we could find, which included cones, horns,
domes, and ribbons. This was a lot of drivers,
drawn from both the consumer and pro audio
Single-Ended Class A power amplifier
devoted to each driver
The bottom end is powered by an Aleph X
balanced single-ended Class A amplifier, a cousin
of the newly released Pass XA series. Using the
two most important patented topologies available
to Pass Labs, it combines the warmth and
sweetness and detail of the Aleph series singleended output stage with the dynamic range and
control of the Supersymmetric topology of the
Pass X amplifiers.
The three mid-bass, mid-range, and highend amplifiers are conventional Aleph series
circuits, best suited to elicit the best qualities of
articulation and imaging, warmth and top end
sweetness of these very dynamic loudspeaker
drivers.
Having only two stages, these amplifiers embody
the audiophile quest for the most realistic sound
through minimalism and purity of signal path.
Single-ended Class A amplifiers have what is
known as a monotonic character; they continue
to improve as the power level goes down.
Thus, the first watt is the best watt, and they
give a great performance at ordinary volume
levels. This character greatly complements very
sensitive drivers, and Rushmore invites you to
listen at normal loudness. You do not have to
play music at abusive levels to achieve detail and
drama with this loudspeaker.
Each amplifier is adjusted for the specific
voltage, current and gain requirements of each
driver. The cone drivers are direct coupled to the
amplifiers with no intervening passive circuitry to
make maximum use of the damping provided by
the amplifier and to assure minimum distortion
and signal loss.
The heat sink for the amplifiers is a 4 ft. long
finned extrusion that embodies the rear of
the loudspeaker from bottom to top. During
operation, it reaches approximately 55 degrees
C. in the area containing the 20 power Mosfets
which populate the 4 channels of amplification.
The power supply to the amplifiers delivers a
continuous constant 300 watts to the Class A
output stage during operation. This draw is
constant regardless of signal level, and drops
only when the speaker is powered down. The
capacitor filter bank contains 240,000 uF
capacitance and uses dual CLC pi filters to
reduce ripple and noise by a factor of 20 dB
lower than a conventional unregulated supply.
The bass amplifier is rated at 80 watts into the
woofer, the mid-bass amplifier is 20 watts, the
mid-range amplifier is 20 watts, and the ribbon
tweeter amplifier is rated at 20 watts, all singleended Class A.
The absolute maximum output of the speaker
at 1 meter is approximately 120 dB with musical
material.
Quad Amp Crossover
The filter characteristics for each driver are
determined in passive filter networks prior to
each of the four amplifier inputs per speaker.
Their specific characteristic has been trimmed
and evaluated through trial and error over a long
period of time in many rooms with many listeners
and confirmed by measurement equipment. For
best transient and phase response, the mid-bass
and mid-range band-pass filters are single-pole
types, while the bass driver sees a 2-pole lowpass filter at 22 Hz and the ribbon tweeter is
crossed with 2 poles at 8 KHz. None of these
filters uses feedback, positive or negative.
The result is a flat response that falls off at 20
Hz on the bottom and 40 KHz, above the top
of the audio band.
The individual level of each driver is calibrated,
but each has an adjustable level control
allowing for +/- 3 dB adjustment for different
environments and taste.
The input of the speaker can be made from a
line level source into either a balanced input or
single-ended input at the rear of the speaker.
The input impedance is 20 Kohm balanced and
10 Kohm single-ended. The balanced input
common mode rejection offers 40 dB of noise
rejection.
The gain of the input through the output has
been calibrated to 100 dB output at 1 meter for
1 volt input.
Granite and Piano Enclosure
We believe that this much achievement should
be housed in the finest temple, so we went to
woodworkers versed in the wood forming and
finishing of pianos. The curved sides of the
Rushmore are made of 9 individual layers of
veneer material formed and cured in a vacuum
press and mated to a 1.25 inch thick solid front
baffle faced with 0.75 inch thick granite. The top
and bottom are panels of 0.75 inch thick granite.
The rear of the loudspeaker consists entirely of
an extruded, machined and anodized heat sink
which holds all the electronic components.
The front baffle of the speaker is made
acoustically dead so that the drivers feel a firm
unyielding mounting surface, but the rest of the
enclosure has been allowed to remain relatively
live, and contributes in its own subtle way to the
overall sound of the speaker.
The Rushmore weighs approximately 300 lbs,
is 50 inches high, 18 inches wide at its widest
point, and 28 inches deep. It has been designed
by Nelson Pass, Kent English, and Desmond
Harrington.
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Pass Laboratories, PO Box 219, Foresthill, CA 95631 - 530.367.3690 - www.passlabs.com
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