Pass Labs SR-1 User Manual

pass

TM

R1.2 -April 16, 2009

SR-1 Owner’s Manual

SR-1 Owner’s Manual

1

Introduction

The SR-1 (Son of Rushmore) is the successor to the Rushmore

loudspeaker system produced by Pass Labs between the years 2003

and 2008. The design goals are similar, but product is different.

The Rushmore aimed for high dynamic contrast and control with

very low distortion. This was achieved with the finest quality

“strong motor” drivers intimately coupled to single-ended Class A

amplifiers. Each driver (deep bass, midbass, midrange, and high

frequency ribbon) operated over a limited frequency band tailored

to its strengths by a passive filter at the input of each of the four

amplifier channels. The high efficiency (98+ dB/watt) cone drivers

also had very high power handling (up to 400 watts), and the ribbon

tweeter extended from the top octave and far beyond the audio

range.

The result was a stunning display of clarity and dynamics. With

each speaker driver enjoying its own amplifier and operated at small

percentage of its capacity, the control was precise and the curtains

of distortion parted to reveal phenomenally subtle inner detail. They

were accurate and musical. They kept their detail and dynamics at

loud and soft levels. Made from Gabro (a deep-sea granite) and

piano-formed Cherry layers, they weighed just under 400 pounds

each and cost a small fortune.

It’s six years later and we no longer make the Rushmore – the cost of

parts, material, processes and shipping have skyrocketed. In this time

we have developed its successor, unimaginatively christened SR-1.

The SR-1 is also a four-way system, and the target is still high

dynamic contrast, control and very low distortion. Realizing

that most customers already have favorite amplifiers or want the

opportunity to experience different electronics and cables, we

designed it with an internal passive crossover.

SR-1 Owner’s Manual

Having a network mediate between the amplifier and driver is not

without its problems. The loudspeaker drivers no longer have access

to source with a high damping factor, and the variable impedance of

2

the driver itself makes non-ideal load for the crossover. To retain

performance comparable to the Rushmore something has to give,

and that something is the overall efficiency of the loudspeaker

system.

First off, the drivers must be intrinsically lower distortion and more

neutral in their character. For this purpose we chose the best pieces

that SEAS has to offer, Nextel coated cones in staggered diameters,

topped off with the Crescendo high frequency dome. The Nextel

coating provides for improved internal damping of the radiating

surface, and we made a point of operating each driver in the “piston

region” below the cone’s modal frequencies. The SEAS drivers

feature long linear excursion, overhung voice coils, and exceptionally

linear magnet assemblies and pole pieces.

These are exceptional drivers, the best we could find, but they

still appreciate being driven by a low impedance source to achieve

the most precise transient quality, the instantaneous “stop-start”

response necessary to impart musical realism. Unfortunately the

very nature of a passive crossover introduces reactive impedances

between the amplifier and drivers, degrading the transient

characteristics and creating lingering artifacts. The result is that the

sonic image is smeared and the little details are obscured.

We experienced these phenomena while comparing various prototype

passive networks against an actively quad-amped version used as a

reference, and the difference was enough to make us choose a brute

force solution – resistively shunting most of the amplifier current

around the driver.

This gives the amplifier a less reactive, lower impedance load. It also

gives the crossover network a less reactive, lower impedance load.

Finally it provides the driver with a less reactive, lower impedance

source.

SR-1 Owner’s Manual

There is a price to pay for this – instead of a speaker whose audio

band impedance varies from about 8 to 30 ohms, the SR-1 varies

3

from 3 to 5.5 ohms. This means that the power amplifier will need

to be able to drive a 3 ohm load (we make amplifiers like that). On

the plus side, the amplifier sees a relatively flat impedance, and much

less of the reactance of the crossover and loudspeaker drivers.

After developing the initial versions of this crossover, we spent the

next two years refining the details of the sound with our five in-

house listeners and their individual environments. We established

the closest “average” desired response, and created adjustments in

mid-bass, midrange, and high end to accommodate a modest range

of room characteristics and taste. The final circuit contains only the

finest components – polypropylene capacitors from Kimber and

Solen, Mills resistors, and high current Erse inductors, all mounted

on thick heavily plated circuit boards. The wire is the same used in

the output stages in Pass amplifiers.

SR-1 Owner’s Manual

4

Unpacking

Caution: These boxes are heavy and require two people using

proper lifting techinques and care when unpacking.

Your SR-1 system comes double boxed with each cabinet in it’s

own double box. Within the double box, each speaker sits in rigid

foam top and bottom trays. We do not suggest that you attempt to

separate inner and outer boxes.

Starting with the two large boxes: Carefully cut the tape securing the

top end-flap on the outer box. Repeat for the inner box. If you

have done this correctly you will now be looking at the bottom of

the speaker, the lower foam tray and a box of hardware containing 4

feet and a jumper cable. Remove and retain the foam tray for future

use.

Once the bottom tray is removed, install the four leveling feet with

the protective washers between the feet and cabinet. The “no-mar”

washers are essential to protect the wood surfaces of the cabinet

from the anodized surface of the leveling feet.

Once the feet are installed, unpack the large (bass) cabinet by tilting

the packing on it’s side first and then titling so the bass cabinet is

resting on it’s feet. At this time lift the outer and inner boxes straight

up and off of the speaker. Retain these boxes as storage containers

for all additional SR-1 packing materials. Remove and retain the

top tray and foam sock. Now place the bass cabinet aas close to the

intended and final location. Once the smaller top cabinet is in place

further movement other than minor positional changes of the large

cabinet will not be convenient.

The top cabinet will be unpacked in a similar fashion. Carefully cut

the tape securing the tops of the outer and inner boxes. Once the

boxes are open you should again be viewing the bottom surface of

the cabinet and a box of hardware.

SR-1 Owner’s Manual

The smaller top cabinets require that the user install several pieces of

adjustment and dress hardware. All of these pieces will be installed

5