Audio Research LS27 User manual

3900 ANNAPOLIS LANE NORTH / PLYMOUTH, MINNESOTA 55447-5447 / PHONE: [phone] FAX: [phone]
www.audioresear h. om
Model LS27
STEREO PREAMPL F ER
Owner' s Manual

Contents
Model LS27 Page No.
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
nstallation of Vacuum Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Description of Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1, 2, 3
Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
nstallation nstructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Operating Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Tape Recording Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Servicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Disposal and Recycling Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Limited Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Model LS27
MODEL LS27
STEREO PREAMPLIFIER
WARNING
TO PREVENT FIRE AND SHOCK HAZARD, DO NOT
EXPOSE THIS DEVICE TO RAIN OR MOISTURE.
UNIT MUST BE OPERATED IN A HORIZONTAL POSITION.
- DO NOT OPERATE WITH COVERS REMOVED-
UNIT CONTAINS VOLTAGES WHICH MAY BE HAZARDOUS.
PLYMO TH, MINNESOTA
MADE IN . S. A.
50 W MAX
50/60 HZ ˜
SERIAL VOLTS
3A SLO-BLO
T2A (230V)
+12VDC
O T
CAUTION
FOR PROTECTION AGAINST FIRE HAZARD
REPLACE ONLY WITH SAME RATING FUSES.
T NER PHONO CD VIDEO A X 1 A X 2 MONITOR PROCESSOR RECORD O T MAIN O T 1 MAIN O T 2
T NER PHONO CD VIDEO A X 1 A X 2 MONITOR PROCESSOR RECORD O T MAIN O T 1 MAIN O T 2
LEFT
RIGHT
VACUUM TUBE PREAMPLIFIER M O D E L L S 2 7 H IG H D EFI NI T IO N ®
POWER MONO BAL/SE PROC MONITOR MUTE
D I S P L A Y
VOLUME INPUT
D E F I N I T I O N S E R I E S
CC

Model LS27
CD
PHONO A X1
MON
A X2
TNER VIDEO
DISPLAY POWER
M TE VOL DN VOL P
BAL L BAL R
BAL/SE HO RS
INVERT
MONO
GAIN PROC
Adjusts illumination of display to
eight levels plus off. Turns power on/off.
Select one of six available audio inputs.
Adjusts volume up. Visible as numeric
readout in display window.
Adjusts volume down. Visible as
numeric readout in display window.
nput for external A–V processor.
Provides unity gain, disabling volume
controls. NA appears in display window.
Sums L and R input for mono output
from any source selected.
Displays number of operating hours
vacuum tubes have been in service.
Mutes output of the LS27.
Select as appropriate for Balanced (BAL)
or Single-Ended (SE) input for each source.
Selects low, medium or high output gain
settings for each input source.
Adjusts balance left. Bar graph
indicates balance in display window.
Adjusts balance right. Bar graph
indicates balance in display window.
nverts absolute phase of all outputs.
Routes the monitor input signal to
the main outputs.
PLYMO TH MINNESOTA .S.A.
ln
NOTE: All LS27 functions are
available on the LS27R hand held
remote controller. All functions
are indicated in the display win-
dow as they are selected. See
“Description of Controls” in this
manual for more information.
MODEL LS27

Preface
Please take time to carefully read and understand
the following instructions before you install or
attempt to operate your Audio Research LS27 pre-
amplifier. Becoming familiar with the product and
its correct operating procedures will help assure
you of maximum musical enjoyment and reliable
operation. The effort you invest now will be well
rewarded in the years ahead.
Warnings
1. To prevent fire or shock hazard, do not expose
this product to rain or moisture.
2. This unit operates on voltages which can cause
serious injury or death. Do not operate with cov-
ers removed. Any necessary servicing should be
carried out by your authorized Audio Research
dealer or other qualified electronics technician.
3. The power cord supplied with this unit is safety-
tested and is equipped with a proper grounding
plug. f used normally, it will provide a safe earth
ground connection of the chassis. Defeat of the
grounding plug or use of a power cord without a
grounding plug, or any unauthorized modification
of the active circuitry or controls of this unit, auto-
matically voids warranty coverage, and could
cause injury or death.
4. For safe operation and protection against fire
hazard, replace fuses only with those of the same
type and rating as those supplied with this unit.
Packaging
Save all packaging accompanying this product. You
have purchased a precision electronic instrument,
and it should be properly cartoned any time ship-
ment becomes necessary. t is very possible that this
unit could be damaged during shipment if repack-
aged in cartoning other than that designed for it.
The original packaging materials help protect your
investment from unnecessary damage, delay and
added expense whenever shipment of this unit is
required.
nstallation of Vacuum Tubes
Note: This unit has been shipped with the vacuum
tubes installed in a protective foam block under the
top cover. Using a phillips-head screwdriver to
loosen the fastening screws, remove the top cover
and set aside. Remove the tubes carefully from the
protective foam block and store the foam block with
the carton.
Warning!
Do not attempt to install or replace vacuum tubes
unless this unit is first turned off and disconnected
from A.C. power outlet. Unit should be connected to
A.C. outlet and turned on only after all vacuum
tubes have been installed and appropriate panels
replaced and fastened. See owner's manual for
installation details.
Before replacing vacuum tubes, turn off unit, discon-
nect from A.C. outlet and allow existing tubes to
cool before attempting to remove and replace with
new tubes. Hot tubes may cause burns or other
injury if not first allowed to cool.
When replacing tubes, do not touch other compo-
nents, as the circuit may retain sufficient levels of
voltage and current to cause injury or death.
Contact Audio Research Customer Service
(763-577-9700) if you have any questions about
installing or replacing vacuum tubes in this unit.
Description of Controls
Front Panel Rotary Controls
The LS27 has two microprocessor-driven rotary
controls.
The rotary control on the left adjusts Volume (output
level) up or down for both R and L channels.
Volume control is also selectable via Vol DN and
Model LS27
1
RE AR
FR ON T
V 2 V 1
TUBE LOCAT ONS
TOP V EW
DO NOT ATTEMPT TO OPERATE TH S
EQU PMENT BEFORE NSTALL NG THE NUMBERED
VACUUM TUBES N THE R RESPECT VE SOCKETS.
Diagram indicates relative positions of two 6H30 tubes
located on circuit board as viewed from the front and
looking down from above the preamplifier.

Vol UP buttons on the remote control. Volume
adjustment is indicated in the display window by
numeric digits on 0–103 scale.
Do not turn volume up beyond normal listening lev-
els when “mute” is engaged to avoid unexpected or
possiblydamaging sound levels. Reduce volume
level whenever changing input sources, even when
muted.
The rotary control on the right selects any of six in-
put sources connected to the preamp: CD, Video,
Aux 1, Aux 2, tuner and phono. Each of these
sources may also be selected via direct access
buttons on the remote control. The selected source
is indicated in the display window.
Front Panel and LS27R Remote
Control Buttons
POWER ON/OFF: Supplies power from A.C. wall
outlet to preamp; indicated by active display win-
dow. Function also on remote control.
For approximately 50 seconds after start up, “Mute”
appears in the display window and flashes until the
automatic muting cycle is completed. This also
occurs in the event of power interruption. Upon
completing the cycle, “Mute” will cease flashing.
The preamp will remain muted until normal opera-
tion is selected by pressing the “Mute” button.
MUTE: When activated, electrically mutes all output
of the preamplifier; indicated by “Mute” in display
window. This control should be activated before
switching inputs, changing connections or shutting
down your audio system to help protect your ampli-
fier and speakers from unexpected signal pulses.
When deactivated, “Mute” disappears from the dis-
play window allowing normal operation. Function
also on remote control.
The LS27 also has automatic muting to help protect
system components during A.C. power interruptions
or low line voltage. When sensing these conditions,
the preamp automatically goes into “Mute” and dis-
ables all outputs. The 50-second warm-up timer will
restart and “Mute” will flash in the display window
when normal power conditions have been restored.
Note that automatic muting is only designed to pro-
tect against power line interruptions or severe volt-
age drop. t will not mute in the event of subsonic
transmissions from a faulty input source, amplifier
failure or speaker malfunction.
PROCESSOR (PROC): May be selected by button on
front panel or on remote control. Selects external
A-V (home theater) processor as input source
and controller of volume level for R and L channels.
LS27 provides Unity Gain when this input is
selected.
When engaged, “PROC” and “UN TY GA N” will
appear in the display, and the volume control is not
adjustable because gain is fixed. The preamp will
also mute the input after “PROC” is engaged.
WARN NG: Before unmuting t he “PROC” input,
be sure to turn the volume down on the home-
theater processor. Never connect a source with
a fixed level output (DVD player, tuner, CD, etc.)
to the PROC input or the amplifier(s) and speak-
er system could be damaged when the LS27 is
unmuted.
VOLUME UP/ DOWN: ncreases/decreases overall
volume level of both channels equally for whichever
input has been selected. Caution: do not turn up
volume beyond normal listening levels when “Mute”
is engaged to avoid unexpected or possibly damag-
ing sound levels. t is a good idea to reduce volume
whenever changing program sources, even when
muted. Function available on remote control buttons
and front panel rotary control on left. .
BAL/SE: Select as appropriate for each input. For
source components having balanced (XLR) connec-
tions to the LS27, select BAL. For single-ended
(RCA) sources, select SE. Once selected for an input
source, the setting is memorized.
MONO: Sums L and R input for mono output from
any source selected. Remembers setting for each
input.
MON TOR (MON): Selecting MON TOR
(“MON TOR” appears on display) routes Monitor
input signal to the Main outputs.
The nput Selector indicates what is routed to the
Record out. To exit the monitor position and return
to program source controlled by input selector (CD,
Tuner, Phono, etc.), press monitor switch again.
Note: The gain of the LS27 through the monitor in-
put is not adjustable by gain selector button (on the
remote), and is fixed at “High” gain (the gain setting
on the display is unlit in the “Monitor” position).
LS27R Remote Control Only
D SPLAY: Selects one of nine display illumination
levels, including off. When off, the display can be
reactivated for 10 seconds at a low level by press-
ing any button (except Power) on remote or control
on front panel.
Model LS27
2

BAL L/ BAL R: Adjusts balance left or right. Adjust-
ment appears in display window as movable mark-
er on bar graph. Useful for compensating for un-
even speaker placement or imbalanced output from
a phono cartridge.
HOURS: When pressed, displays number of operat-
ing hours vacuum tubes have been in service.
WARN NG: after replacing tubes, this function
should be reset to “0” only by a qualified technician.
Hours display can be reset to “0” by pressing black
button labeled “Reset Hours” located on
lower right corner of circuit board directly behind
display window. The preamp must be “On” to reset
function–the service technician must be careful to
avoid any contact with other circuit components
during reset operation. Contact Audio Research
customer service (763-577-9700) for information
regarding this procedure.
GA N: The Gain selector limits the input level to the
LS27, with resulting Low, Medium and High labeled
gain option settings to the outputs. Note that the
gain to the single ended outputs at each of the three
settings is half of the gain to the balanced outputs
at the same setting. The gain options allow limiting
the output of “hot” high-gain signal sources to the
preamplifier's input circuitry and prevents potential
overloading. Properly used, this results in extremely
wide signal input range capability while preserving
optimum use of the Volume control. For each input
source, simply set the Gain selector at the highest of
the three gain settings viewed on the display that al-
lows you to use the Volume control over the desired
range without audible distortion on peak levels
from your program source. Once you have selected
the optimum gain setting for a given input, the LS27
will “memorize” it and return to that gain setting
each time you select that input. Note that the Gain
selector defaults to the “High” setting for each input
unless another is selected.
NVERT: nverts absolute phase of the output signal.
Remembers setting for each input.
Connections
NPUT CONNECTORS: The LS27 provides eight
pairs of BAL/SE input connectors: CD, Tuner, Phono,
Video, Aux 1, Aux 2, Monitor and Processor. The
BAL/SE selector button (front panel and on remote)
should be selected to match the type of connection
used (BAL or SE) on each input.
Connect either the BAL or SE connectors for each
input. Never connect the BAL and SE connectors
simultaneously for any input.
OUTPUT CONNECTORS: Two pairs of BAL/SE
main outputs are provided.
RECORD OUTPUT: The BAL/SE tape outputs should
be connected to your recorder's “Record” or “Line”
inputs. These outputs provide a fixed-level two-
channel signal (R, L) to your recorder from whichev-
er input is selected. The non-variable output level
will be the same as the output of the selected
source.
t is possible to dub or copy from one recorder to
another by connecting the output of the source
recorder to an unused set of stereo inputs (Aux 1,
Aux 2). The signal will then be routed to the Record
Out connectors when that input source is selected.
12V N/OUT REMOTE START JACKS: The +12V DC
input/output jacks provide the ability to remotely
turn on and off other linked components such as
power amplifiers having similar capabilities.
nstallation nstructions
While the LS27 does not dissipate an unusual
amount of heat, it is important that it be provided
with reasonable airflow to assure long, trouble-free
operation. n addition, the following installation
guidelines will help insure maximum sonic perfor-
mance as well as reliable service:
1. Operate unit in upright and horizontal position,
preferably on solid, non-metal shelving.
2. Do not stack the preamp on top of a power am-
plifier: not only could this cause overheating, but
hum may be introduced into the preamp from the
proximity of the amplifier's power transformer.
Do not stack components or other objects directly
on top of the LS27.
3. Do not place or operate your preamp on a soft
or irregular surface such as a rug. This will pre-
vent proper ventilation.
4. Do not operate your preamp without the top and
bottom covers installed. These are required both
for safety as well as shielding from interference
(except in service operations by qualified person-
nel).
Operating Procedure
Start-Up:
1. Secure all rear-panel connections between LS27,
power amplifier(s) and input sources.
2. Plug three-prong power cord from rear of chassis
into grounded AC wall outlet. The Power switch
defaults to “off ” when the unit is plugged into a
power receptacle.
3. Press power switch (either remote or front panel).
Model LS27
3

“Mute” will will flash in the display window for ap-
proximately 50 seconds, then cease flashing and re-
main in the display. Press “Mute” to initiate opera-
tion.
4. Select input source, then BAL or SE as necessary.
Shut-Down:
1. Activate “Mute” function.
2. Set Volume level to “0”.
3. Turn off power amplifier(s).
4. Press Power switch to “off ”.
5. Turn off input sources.
Tape Recording Procedure
When using the LS27 as a control center for record-
ing, the program source to be recorded must be
connected to one of the six inputs controlled by the
nput selector switch or to the Processor input. This
routes the selected program to the Record output.
Note: f the Processor input is activated by selecting
the “Processor” switch position, all other inputs will
be deactivated. When recording from the Processor
input to the Record out of the LS27, the level of the
recording will be dependent on the level setting of
the processor.
f you own a three-head tape deck, and wish to
monitor the actual tape while making a recording
(for a true “A-B” comparison of signals before and
after recording), connect the tape deck output to the
Monitor input.
t is also possible to dub from one tape deck to an-
other. Simply connect the output from one tape deck
to an unused set of inputs controlled by the nput se-
lector (Tuner, Phono, CD, etc.) on the LS27. This sig-
nal will then be routed to the second tape deck
through the Record output when the appropriate in-
put is selected on the nput selector.
Troubleshooting:
t is possible for a static discharge to temporarily
disable the microprocessor that allows the controls
to function. f this should occur, depress the “Mute”
button, then turn off the preamp and unplug the pre-
amp from the A.C. receptacle for 60 seconds. Re-
connect the A.C. plug and turn the preamp back on,
along with other components. The controls should
resume normal operation. f the problem persists,
contact your dealer or Audio Research Customer
Service at 763-577-9700.
Servicing
Because of its careful design and exacting stan-
dards of manufacture, your LS27 should require no
routine maintenance. The remote control's batteries
(AAA) should be replaced once a year.
CAUT ON: Your LS27 contains sufficient levels of
voltage and current to be lethal. Do not tamper with
a component or part inside the unit. Refer any need-
ed service to your authorized Audio Research deal-
er or other qualified technician.
The vacuum tubes inside your LS27 are quality 6H30
dual triode tubes and with normal use should not
need to be changed for approximately 4,000 hours
of use. Replacement tubes should be of equivalent
quality and are available from Audio Research.
Note that tube numbers (V1 and V2) match printed
numbers next to tube sockets on circuit board.
Should service be necessary, please contact your
Audio Research dealer, or Audio Research Cus-
tomer Service at 763 577-9700.
Cleaning
To maintain the new appearance of this unit, occa-
sionally wipe the front panel and top cover with a
soft, damp (not wet) cloth to remove dust. A mild,
non-alkaline soap solution may be used to remove
fingerprints or similar smudges. Cleaners containing
abrasives should not be used as they will damage
the anodized finish of the front panel. A small, soft
paint brush is effective in removing dust from
bevels, the recessed nameplate and other features
of the front panel.
Disposal and Recycling Guidelines
To dispose of this electronic product, do not place
in landfill. n accordance with the European Union
Waste Electrical and Electronic Equipment (WEEE)
directive effective August 2005, this product may
contain regulated materials which upon disposal re-
quire special reuse and recycling processing.
Please contact your dealer or importing distributor
for instructions on proper disposal of this product in
your country. Or, contact Audio Research Corpora-
tion (763-577-9700) for the name of your importing
distributor and how to contact them.
Packing and shipping materials may be disposed of
in a normal manner.
4
Model LS27

Limited Warranty
Audio Research Corporation products are covered
by a 3-Year Limited Warranty, or a 90-Day Limited
Warranty (vacuum tubes). This Limited Warranty ini-
tiates from the date of purchase, and is limited to
the original purchaser, or in the case of demonstra-
tion equipment, limited to the balance of warranty
remaining after original shipment to the retailer or
importer.
n the United States, the specific terms, conditions
and remedies for fulfillment of this Limited Warranty
are listed on the warranty card accompanying the
product in its shipping carton, or may be obtained
from the authorized retailer or from the Audio
Research Customer Service Department. Outside the
United States, the authorized importing retailer or
distributor has accepted the responsibility for war-
ranty of Audio Research products sold by them. The
specific terms and remedies for fulfillment of the
Limited Warranty may vary from country to country.
Warranty service should normally be obtained from
theimporting retailer or distributor from whom the
product was purchased.
n the unlikely event that technical service beyond
the ability of the importer is required, Audio
Research will fulfill the terms and conditions of the
Limited Warranty. Such product must be returned at
the purchaser's expense to the Audio Research fac-
tory, along with a photocopy of the dated purchase
receipt for the product, a written description of the
problem(s) encountered, and any information nec-
essary for return shipment. The cost of return ship-
ment is the responsibility of the purchaser.
Specifications
FREQUENCY RESPONSE: +0-3dB, 0.2Hz to 160kHz at
rated output (Balanced, 200k ohms load)
D STORT ON: Less than .01% at 2V RMS BAL output.
GA N: Main output: Selectable for each input: 24dB,
18dB,
12dB Balanced output. (18dB, 12dB, 6dB SE
output.)
Record output: 0dB (Processor input: 0dB
balanced).
NPUT MPEDANCE: 120K ohms Balanced, 60K ohms SE.
nputs (8): Tuner, Phono, CD, Video, Aux 1, Aux 2, Moni-
tor, Processor (XLR and RCA connectors).
OUTPUT MPEDANCE: 700 ohms Balanced, 350 ohms SE
Main (2), 20K ohms minimum load and 2000pF maximum
capacitance. Outputs (3): 2 Main, 1 Record (XLR and RCA
connectors).
OUTPUT POLAR TY: Non-inverting.
MAX MUM NPUT: Low Gain: 48V RMS BAL, 24V RMS
SE, Medium Gain: 24V RMS BAL, 12V RMS SE, High
Gain: 12V RMS BAL, 6V RMS SE.
RATED OUTPUTS: 2V RMS 1Hz to 100kHz into 200K ohm
balanced load (maximum balanced output capability is
15V RMS at less than 0.5% THD at 1kHz).
CONTROLS: Rotary volume selector (104 steps) and
rotary input selector.
Push buttons: Power, Mono, BAL/SE, Proc, Monitor, Mute.
Also remote buttons: Display, Bal L, Bal R, Hours, nvert,
Gain.
POWER SUPPL ES: Electronically-regulated low and high
voltage supplies. Automatic 50 sec. warm-up/brown-out
mute. Line regulation better than .01%.
NO SE: 1.3uV RMS residual HF weighted balanced
equivalent input noise with volume at 1 (100dB below 2V
RMS output).
TUBE COMPLEMENT: 2-6H30P dual triode, (Hybrid
JFET/ tube audio circuit, solid-state power supply.)
POWER REQU REMENTS: 105-130VAC 60Hz (210-
260VAC 50/60Hz) 50 watts maximum.
D MENS ONS: 19" (48 cm) W x 5.25" (13.4 cm) H
(standard rack panel) x 13.3" (33.8 cm) D. Handles
extend 1.50" (3.8 cm) forward of the front panel.
WE GHT: 16.3 lbs. (7.4 kg) Net; 23 lbs. (10.5 kg)
Shipping.
Specifications subject to change without notice.
©2010 Audio Research Corporation.
Model LS27
5