Conrad-Johnson LP70S User manual

Conrad-Johnson Owner’s Manual:
LP70S Vacuum-Tube Amplifier
conrad-johnson It just sounds right.

LP70S Owner’s Manual 1
Thank you for selecting the LP70S to power your audio system.
The LP70S draws on three decades of research at conrad-johnson
into the reproduction of recorded music. The result is a power am-
plifier with a remarkable ability to recreate the dynamics, textures,
tonalities, and ambience of live musical performances. With the
LP70S, we believe that you will experience the excitement of dis-
covery in hearing more from your favorite recordings than ever be-
fore.
At conrad-johnson, we expect our products to be a source of satis-
faction and of pride to their owners for many years to come. Ac-
cordingly, circuit designs, parts and materials for all conrad-
johnson products are selected with a view to maintaining optimal
performance over the years. Our reputation for producing among
the industry's most reliable components is a natural consequence
of this engineering approach.
Although operation of the LP70S is quite straightforward, please
take a few minutes to read this manual for useful information on it's
installation and operation.
In closing, we’d like to welcome you to the family of conrad-
johnson owners. We want you to enjoy your conrad-johnson prod-
uct to the fullest. To this end, our staff stands ready to answer any
questions you may have about the function and application of your
LP70S, and to provide any needed service both during, and after
the warranty period. Our goal is to heighten your enjoyment of re-
corded music.

LP70S Owner’s Manual 2
imited Warranty For Conrad-Johnson Components
Conrad-Johnson Design, Inc. will provide service under warranty to
the original owner on products sold new in the United States for the
lesser period of three years from the date of purchase by the origi-
nal purchaser, or five years from the date of shipment to the au-
thorized conrad-johnson dealer. During the warranty period, con-
rad-johnson will repair defective units without charge for labor or
parts (with the exception of vacuum tubes and batteries).
Exclusions. The following are not covered under this warranty:
a) Units which have been damaged by misuse, abuse, or acci-
dent.
b) Units which have been modified, altered, or improperly re-
paired by anyone not specifically authorized by conrad-johnson
design, inc.
c) Units not purchased from an authorized conrad-johnson
dealer in the United States for use in the United States.
d) Normal wear
e) Incidental or consequential damages are not covered under
this warranty. Some states do not allow the exclusion of inci-
dental or consequential damages, so this exclusion may not
apply to you.
Obtaining Warranty Service: To obtain warranty service, the unit
must be shipped, along with evidence of purchase, in factory pack-
ing to conrad-johnson design (or designated service center) with
freight and insurance prepaid by the owner. After repair, the unit
will be returned with freight and insurance prepaid by conrad-
johnson design to any destination in the United States.
All implied warranties, including merchantability and fitness for a
particular purpose are limited in duration to the duration of this ex-
press warranty. Some states do not allow limitations on the dura-
tion of implied warranties so the above limitations may not apply to
you.
This warranty gives you specific legal rights and you may also
have other rights which vary from state to state.
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LP70S Owner’s Manual 3
Conrad-Johnson products purchased outside the United States are
covered by warranty terms of the importing distributor in the coun-
try in which the product was originally purchased, which may differ
from the terms set out herein. Importing distributors are not obli-
gated to provide warranty service for products originally purchased
outside their country. Conrad-Johnson will provide warranty service
for products outside the United States, but in these cases, the cus-
tomer must pay all shipping, handling and customs costs both to
and from our Service Department.
Questions about this warranty should be addressed to:
Service Department
conrad-johnson design, inc.
2733 Merrilee Drive
Fairfax, VA 22031
The conrad-johnson service department can also be reached by e-
mail at [e-mail], by phone at 703-698-8581,
or by fax at 703-560-5360.
Service
If your conrad-johnson audio component requires service, repack it
using the original box and packing material and ship to the Service
Department address above. Boxes and packing materials can be
purchased through our service department if you no longer have
yours. Include with the unit a note describing the problem you are
having in as much detail as possible. It is especially important for
our technician to know if the problem is intermittent. If you want an
estimate of cost for out of warranty service, be sure to request it in
this note. Be aware that requesting an estimate will delay service
to your unit, as we will have to contact you for approval before
commencing service.
Registering The Warranty
Please return the enclosed card to the factory within 30 days of
purchase to register the warranty

LP70S Owner’s Manual 4
nstallation
As is the case with any vacuum-tube power amplifier, the LP70S
dissipates a significant amount of heat. Provision must be made for
adequate ventilation. Mount the unit on a flat, hard surface, taking
care that the ventilation holes in the bottom are unobstructed.
Maintain at least four inches of clearance above the unit, and at
least three inches on the sides. The cabinet or shelf should be
open at both front and back. For more restricted installations, cool-
ing fans are essential.
All LP70Ss sold in the United States are configured for operation
on a 60Hz ac power line producing between 108 and 126 volts.
Export versions of the LP70S will have the correct operating volt-
age and frequency clearly marked on the back panel of the unit,
near the ac power cord. In all cases, the actual line voltage should
be within + 5/-10% of the nominal rated voltage.
Electromagnetic Interference (EMI)
Considerable care has been taken in the design of the LP70S to
minimize its susceptibility to radio frequency interference and other
forms of EMI. Choice of materials, physical layout, grounding prac-
tice, and power supply design have all been specified with a view
to reducing the impact of electromagnetic fields on the perform-
ance of this unit. At the same time, however, our primary goal is
the accurate reproduction of recorded music in the normal home
environment, and we have elected not to compromise this objec-
tive by the application of heavy-handed RFI filters, or by using
grounding practices that reduce RFI at the expense of degraded
audio performance. We find that the approach we have taken has
worked extremely well, resulting in only rare instances of EMI prob-
lems which could be treated locally as needed, rather than com-
promising the performance of our product in the 99.9% of installa-
tions where EMI is not a problem.
Care in installation can often avoid EMI induced problems. The fol-
lowing practices should generally be observed in any application,
and will be especially important where EMI may be a problem.
Interconnect cables should be kept as short as possible (3 meters
or less), and shielded cable should be used (cable which has two
center conductors, and a separate external shield connected at
only one end).
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LP70S Owner’s Manual 5
Physical location and cable “dress” can be an important factor in
minimizing hum pickup. The installation should situate the pream-
plifier well away from the power amplifier, and power (ac mains)
cords should be dressed to remain at least 4" (100mm) away from
input/output cables.

LP70S Owner’s Manual 6
onnection
INPUT: Connect the preamplifier to your LP70S amplifier at the
RCA type connectors located on the back of the amplifier.
OUTPUT: Five-way binding posts are provided for connection of
your loudspeakers, with correct spacing to accept a standard "dou-
ble banana" plug. They will also accept spade lugs, bare wire, or
pin connectors.
The two channels should be connected in correct "relative phase".
This means that when the same signal is applied to both channels,
the right and left channels speaker diaphragms will move synchro-
nously -in and out together. Terminals on speakers are usually
coded -one designated "C", "ground", "-" or black, the other des-
ignated "+" or red. "In phase" connection of the speakers can nor-
mally be achieved by taking care to connect the wire from the "+"
amplifier terminal to the red or "+" coded terminal on each speaker,
and connecting the "-" terminal to the black or "-" coded terminal on
each speaker. In phase connection of the speakers can be readily
ascertained by ear. Listen to a recording of a solo vocalist (use a
mono recording if available). With the speakers in phase the voice
should be clearly focused between the two speakers. With the
speakers connected out of phase, the voice will be diffused, with
no identifiable source. Relative phase may be reversed by switch-
ing the "+" and "-" leads at one speaker only.
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LP70S Owner’s Manual 7
acuum Tube Replacement
The LP70S uses seven vacuum tubes of two different types: three
type 6922 (V1, V2 and V3), and four type 6550 (V4 through V7).
The brands of tubes we supply have been chosen by first selecting
those brands which are known to be most reliable, then by exten-
sive auditioning of these acceptable brands with the final choices
being made solely on the basis of sonic performance. We know of
no vacuum tubes available which will improve the sonic perform-
ance of your LP70S. We highly recommend that you purchase re-
placement tube sets from conrad-johnson design.
We anticipate tube life to accommodate two to three years of op-
eration without degradation in normal use -if the amplifier is
switched off when not in use. If it is left on at all times, tube life can
be exhausted in a matter of a few months.
The LP70S has been designed to make tube replacement as sim-
ple as possible. Because the output tubes are individually biased,
replacement tubes need not be matched pairs. No a-c balance ad-
justment is necessary. It is necessary to re-bias the amplifier when
the output tubes are replaced. The LP70S has built in bias indica-
tors so that no instruments are required to make this adjustment.
V
V5
V7
V1 V3V2
V4
V6

LP70S Owner’s Manual 8
Adjusting Output Tube Bias
Biasing the amplifier is a simple procedure that can easily be done
by the owner. Only a plastic screwdriver is required (one has been
included with your amplifier for this purpose). After replacing the
tubes, the amplifier should be connected to a load (your loud-
speakers will work fine) and there should be no signal applied to
the amp. It should be connected to you preamplifier with the vol-
ume control turned fully down. Turn the amplifier on. After about
one minute, turn each of the four screwdriver adjustable controls
(located near the output tube sockets) counterclockwise until the
associated indicator LED just goes off. Clockwise rotation may be
necessary first to turn the LED on; do not turn the adjusting screw
any further in the clockwise direction once the LED has come on.
After the amplifier has warmed up thirty minutes, the procedure just
described should be repeated. NOTE: It is normal for these LEDs
to flash or flicker when the amplifier is in use. This biasing proce-
dure should be performed whenever the amplifier's output tubes
are replaced and after each six months of average use.
PLATE FUSE: Your LP70S has been fitted with plate supply fuses
that are installed to protect the amplifier circuitry in the event of
output tube failure. The fuse holders are physically located on the
back panel, near the speaker connectors. The cap unscrews
counter-clockwise to remove the fuse (KTK-1). An LED near the
fuse holder will light when the fuse is blown.
What happens when the fuse blows? If one of these fuses should
blow, it usually indicates a marginal output tube (one of two for
each channel) and an attempt should be made to identify and re-
place it, by either (a) replacing the fuse and turning on the amp
while watching the output tubes carefully; look for a bright white or
blue flash inside the tube. If nothing happens, then (b) lightly tap
the output tubes one by one with the plastic handle of a screw-
driver while watching for flashes inside the tube. If efforts to identify
the bad tube fail, the best course of action is to replace both tubes
in the channel that blew the fuse. If the tubes are over two years
old, or have more than 1,500 hours on them, a complete replace-
ment tube set is probably in order.
The sonic effects of a blown fuse are very low, very distorted
sounds from the speaker being driven by that channel.
TO AVOID SHOCK, ALWAYS TURN OFF THE AMPLIFIER
BEFORE REMOVING THE FUSE HOLDER CAP
Replacement fuses are available from conrad-johnson dealers.

LP70S Owner’s Manual 9
etting The Most From Your LP70S
In a system of commensurate high quality components, the
conrad-johnson LP70S offers an unparalleled level of
sophistication and refinement in music reproduction. To get the
best performance out of any audio system, there are a number of
important details that must be attended to.
Absolute Phase
Musical notes are heard through the ear’s response to waves of al-
ternating rise and fall of air pressure. Musical transients are almost
exclusively positive: that is, the initial effect is a rise in pressure.
The ear is capable of distinguishing these positive transients from
the musically unnatural alternative of a negative transient (an initial
fall in air pressure). In terms of your stereo system, these tran-
sients are created by your loudspeakers. If the speakers respond
to musical transients by first moving out, they are creating a rise in
pressure, and the system is said to be phase correct. If they re-
spond by moving in, they create a fall in pressure and the system
is said to be phase inverting. Each component in the stereo system
either preserves the phase of the incoming signal, and is said to be
phase correct, or inverts the phase and is said to be phase invert-
ing. It is unimportant whether an individual component is phase
correct or phase inverting, as long as the system as a whole is
phase correct. This will be the case if the number of phase inver-
sions is even (or zero).
The LP70S is phase correct (non-inverting). If your system has an
odd number of inversions, (for example, if you have a phase invert-
ing preamplifier) then you must add one phase inversion. This is
conveniently done by reversing the positive and negative connec-
tions to your speakers (be sure to reverse both channels). If you
are not sure about the phase of every piece in your system, you
can establish correct absolute phase by careful listening. When the
system is in correct phase, transients will be noticeably cleaner
and more sharply defined. The effect is especially apparent on
plucked string sounds. A final warning -not all recordings are
phase correct (including some “audiophile” recordings), so listen to
several before concluding your investigation of absolute phase.
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LP70S Owner’s Manual 10
The Importance Of Wires
Interconnect and speaker wires are an important element in your
stereo system. Interconnects are available which will permit a ref-
erence quality system to blossom and fulfill its promise of musical
reality. Others will strangle the system to the point where it be-
comes little better than average. To complicate matters, our ex-
perience suggests that the choice of interconnects will be system
dependent -those that are top ranked on one system may be a
poor choice for a different system. Consult your conrad-johnson
dealer for recommendations for your specific system.
Performance Tip
Warm up the LP70S before listening: The sonic performance of the
LP70S improves noticeably as the unit warms up. The midrange
becomes more lucid, the highs smoother, and the soundstage ex-
pands. The warm up period can be expected to last about fifteen
minutes.
Questions: If you have questions about the installation or function
of your LP70S do not hesitate to call Customer Service at (703)
698-8581.

LP70S Owner’s Manual 11
pecifications
Power: 70 watts per channel RMS both channels driven into 4
ohms from 30Hz to 15KHz at no more than 1.5% total harmonic
distortion or intermodulation distortion (can be re-connected in-
ternally for 8 or 16 ohm loads).
Sensitivity: .5 V rms to rated power
Small Signal Distortion: less than .2% at mid-band
Frequency Response:20Hz to 20 kHz +0/-.25 dB
Hum and Noise:102 dB below rated power output
Phase:phase correct
Input Impedance:100 kOhms
Mechanical
Dimensions: 16.125"D x 19"W x 6.375"H
Net Weight: 54 lb net
Fuses
The LP70S power transformer is protected by a fuse on the ac
power line (F5, located in a compartment in the ac line-cord recep-
tacle mounted on the back of the amplifier), and by four separate
internal fuses on the secondary circuits of the transformer (F1 –
F4) located on the bottom of the pc board. A failure of any of these
fuses is a symptom of a more serious problem, and a competent
service technician should be consulted. In no event should fuses
be replaced with a value or type different than that originally sup-
plied. The correct fuse values and types are:
F5: 5x20mm 5 amp T-type if configured for 100 or 120V:
5x20mm 3 amp T-type if configured for 220 or 240V.
F4: 3AG type 4 amp, slow blow
F1, F2, and F3: ¼ amp, slow blow
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