Conrad-johnson design MV125 User manual

Congratulationson your purchase of the conrad-johnson MV125 ampli-
fier. You have acquired one of the finest pieces of music reproduction
equipment available today. You will find it to be capable of reproducing
with remarkable accuracy the dynamics, tonalities, and textures of live
music performances. The design and materials have been carefully
selected to yield years of unvarying excellence.
Please take a few minutes to read this manual so that you will better
understand the featues and capabilities of your MV125.
MV125 Owner’s Manual 1

LIMITED WARRANTY FOR THE CONRAD-JOHNSON
MV125
1. conrad-johnson design, inc. warrants to the original owner that the
conrad-johnson MV125 will be free from defects in material and work-
manship for a period of three years from the date of original purchase.
2. During the warranty period, conrad-johnson will repair defective units
without charge for labor or parts (with the exception of the vacuum
tubes) subject to the following conditions:
a) The unit must not have been altered or damaged through
misuse, abuse, negligence, accident or improper operation.
b) The purchaser must provide evidence of purchase at the time
service is requested.
c) All repairs must be undertaken at the factory or other service
center designated by conrad-johnson design, inc. Units submitted
for warranty repairs must be shipped in the factory packing carton
to conrad-johnson design or its designated service center, freight
and insurance prepaid by the owner, and will be returned to the
owner, freight and insurance prepaid by conrad-johnson design.
Replacement cartons are available from the factory for a nominal
charge.
d) Normal wear and maintenance are not covered by this warranty.
3. The above warranty may be transferred to subsequent owners pro-
vided that the warranty registration card has been returned to conrad-
johnson design, inc. within 30 days of the original purchase and that the
registered owner provides the factory with a signed notice giving the
newowner’snameandaddressandthemodelandserialnumberofthe
unit.
4. Where permitted by law, conrad-johnson design’s liability shall be
limited to that set forth in this warranty. No other warranty of any kind,
expressed or implied, is made by conrad- johnson design, inc., and all
implied warranties, including merchantability and fitness for a particular
"purpose", which exceed the obligations and time limits set forth herein
are hereby disclaimed. conrad-johnson design, inc. shall not be liable
for incidental or consequential damages. Some states do not allow the
exclusion or limitations on how long an implied warranty lasts, so the
above limitation or exclusion may not apply to you.
MV125 Owner’s Manual 2

5. This warranty gives you specific legal rights, and you may also have
other rights which vary from state to state. Questions about warranty
service should be addressed to:
Service Department
conrad-johnson design, inc.
2800 R Dorr Avenue
Fairfax, VA 22031
REGISTERING THE WARRANTY
Please return the enclosed card to the factory within 30 days of pur-
chase to register the warranty. Proper registration of the warranty is
important! It will enable us to inform you of any alterations in the design
of a given model and of any new products we offer which are likely to be
of interest to you.
MV125 Owner’s Manual 3

CIRCUIT DESIGN
It has been our experience that simple designs with few active stages
when executed with care and great attention to detail will be both more
musical and more reliable than overcomplicated alternatives. Accord-
ingly, the audio circuit of the MV125 is remarkably straightforward. A
triode input amplifier comprised of paralleled sections of a 5751 is direct
coupled to a cathode coupled differential phase inverter using high
current triodes to provide balanced, low impedance drive to the push-
pull output stage. The output stage features ultralinear operation of two
pairsof6550s for high power levels with reducedsourceimpedance.As
a result, the amplifier is capable of supplying the large current demands
of high amplitude musical transients driven into reactive speaker loads.
A substantial main power supply of massive polypropylene capacitors
enables the MV125 to easily meet these extraordinary current de-
mands.Voltageregulatorsforthevoltageamplifiersandphaseinverters
provide nearly absolute isolation of these sensitive stages from the
output stages.
MV125 Owner’s Manual 4

INSTALLATION
Because of the number and type of vacuum tubes, the MV125 dissi-
pates considerable heat. It is essential, therefore, that proper attention
be paid to ventilation. The unit should be mounted horizontally on a flat
hardsurfacesothattheventilation holesin thebottomareunobstructed.
At least six inches of clearance should be provided 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, cooling fans are
essential.
The front panel of the MV125 will fit in a standard 19" equipment rack
and is designed to carry the weight of the unit so that it may be rack
mounted. Again, it is important that adequate clearance be provided
above so that ventilation is not impeded. About eight inches of the front
of the rack should be left open above the unit.
To minimize hum pickup, keep the unit well away from the preamplifier
and turntable and route power cords away from input cables.
THE MV125 IS TO BE OPERATED ONLY ON AC LINE VOLTAGE OF
108 TO 125 VOLTS.
The MV125 is optionally available wired for 100 volt, 120V, 220V or 240
volt, 50/60 Hz current.
CONNECTION
INPUT: Connect the preamplifier to your MV125 amplifier at the RCA
type connections located on the back of the amplifier.
OUTPUT: Terminal strips are provided for connection of your loud-
speakers. They will accept bare wire or spade lug connectors.
Three different taps- 4 ohm, 8 ohm, and 16 ohm are provided in addition
to the "C" or ground tap.
Matchingthe rated nominalimpedanceofyour speakers withtheampli-
fier output taps will not necessarily guarantee optimum performance.
The sound with a given set of speakers will vary with the different
MV125 Owner’s Manual 5

impedancetaps. We suggest listening to eachof the three taps to select
the best match for your speakers.
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 should move synchronously-in
and out together. Terminals on the speakers are usually coded-one
designated "C", "earth", "ground", "-" or black, the other designated "+"
or red.
"In phase" connection of the speakers can normally be achieved by
takingcare to connectthewirefrom the4,8,or16 ohm amplifierterminal
marked on each channel to the red or "+" coded terminal on each
speaker, and connecting the "C" tap to the black or "-" coded terminal
on each speaker.
In phase connection of the speakers can be readily ascertained by ear.
With the preamplifier mode switched into the MONO position, play a
recordingof asolovocalist. Withthespeakers inphasethe voiceshould
be clearly focused between the two speakers. With the speakers con-
nected out of phase, the voice will be diffused, with no identifiable
source. Relative phase may be reversed by switching the "+" and "-"
leads at one speaker only.
MV125 Owner’s Manual 6

GETTING THE MOST FROM YOUR
CONRAD-JOHNSON MV125
The conrad-johnson MV125 offers an unparalleled level of sophistica-
tion and refinement in music reproduction, but it cannot transform an
inferior stereo system. Only the very best loudspeaker systems will be
capable of fully revealing the capabilities of this precision instrument.
Proper set-up is important in any high-fidelity system, but with state-of-
the-art components it is essential. Given a system of high quality
components, there remain a number of important details which must be
attended to.
ABSOLUTE PHASE
Musical notes are heard through the ear’s response to waves of alter-
nating rise and fall of air pressure. Musical transients are 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 your
stereo system, these transients 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 respond by moving in, they create a fall in pressure and the
system is said to be "phase inverting". Each component in the stereo
systemiseitherphasecorrectorphaseinverting(includingthecartridge
and speakers-these are normally phase correct). Your MV125 is phase
correct. It is of no consequence if an individual component is phase
inverting, as long as the system as a whole is phase correct. This will be
the case as long as the number of phase inversions is even (or zero). If
your system has an odd number of inversions, then you must add one
phase inversion. This is conveniently done by reversing the positive and
negative connections on either your cartridge or your speakers (be sure
to reverse both channels).
If you are not sure about the phase of every component 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
MV125 Owner’s Manual 7

(including some "audiophile" recordings), so listen to several before
concluding your investigation of absolute phase.
THE IMPORTANCE OF WIRES
Interconnect and speaker wires are an important element in your stereo
system. Interconnects are available which will permit a reference quality
system to blossom and fulfill its promise of musical reality. Others will
stranglethesytemtothepointwhereitbecomesnobetterthanaverage.
To complicate matters, our experience suggests that the choice of
interconnects will be system dependent-those that are ranked first on a
given system may be a poor choice for a different system. Consult your
conrad- johnson dealer for recommendations for your specific system.
QUESTIONS?
If you have any questions about the installation or function of your
MV125 do not hesitate to call Customer Service at (703) 698- 8581.
MV125 Owner’s Manual 8

TUBE REPLACEMENTS
TheMV125 usestwelvevacuumtubes ofthreedifferent types: two5751
(V1 and V2), two 6FQ7 (V3 and V4) and eight 6550s (V5 through V12).
Each type has been carefully chosen for its circuit application. It is well
known that tubes of a given type vary sonically depending on the
manufacturer. We have chosen the brands of tubes that we supply
basedon extensive auditioningofavailablebrands. The choiceofbrand
has been made solely on the basis of sonic performance and reliability
without regard to either cost or prestige. We know of no vacuum tubes
availablewhichwillimprovethesonicperformanceof yourMV125.Also,
the tubes in your amplifier have been tempered by a controlled burn in
procedure that permits them to perform for a greatly extended period
without degradation in normal use. We highly recommend that, when
thetime comes toreplacethevacuum tubes, youpurchasereplacement
tube sets from conrad-johnson design.
The MV125 has been designed to make tube replacement as simple as
posssible. Beacuase the output tubes are individually biased, replace-
ment tubes need not be matched pairs. No a-c balance adjustment is
necessary. It is necessary to re-bias the amplifier when the output tubes
are replaced. The MV125 has built in bias indicators so that no instru-
ments are required to make this adjustment.
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. A red plastic screw-
driver has been included with your amplifier for this purpose. After
replacing the tubes, the amplifier should be connected to a load (your
loudspeakerswillwork fine) andthereshouldbeno signal appliedtothe
amp. It should be connected to you preamplifier with the volume control
turned fully down. Turn the amplifier on. After about one minute, turn
each of the four screwdriver adjustable controls 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
MV125 Owner’s Manual 9

flash or flicker when the amplifier is in use. This is normal and no cause
for alarm.
This biasing procedure should be performed whenever the amplifier’s
output tubes are replaced and also after six months of average use.
FUSES
LINE FUSE: The AC line is fused to protect the power transformer. This
fuse will not fail in normal operation. Failure of the fuse is a symptom of
a more serious problem, so the unit should be taken to a qualified
service center. IN NO EVENT SHOULD THE FUSE BE REPLACED
WITH A FUSE OF DIFFERENT TYPE OR RATING THAN THAT SUP-
PLIED BY THE FACTORY.
PLATE FUSE: Your MV125 has been fitted with plate supply fuses
whicharedesigned to protecttheamplifiercircuitryin the eventofoutput
tube failure. The fuse holders are physically located near the speaker
connectors. The cap unscrews counter clockwise to remove the fuse (
a BUSS BBS-2). 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 6550 tube (one of four for each channel)
and an attempt should be made to identify and replace it, by either (a)
replacing the fuse and turning on the amp while watching the 6550s
carefully; look for a bright white or blue flash inside the tube. If nothing
happens, then (b) lightly tap the 6550s one by one with the plastic
handle of a screwdriver while watching for flashes inside the tube. If all
efforts to identify the bad tube fail, the best course of action is to replace
all 6550s 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 and tubes are available from conrad-johnson deal-
ers.
MV125 Owner’s Manual 10

SPECIFICATIONS:
Power: 125 watts per channel RMS both channels driven into 8 ohms
from 30Hz to 15KHz at no more than 1% total harmonic distortion or
intermodulation distortion.
Sensitivity: 880 mv to full power.
Small Siganal Distortion: Less than .1% at midband
Phase: Phase correct (non-inverting)
Frequency response: 20Hz to 20KHz +0, -.5dB
Hum and noise: 96dB below full power output.
Input impedance: 100K ohms.
Dimensions: 20.5D x 19W x 8.75H inches.
Weight: 80 lbs.
MV125 Owner’s Manual 11

SERVICE
In the event your conrad-johnson MV125 stereo amplifier requires
service, repack it with the original packing material and ship to:
conrad-johnson design, inc.
Service Department
2800R Dorr Avenue
Fairfax, VA 22031
Be sure to enclose a note explaining the problem you are having in as
much detail as possible. If the problem is intermittent, please indicate. If
you do not have the original packing material, replacements are avail-
able from conrad-johnson design at a nominal charge. If the unit is
under warranty, be sure to enclose a copy of your receipt as proof of
purchase.
MV125 Owner’s Manual 12