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C11 STEREOPHONIC PREAMPLIFIER
GENERAL DESCRIPTION
The Mcintosh C11 Stereophonic Pre
amplifier is a control center for any stereo
phonic sound system. To increase the
enjoyment of stereo, this control center does
four jobs with precise control.
First, the control center amplifies weak
electrical impulses. As the record rotates on
the turntable, undulations in the grooves
move the pick-up stylus approximately one-
thousandth of an inch, in any direction, from
the rest position. From this slight mechanical
movement, the pick-up stylus generates a
weak electrical impulse on the order of a few
thousandths of a volt. To amplify and pre
serve the information in such an electrical
impulse, the finest amplifier performance is
required.
Second, every sound system is used in a
different acoustical environment. The tone
balance of the music is affected by variations
The C11 Stereophonic Preamplifier com
bines excellence in performance with ease of
operation. The controls used the most have
large diameter knobs. A new type of rocker
switch indicates those controls with simple
off-on functions. The C11 has an illuminated
front panel with light intensity control to
provide convenient reading under low-level
lighting conditions.
The C11 circuit consists of three amplify
ing sections in duplicate for the left and right
stereo channels and a power supply. The
first section is the input preamplifier used to
amplify and equalize the input signals com
ing from phonograph pickups, microphones,
or tape heads. Skillful layout, grounding, and
shielding for low hum pickups, metal film and
wire wound resistors, terminal boards with
high specific resistivity, low noise tubes and
extreme care in manufacturing combine to
achieve a new low in noise and hum.
The second amplifying section follows the
volume control, and thus it is impossible to
overload the circuit or any following circuits.
The same techniques as were used in the
preamplifier stage assure low hum and noise.
The bass and treble tone controls are feed
back type circuits and operate in connection
in environment. Also, people listening to the
music have varying ideas of correct tone bal
ance. To compensate for these conditions, a
high-quality control center is needed.
Third, there is a great variety of stereo
phonic and monophonic records manufac
tured today, both domestic and foreign. The
control center must accurately compensate
for the equalization introduced in the re
cording process.
Fourth, with programs originating from
several sources such as tuners, records, tape
machines, microphones, etc., a control cen
ter is needed to select and switch these
sources separately or in combination.
All of these jobs are performed with ex
cellence by the C11 and yet this instrument
is simple to operate, easy to mount, and is
the finest in appearance.
with this section. Exceedingly low distortion
and correct frequency response are assured
using this arrangement.
The third section is the cathode follower
output. The sharp cut-off (18db per octave)
rumble and high frequency filters are asso
ciated with this section. Output level set con
trols are located at the input of the cathode
follower to allow simple balance of the entire
amplifying and speaker system. These con
trols are conveniently accessible at each side
of the front panel under the end caps.
The power supply deserves special men
tion. The power transformer is constructed
with "core" type grain oriented laminations
and magnetic shielding for low hum field.
Long life rectifiers, full wave rectification,
filter condenser sectionalizing and careful
grounding add to the hum free and long life
characteristics of the C11.
Special attention has been given to the
mechanical design also. The C11 may con
veniently be installed in furniture cabinets,
custom built installations, or professional
relay racks. The tubes and the electrical con
nections are conveniently located on the
back panel of the C11 for ready access.
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TECHNICAL DESCRIPTION
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treble response, turning it counterclockwise
decreases the treble response. Use the con
trols to compensate for 3% tape speed for
individual tape head characteristics.
A left plus right output jack is next to the
MAIN output. It is marked L+R. A mono
phonic signal can be distributed to other
rooms by connecting a third power amplifier
to the jack marked L+R. This amplifier is
used to drive the monophonic loudspeakers.
The cable connecting to this output should
not have a capacity of more than 200 mmf.
The input impedance of the power amplifier
connecting to this output should not be less
than 150,000 ohms (150K). The generator
impedance at this output is approximately
25,000 ohms (25K).
LOUDSPEAKER PHASING CONNECTIONS
Two pairs of screw terminals are located
on the lower left section of the C11 back
panel. The speaker leads from the "left"
power amplifier should be connected to the
screw terminals marked FROM AMPLIFIER.
The pair of terminals marked TO SPEAKER
The ultimate in stereo performance and
listening enjoyment is obtained through the
proper balancing of the stereo system. A
properly balanced stereo system must be in
phase. Each channel must be equal in loud
ness and similar in frequency response.
Do not use the C11 BALANCE control for
this procedure. The C11 BALANCE control
is used to adjust for any unbalance in the
source material, the phono cartridge, and
tape machine.
Before attempting the balancing of the
C11 make sure the controls on the amplifiers
are set properly. On the MC240 and MC275
turn the control marked BALANCE to its
center position. On the MC225 set the input
level controls for each channel to the black
dot above the control.
To properly balance the C11, proceed as
follows: A familiar recording, either stereo
phonic or monophonic, should be used in
balancing the C11.
1. Turn the INPUT SELECTOR to the posi
tion corresponding to the program selected.
2. Turn the BASS CONTROLS and TREBLE
CONTROLS to their 0 positions.
3. Turn the MODE SELECTOR to the L+R
TO L position.
4. Place the COMPENSATOR switch in the
RIAA position.
5. Place the TAPE switch in the NORMAL
position.
6. Place the PHASE switch in the NORMAL
position.
7. Place the POWER switch in the ON
position.
8. Place the RUMBLE switch in the FLAT
position.
9. Place the H.F. cutoff filter switch in the
FLAT position.
10. Place the LOUDNESS switch in the
NORMAL position.
11. Turn the BALANCE control to the 0
position.
12. While the program is playing, alternate
the MODE SELECTOR between the L+R TO
L position and the L+R TO R position. Adjust
the thumb wheel level controls under the
end caps until the loudspeakers are of equal
loudness.
When the proper balancing of the Mcin
tosh C11 is accomplished, the stereo system
will remain balanced throughout all modes
of operation.
OPERATING INSTRUCTIONS
BALANCING A STEREO SYSTEM
should be connected to the "left" loud
speaker.
These connections allow the front panel
PHASE switch to reverse the phase on the
left loudspeaker. This arrangement is con
venient when setting up a stereo system.
GROUND CONNECTION
A single ground post is provided. The
chassis ground from turntable, record chang
ers (motors used with each), tape decks, etc.,
should be returned to this post. Do not dupli
cate this ground circuit. Hum is likely to be
heard in the system if duplicate ground re
turns are used.
The left and right program cables from
each source should be twisted together and
the ground wire from each source can be
wound or twisted in with these cables. To
avoid hum, make sure the ground wire does
not make any connections to shields of the
left and right channel cables except for the
connection provided with the C11 ground
post.
