McIntosh C-27 Owners manual

THE McINTOSH C27 SOLID STATE STEREO PREAMPLIFIER

Reading Time: 28 Minutes

Price: $2-00

WARNING: TO PREVENT FIRE OR SHOCK

HAZARD, DO NOT EXPOSE THIS UNIT TO
RAIN OR MOISTURE.

The Mclntosh you have purchased has a serial number located on
the rear panel of the chassis.
Record that serial number here

for future references. The model and serial number are important

to you for any future service.

Your C 27 Stereo Preamplifier will give you

many years of pleasant and satisfactory
performance. If you have any questions,

please contact:

CUSTOMER SERVICE

Mclntosh Laboratory Inc.
2 Chambers Street
Binghamton, New York 13903
Phone: [phone]

Take Advantage of 3 years

of FREE Service ...
Fill in the Application NOW.

Contents

SERVICE CONTRACT . . . . 1

INTRODUCTION . . . . 2

HOW TO INSTALL .... 2

HOW TO CONNECT .... 4

WHAT THE FRONT PANEL CONTROLS

AND HOW TO USE THEM .... 8

BALANCING YOUR STEREO .... 11

PERFORMANCE LIMITS AND RATINGS . . . . 12

PERFORMANCE CHARTS . . . . 13

TECHNICAL DESCRIPTION .... 14

BLOCK DIAGRAM .... 15

THREE YEAR SERVICE CONTRACT

An application for a FREE THREE YEAR SERVICE CONTRACT is included with this manual.

The terms of the contract are: mishandling is not covered by the SERVICE

CONTRACT.

4. The SERVICE CONTRACT is issued to you as

1. Mclntosh will provide ail parts, materials and
labor needed to return the measured perfor­mance of the instrument to the original per­formance limits free of any charge. The

SERVICE CONTRACT does not cover any
shipping costs to and from the authorized
service agency or the factory.

2. Any Mclntosh authorized service agency will
repair all Mclntosh instruments at normal

service rates. To receive the free service

under the terms of the SERVICE CONTRACT,

the SERVICE CONTRACT CERTIFICATE must
accompany the instrument when taken to
the service agency.

3. Always have service done by a Mclntosh

authorized service agency. If the instrument

is modified or damaged, as a result of un­authorized repair the SERVICE CONTRACT
will be cancelled. Damage by improper use or

the original purchaser. To protect you from

misrepresentation this contract cannot be

transferred to a second owner.

5. For your protection Mclntosh selects only
dealers who have technical competence to

guide purchasers fairly, and provide service
when necessary. To receive the SERVICE
CONTRACT your purchase must be made
from a Mclntosh franchised dealer.

6. Your completely filled in application for a
SERVICE CONTRACT must be postmarked
within 30 days of the date of purchase of the
instrument.

7. To receive the SERVICE CONTRACT all in­formation on the application must be filled
in. The SERVICE CONTRACT will be issued
when the completely filled in application is

received at Mclntosh Laboratory Incorporated

in Binghamton, New York.

Copyright © 1977 by Mclntosh Laboratory Inc.

How to Connect

CONNECTING A TURNTABLE TO PHONO 1

Connect the cable from the "left" channel of the turn-

table into the L PHONO 1 INPUT jack.

Connect the cable from the "right" channel of the turn-

table into the R PHONO 1 INPUT jack.

PHONO 2 is provided for the use of a second turntable.

Connect the cable from the "left" channel of the turn-

table into the L PHONO 2 INPUT jack.

Connect the cable from the "right" channel of the turn-

table into the R PHONO 2 INPUT jack.

Plug the AC power cable from the turntable into the red
unswitched POWER outlet on the rear panel. The red
receptacle is on at all times. It is necessary to turn off the
turntable with its own AC power switch. This arrangement

protects the turntable.

The four black AC power outlets are controlled by the
front panel POWER switch. Use these outlets for a tuner,
tape recorder, etc.

GROUND CONNECTION

A single ground post is provided. Grounds for turntables,

record changers, tape decks, etc. are to be connected to

this post. The left and right program cables and the ground

wire from that source should be wound or twisted together.

To avoid hum make sure the ground wire does not make

any connections to the shields of the left and right program
cables between the program source and the C 27.

CONNECTING A STEREO TUNER

Connect the cable from the "left" channel tuner output

to the L TUNER INPUT jack.

Connect the cable from the "right" channel tuner output

to the R TUNER INPUT jack.

AUX—Any high level program source such as a tuner,

a TV set or a tape recorder can be connected to the AUX 1

and 2 INPUT jacks. The connecting procedure is the same
as for the tuner input.

CONNECTING TAPE RECORDERS
To Record:

Connect a cable from the L TAPE 1 OUTPUT jack to the

left high level input of the tape recorder.

Connect a cable from the R TAPE 1 OUTPUT jack to the

right high level input of the tape recorder.

Connect a second recorder in the same manner to the

TAPE 2 OUTPUT jacks.
To Playback/Monitor:

Connect a cable from the left channel output of a tape

recorder to the L TAPE 1 INPUT jack.

Connect a cable from the right channel output of a tape

recorder to the R TAPE 1 INPUT jack.

Connect a second recorder in the same manner to the

TAPE 2 INPUT jacks.

4

FM ANTENNA

TUNER

TURNTABLE #1

TURNTABLE #2

AC POWER TO
TUNERS, TAPE

RECORDERS AND
POWER AMPLIFIERS

AC POWER
TO TURNTABLES

TAPE
RECORDER

1

CONNECTIONS FOR TUNERS, TURNTABLES, AND TAPE RECORDERS

5

TAPE
RECORDER

2

POWER AMPLIFIER

LOUDSPEAKER
ENVIRONMENTAL
EQUALIZER

SPEAKER SYSTEM #1

SPEAKER SYSTEM #2

CONNECTIONS FOR POWER AMPLIFIER AND LOUDSPEAKERS

6

CONNECTING THE C 27 TO POWER AMPLIFIERS

Connect the MAIN OUTPUT jacks to the input of a
stereo power amplifier. The Left MAIN jack is connected
to the left amplifier input jack. The Right MAIN jack is
connected to the right amplifier input jack.

A second pair of MAIN OUTPUT jacks have been supplied
to feed a second power amplifier. In the event it is desired
to tape record after the tone filter and volume controls
the second set of MAIN OUTPUT jacks can be connected
to the tape recorder input. The controls are then all avail­able to alter the signal before recording. Remember, any
change in the setting of the controls will change the pro-

gram to the tape recorder. Since the recording signal
available at the MAIN OUTPUT is much higher than the
output available from the TAPE 1 or TAPE 2 OUTPUT
jacks, be sure not to overload the tape recorder input. It

will not be possible to monitor from the tape recorder via
the TAPE MONITOR pushbuttons while feeding the recorder
from the MAIN OUTPUT jacks. Depressing the TAPE

MONITOR pushbutton will disconnect the program source

and may cause feedback oscillation.

Cables are supplied with the C 27 to interconnect to
the power amplifier. Longer cables can be used if needed.
The length of the cable is limited by the capacity of the

cable. The total capacity must not exceed 1,000 pF. For
instance: cables with a capacity of 25 pF per foot may be

40 feet long. 13.5 pF per foot cable may be 75 feet long.

The input impedance of the amplifiers should be 22k
ohms or greater.

Connect a wire from the proper impedance output

connection on the power amplifier to the FROM AMPLIFIER

push connectors on the C 27 rear panel. Connect the first
pair of speakers to the rear panel SPEAKER 1 push con­nectors and a second pair of speakers to the SPEAKER 2

push connectors. When connecting the speakers and power
amplifier it is important to connect the power amplifier
output impedance tap for the parallel impedance com-

bination of the two speaker systems used. The front panel
SPEAKER switch permits routing program to either
SPEAKER 1, SPEAKER 2, both 1 and 2, or OFF. In the
SPEAKER OFF position, the program is fed only to the
HEADPHONE jack for private listening.

AC OUTLETS

There are 4 black AC outlets and one red AC outlet. The
power to the black AC outlets is controlled by the front
panel AC power switch. Use these outlets for a tuner, tape
recorder, etc. The total capacity of all AC power outlets is

600 watts. The red receptacle is on at all times to feed
AC power to a turntable so that it can be turned on or off

with its own AC switch.
FUSE

A 0.5 AMP fuse protects the preamplifier circuits. The

fuse does not protect additional equipment connected to

the back panel AC outlets.

CTR CHAN

Use the CenTeR CHANnel output to feed left plus right
signal to a separate power amplifier for monophonic back­ground music or for a center channel.

CONNECTING FOR SPEAKER SWITCHING AND

HEADPHONE

When the power amplifier output is properly connected
to the rear panel FROM AMPLIFIER push connectors,

speakers in two separate areas can be turned on or off
and the C 27 front panel HEADPHONE jack will receive
the program signal.

7

What the Front Panel controls Do

and How to use Them

INPUT SELECTOR

The INPUT SELECTOR is a five position switch that

connects the chosen input program to the proper circuits

in the C 27.

AUX 1: Connects to the high level input, any output
from a high level program source requiring flat amplifica­tion. Such a source could be a television set, playback from
another tape recorder, an additional tuner, etc. In the
AUX 1 position the gain is 20 dB to the MAIN outputs,
0 dB to the TAPE outputs. The input impedance is 100 k
ohms.

AUX 2: Same as AUX 1 but for a second high level
program source.

TUNER: Connects the output from any AM, FM or MPX
FM tuner to the high level input stage. In the TUNER posi-

tion the C 27 has flat amplification. There is 20 dB of

gain to the MAIN outputs, 0 dB to the TAPE outputs. The
input impedance is 100k ohms.

PHONO 1: Connects the output of any magnetic phono
cartridge to the low level input stage. The response has
been shaped according to the RIAA standard to compensate

for the characteristics of a magnetic phono cartridge. The
gain at 1K Hz is 62 dB to the MAIN outputs, 42 dB to the
TAPE outputs. The input impedance is 47k ohms and

100 pF.

PHONO 2: Same as PHONO 1 but for a second phono

system.

MODE SELECTOR

L to L & R: Connects the left input to both loudspeak-

ers. R to L & R: Connects the right input to both loud-

speakers. Stereo Rev: Connects the left input to the right
loudspeaker and the right input to the left loudspeaker.

Stereo: Connects the left input to the left loudspeaker

and the right input to the right loudspeaker.

Mono (L + R): Adds the left input and the right input
and then connects the L + R program to both loud­speakers.

L + R to L: Adds the left and right program together
and connects the combined program to the left loud­speaker only.

L + R to R: Adds the left and right program together
and connects the combined program to the right loud-

speaker only.

PRECISE TRACKING VOLUME CONTROL

The VOLUME control is a precision step attenuator

which has left to right tracking accuracy within 1 dB
throughout its entire range. Such extremely accurate match­ing is achieved through electronically controlled trimming
of the resistance material deposited on pairs of printed

circuits. Since the switch commutator touches only contact

pads and not the actual resistance element, tracking ac­curacy is not degraded with use as in ordinary volume
controls. Clockwise rotation increases volume. The VOLUME
control in the C 27 has been designed and manufactured
for Mclntosh Laboratory.

FILTERS/SPEAKERS

The small center knob of the concentric FILTER/

SPEAKERS control shapes the C 27 frequency response to

reduce or filter objectionable low frequency noise and/or

objectionable high frequency noise. The large outer knob

8

will turn main or remote loudspeakers on or off when a

power amplifier has been properly connected to the C 27
rear panel.

FILTER OPERATION

OUT: Filter disconnected, program not modified by
either high or low frequency filters.

LF (LOW FREQUENCY FILTER)

Use the LF position to reduce objectionable low­frequency noise created by a turntable or record changer
or acoustically coupled feedback. Response is rolled off
below 50 Hz.

HF (HIGH FREQUENCY FILTER)

Use the HF position to reduce objectionable high­frequency noise such as record scratch. Response is

rolled off above 5,000 Hz.

LF-HF

Both high and low frequency filters are activated.

SPEAKERS OPERATION

OUT: All speakers are disconnected. Program is fed only
to the HEADPHONE jack for private listening.

1: Program is fed only to the rear panel SPEAKER 1 con-

nectors and the HEADPHONE jack. SPEAKER 2 is
disconnected.

1 & 2: Program is fed to the rear panel SPEAKER 1 and

2 connectors and the HEADPHONE jack.

2: Program material is fed only to the rear panel

SPEAKER 2 connectors and the HEADPHONE JACK.
SPEAKER 1 is disconnected.

Right: Has the same effect on the sound from the right

loudspeaker.
TREBLE

Concentric, 11 position, tone control switches adjust
the treble. The outer knob adjusts the right channel re­sponse. The center knob adjusts the left channel treble

response.

Left: Adjusts the treble loudness from the left loud­speaker. Clockwise rotation increases the treble loudness
while counterclockwise rotation decreases the treble loud-

ness. Each step of the tone control adjusts the treble

loudness about 4 dB.

Right: Has the same effect on the sound from the right

loudspeaker.

LOUD/BALANCE
LOUDNESS:

Use the loudness control for full frequency range listen­ing at even the softest listening levels. The LOUDness
control (the small center knob of the concentric control)
provides low-frequency boost to compensate for a hearing
characteristic of the human ear at low listening levels.
The ear becomes less sensitive to low frequencies as volume
is reduced. Increased loudness compensation is therefore
desirable at lower listening levels. The loudness control

boosts the low frequencies so that the ear hears approxi­mately the same musical balance as it would at higher
listening levels.

FLAT: (Maximum counterclockwise rotation) Full volume
with flat frequency response. The loudness compensation
is inoperative in this position.

BASS

Concentric, 11 position, tone control switches adjust

the bass. The outer knob adjusts the right channel bass

response. The center knob adjusts the left channel bass
response.

Left: Adjusts the bass loudness from the left loudspeaker.
Clockwise rotation increases the bass loudness while coun­terclockwise rotation decreases the bass loudness. Each
step of the tone control adjusts the bass loudness about 4

dB.

Turning the control clockwise toward MAX reduces the
listening volume, and automatically increases the compen­sation by boosting bass.

BALANCE

The BALANCE control adjusts for unequal volume in

either the left or right channels. The volume of the chan­nels can be varied relative to each other without affecting

their combined volume.

Turning the control to the left accents the left channel

9

by reducing the right channel output and turning the con-

trol to the right accents the right channel by reducing the

left channel output.

TAPE 2 MONITOR

A second tape recorder can be operated in the same

manner using pushbutton TAPE 2 MONITOR.

PANLOC

Depressing the two PANLOC buttons on the front panel
permits the C 27 to be either locked firmly in place or
unlocked so that the chassis can be slid forward, until it
is stopped by the mid-position latches. Press the latches
on the side of the C 27 inward to permit the instrument to

slide in or out of the cabinet. With the C 27 front panel

flush with the cabinet panel, pressing the PANLOC buttons
will lock the unit firmly in the cabinet.

THE PUSHBUTTONS

The C 27 is designed to be used with one or two tape
recorders. The front panel pushbuttons permit normal
playback of either recorder, monitor as recordings are
being made on either recorder, or copying tapes from one
recorder to another while listening to a separate program.

TAPE MONITOR

To monitor while recording from any source chosen on

the INPUT SELECTOR, your tape recorder must have sepa-

rate record and playback heads and separate electronics.

The MONITOR switches let you monitor the quality of tape

recordings made during the recording process. When the
chosen TAPE MONITOR pushbutton is in the IN position

the sound from the tape as it passes the playback head is

heard a moment after it is recorded. The recording process
continues as usual. When the switch is in the OUT position,

the program that is being recorded is heard.

TAPE 1 MONITOR

PUSHBUTTON OUT ... The program source selection by

the INPUT SELECTOR is fed to the power amplifiers and

heard through the loudspeakers.

PUSHBUTTON IN ... The program source becomes the
recorded tape on the recorder connected to TAPE 1 INPUT.
The recorded program from tape recorder 1 is fed to the

power amplifiers and heard from the loudspeakers.

In the PUSHBUTTON IN position, a rectangle is lighted
above the pushbutton. When the light is on only the tape
will be heard. To listen to other sources the pushbutton

must be out and the light off.

Use the front panel pushbutton TAPE 2 to playback from
a tape recorder plugged into the rear panel TAPE 2 INPUT
jacks.

HOW TO COPY TAPE, TAPE COPY 1 -> 2

Put the tape to be copied on the recorder connected to
TAPE 1 INPUT. Press in the TAPE COPY 1 -> 2 pushbutton.
The signal available at the TAPE 2 OUTPUT jacks is the

playback of TAPE 1. Record on the recorder connected to

TAPE 2 OUTPUT. The source tape on tape recorder 1 can

be heard by pressing in the TAPE 1 MONITOR pushbutton.
The recording can be monitored by pressing in the TAPE 2
MONITOR pushbutton. Instantaneous comparison on the

recorded program with the original can be heard.
TAPE COPY 2 -> 1

When pushed IN the TAPE 2 OUTPUT jacks are con­nected to the TAPE 1 INPUT jacks without affecting the
program heard from the speakers. In this position a copy

of the program on tape recorder 2 can be made on tape

recorder 1.

To monitor the tape copy procedures use the TAPE

MONITOR pushbuttons.
POWER

The POWER switch energizes the C 27 circuits and con-

nects AC power to the black rear panel AC power outlets.
The black AC power outlets are not fused. A lighted rec­tangle above the ON POWER switch indicates the AC

power to the system is turned on.

HEADPHONES

The front panel HEADPHONE jack has been designed to

feed low impedance dynamic headphones. Electrostatic

headphones generally require higher power than dynamic
headphones. They must be connected to the output con­nectors on the power amplifier.

Plug headphones into the front panel HEADPHONE

jack. Adjust the VOLUME control for comfortable head-

phone listening. The output of an external power amplifier
must be properly connected to the C 27 back panel for
program material to be available at the HEADPHONE jack.

10

Balancing your Stereo

The performance and enjoyment of a stereo system is
greatly increased when the sound is properly balanced.
The balance of the stereo system is affected by many
things including room acoustics, furniture placement, room
shape, small differences in loudspeakers etc.

TO BALANCE LOUDNESS

1. Set the MODE switch to MONO (L + R]

2. Play a familiar recording.

3. Turn the BALANCE control to the 10 o'clock position.

While the program is playing, stand between the two
loudspeakers. Listen for a difference in loudness between

speakers. If there is a difference in loudness turn the
BALANCE control to adjust for equal loudness.

Clockwise rotation balances for lack of loudness on the
right—conterclockwise balances for lack of loudness on
the left, Next, set the MODE selector to STEREO. If there
is then a difference in loudness turn the BALANCE control
toward the speaker that is not as loud. Adjust until the
sound is balanced from both speakers.

11

Performance Limits and Ratings

PERFORMANCE LIMITS are the maximum deviation from
perfection permitted for a Mclntosh instrument. We promise
you that your C 27 must be capable of performance at or
exceeding these limits or you get your money back. Mc­lntosh is the only manufacturer that makes this guarantee.

FREQUENCY RESPONSE

+0

-0.5

dB 20 Hz to 20k Hz

DISTORTION
Will not exceed 0.05% at rated output level, 20 Hz to 20k

Hz

INPUT SENSITIVITY AND IMPEDANCE
AUXILIARY i and 2, TUNER, TAPE 1 and 2: 250 millivolts
at 100k ohms. PHONO 1 and 2: 2 millivolts at 47k ohms

and 100 pF.

HUM AND NOISE
AUXILIARY 1 and 2, TUNER, TAPE 1 and 2: 85 dB un­weighted; 90 dB IHF A weighted.
PHONO 1 and 2: 80 dB below 10 mV input, unweighted;
85 dB IHF A weighted.
OUTPUT LEVEL AND IMPEDANCE
MAIN OUTPUT 2.5 volts with rated input, less than 1k
ohms source impedance, to operate into 22k ohms load

or higher. Maximum output is greater than 10 volts
TAPE OUTPUT 0.25 volts with rated input, less than 1.5k
ohms source impedance, to operate into 22k ohms load
or higher. Maximum output is greater than 10 volts
CENTER CHANNEL OUTPUT (L + R) 2.5 volts with rated
input to both channels, less than 1.2k ohms source im-

pedance, to operate into 22k ohms load or higher.
VOLTAGE AMPLIFICATION IN DECIBELS

AUXILIARY 1 and 2, TUNER, TAPE 1 and 2
to MAIN OUTPUT 20 dB
to TAPE OUTPUT 0 dB

PHONO 1 and 2

to MAIN OUTPUT 62 dB

to TAPE OUTPUT 42 dB
SEMICONDUCTOR COMPLEMENT

18 Silicon—Planar transistors
4 Silicon Diodes
5 Light Emitting diodes
AC POWER OUTLETS

1 unswitched (Red)
4 switched

POWER REQUIREMENT

120 volts, 50/60 Hz, 15 watts

MECHANICAL INFORMATION

SIZE: Front Panel measures 16 inches wide (40.6 cm) by 5
7/16 inches high (13.8 cm). Chassis measures 14 3/4 in­ches wide (37.5 cm) by 4 13/16 inches high (12.2 cm) by 13
inches deep (30.0 cm), including PANLOC shelf and back
panel connectors. Knob clearance required is 1 1/4 inches
(3.2 cm) in front of the mounting panel.

FINISH: Front panel is anodized gold and black with
special Mclntosh gold/teal panel nomenclature illumina­tion. Chassis is black.

MOUNTING: Exclusive Mclntosh developed professional
PANLOC.

WEIGHT: 20 pounds (9.1 kg) net, 32 pounds (14.5 kg) in

shipping carton.

FACILITIES AND FEATURES

BASS
Separate 11 position rotary switches for each channel.

-17 dB to +16 dB at 20 Hz

TREBLE
Separate 11 position rotary switches for each channel

-20 dB to +20 dB at 20k Hz

LOUDNESS
Flat response, or continuously variable loudness equaliza-

tion.
BALANCE

Natural balance at center position, attenuation of left or
right channel by rotating control

VOLUME

Precision step attenuator with matched elements for proper

tracking at all listening levels. Does not change stereo

balance as volume is changed.

INPUT
Five positions—AUXILIARY 1 and 2, TUNER, PHONO 1

and 2

MODE
Seven positions—Left channel only to both speakers. Right
channel only to both speakers, STEREO REVerse, STEREO,

MONO (L + R), L + R to Left speaker only, and L + R to

Right speaker only

TAPE MONITOR

Two pushbutton switches. Either of two tape recorders can

be monitored. They are mechanically interlocked to accept

only one pushbutton at the IN position at one time

TAPE COPY SWITCH
Two pushbutton switches. Either of two tape recorders can

be connected to copy from one to the other or vice versa.
They are mechanically interlocked to accept only one
pushbutton at the IN position at one time.

LF FILTER (Rumble Filter)
Flat or roll-off 12 dB per octave below 50 Hz, down 12 dB at

20 Hz

HF FILTER (Scratch Filter)
Flat or roll-off 12 dB per octave above 5k Hz. down 12 dB at

20 k Hz

SPEAKER SWITCH

Facilities are provided to connect the power amplifier to

the rear panel of the C 27 to permit switching two pairs

of speakers on and off.
HEADPHONE JACK

For low impedance headphones; powered by properly in­terconnected external power amplifier.

12

13

Technical Description

The accompanying BLOCK DIAGRAM indicates the
various signal paths within the C 27. Circuits represented
by each block are discussed below.

INPUT SELECTOR SWITCH

The INPUT SELECTOR routes the various input signals
within the C 27. The switch is robust in construction and
has large double wiping silver contacts. Isolating networks
at each high level input and shorting switch action ground
all unused inputs preventing undesired program feed­through. The same isolating networks block any DC that
might be present on a high level input to eliminate pops
or clicks when changing inputs.

PHONO PREAMPLIFIER

Each channel of the phono preamplifier uses three
discrete transistors especially selected, and circuit op­timized, to produce low noise and distortion free operation.
Of particular benefit is the use of low impedance com-

ponents in the RIAA feedback equalization networks. The

network drive is provided by the final stage transistor
which operates at an adequate current level to drive the
low impedance without slew rate limiting.

MONITOR AND TAPE COPY SWITCHES

Tape switching uses pushbutton switches. The switches
are mounted directly to a printed circuit board that in­corporates the necessary inter-connection wiring. Isolating

networks are included at each tape input and output
which eliminates the possibility of DC leakage from asso­ciated tape recorders which prevent pops or clicks when
switching. The pushbutton switches have reliable push-

push action ratchets to prevent switch hangup. Light
emitting diode (LED) lamps are activated by the switches
to indicate the particular switch function being used.

a 32-step control with a 70 dB range, plus volume off.
Left and right channel tracking are within 1 dB. This ex­treme accuracy is obtained through special electronically
controlled resistance element trimming. Since the switch
commutator touches only contact pads and not the preci­sion resistor elements, tracking accuracy is permanently
maintained and is noise free.

VOLTAGE AMPLIFIER

A voltage amplifier with 14 dB gain follows the VOLUME
control. This amplifier plus the following amplifier with
6 dB gam constitute the main amplifier section of the C 27.

The amplifier has high input impedance, low output im-

pedance, very low noise level and wide dynamic range.

HIGH FREQUENCY AND LOW FREQUENCY FILTERS

The HF and LF FILTER networks are switch controlled.

The high-frequency filter network reduces treble response

above 5,000 Hz. The low-frequency filter reduces bass
response below 50 Hz. The slope of the filter frequency
response is selected for maximum rejection of objectionable
noises. Careful design has kept the loss of usable program
material to a minimum.

LOUDNESS CONTROL

The LOUDness control is continuously variable. Full

volume and flat frequency response is obtained in the

counterclockwise or FLAT position of this control. As the
control is advanced clockwise the volume level reduces
but loudness compensation is introduced which maintains
the low-frequency output at a higher level than the mid
and high frequency output. This action complements the
human ear response characteristics, the Fletcher Munson
effect. Loudness compensation is provided for a wide range
of listening levels.

MODE SWITCH

A seven-position MODE SELECTOR switch allows routing

of the left and right channel signals in various combina-

tions for system balancing and operation. The MODE SE-

LECTOR functions only in the signal path directed through
the MAIN OUTPUT jacks and does not control the TAPE
OUTPUT.

VOLUME CONTROL

The VOLUME control is a precision step attenuator

designed and manufactured for Mclntosh Laboratory. It is

BALANCE CONTROL

Rotating the BALANCE control to the left or right allows
you to balance the level of the two channels. At the de­tented center position the gain of the two channels is equal.

BASS AND TREBLE CONTROLS AND ASSOCIATED
AMPLIFIER

A high gain three stage linear amplifier is used in the

bass and treble control circuits. Negative feedback reduces

the gam to 6 dB when the controls are set flat. The feed-

back path incorporates the control response shaping net-

14

15

works. Separate BASS and TREBLE controls route the

feedback signal to shape the frequency response by varying

the amount of feedback used at different frequencies.

Accurate control is accomplished by using precision capa­citors and resistors. The output of this amplifier is fed to

the MAIN OUTPUT jacks.

CENTER CHANNEL AMPLIFIER

The CTR CHAN (center channel) amplifier is an emitter
follower arranged for very high input impedance. Unity
gain is provided at the CTR CHANnel OUTPUT jack when

both the LEFT and RIGHT channels have equal output

signals.

POWER SWITCH AND POWER OUTLETS

The pushbutton POWER switch controls AC power to
the C 27 and the four black switched outlets. Power to
the red unswitched outlet is present all the time and is
not controlled by the power switch. The red outlet is in­tended to power turntables. The switched outlets are used
to power tuners, power amplifiers, and similar components.

POWER TRANSFORMER AND POWER SUPPLY

The C 27 power transformer uses a grain oriented silicon
steel core plus copper and magnetic steel shield bands to
eliminate any radiated magnetic hum field. The extra care

in shielding assures that the power transformer will not
radiate an external magnetic field that could introduce
hum into the amplifier circuits or an adjacent phono pickup

cartridge or tape recorder playback head.

The power supply uses zener diode reference voltage

regulation and electronic filtering. Normal power line
variations do not affect the performance of the C 27. The
turn on and off time characteristics of the power supply
are controlled to prevent turn on and off transients.

SPEAKER SWITCH AND HEADPHONE JACK

The SPEAKER switch is arranged to control two sets of

loudspeakers powered by an external power amplifier.
When connecting the speakers and power amplifier it is

important to connect the power amplifier output impe­dance tap to the parallel impedance of the two speaker
systems used. The headphone jack is also fed from the

external power amplifier. The power amplifier is required
to make the HEADPHONE jack operate.

16

MclNTOSH LABORATORY INC.

2 CHAMBERS ST. BINGHAMTON, N. Y. 13903

607-723-3512

The continuous improvement of its products is the policy of

Mclntosh Laboratory Incorporated, who reserve the right to

improve design without notice.

Printed in U.S A.

339091