McIntosh C-20 Owners manual

STEREO

TABLE OF CONTENTS

PREAMPLIFIER

GENERAL DESCRIPTION

TECHNICAL DESCRIPTION

Mechanical Specifications
Electrical Specifications

FRONT PANEL FACILITIES

INSTALLATION

CONNECTING

A-C Connections

Input Connections
Output Connections
Ground Connections (Program Sources)

OPERATING INSTRUCTIONS

Balancing a Stereo System
To Adjust for Balance in the Treble

Range

To Adjust for Amplitude Balance in

the Bass Range
Adjusting Phase
Listening to a Stereo Record
To Adjust the Balance Control After the

System has been Balanced
Adjusting for Special Effects
Using the C20 with a Stereo Tuner
Using the C20 with a Stereo Tape Machine_
Using the C20 Front Panel Tape Jacks
Using the C20 with Tape Decks
Using the C20 with Microphones for Stereo.

Monophonic Record Settings

Operating Curves

GUARANTEE

C20

1
1

2

2

2

5
6

6

7
9
9

10

10

10

10

10

10

11

11

11

11

12
12

12

12

14

16

C20

C20 STEREOPHONIC PREAMPLIFIER

GENERAL DESCRIPTION

The Mclntosh C20 Stereophonic Pre-amplifier is a
control center for any stereophonic sound system. This
control center is necessary to accurately perform four
specific functions to increase the enjoyment of stereo.

First, the control center amplifies weak electrical

impulses. As the record rotates on the turntable, un-

dulations in the grooves move the pick-up stylus ap-

proximately 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 preserve the character of 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

TECHNICAL DESCRIPTION

You now own the finest expression of stereo crafts-

manship . . . designed and built to fulfill your needs.
Through direct consumer contact, a unique Mclntosh
marketing research program determines the application
of electronic and mechanical engineering to fit your

needs. Relentless research by Mclntosh engineers im-

proves the selection of materials and the circuits in
which they are used. It is this total concept of crafts-

manship directed to consumer satisfaction that has

maintained the Mclntosh reputation for excellence in

performance, quality, and reliability.

Perhaps you would like to know a few of the
features developed through research based on con­sumer needs that place your C20 Pre-amplifier above
all other pre-amplifiers.

An example of the unique mechanical construction
throughout is emphasized on the back panel where low­voltage program sources are connected to the C20. This

portion of the panel is cut out and mounted on small

electrical insulators. The ground circuit is connected

to the main chassis at only one point. By breaking the
path for circulating hum currents in this manner, the
hum and unwanted noises induced in other stereo
systems when used with various program sources and
combinations of program sources are eliminated.

Hum noises are also introduced into stereo sys­tems by the magnetic fields produced by the power
transformer windings. The greatest care went into the
design and construction of the power transformer in

your C20. It is wound in such a way that the magnetic
fields produced by the windings tend to cancel. This is

accomplished by splitting the windings into two seg-

ments. Further reduction of the magnetic field intensity
is achieved through the use of additional magnetic and

electrical shielding. With these construction improve-

is affected by variations in environment. Also, people
listening to the music have varying ideas of correct
tone balance. To adequately compensate for these con­ditions, a high-quality control center is needed.

Third, there is a great variety of stereophonic and

monophonic records manufactured today, both domestic
and foreign. The control center must provide record
compensation for a great variety of recordings.

Fourth, with programs originating from a variety of
sources such as tuners, records, tape machines, micro­phones, etc., a control center is needed to select and
switch these sources separately or in combination.

All of these jobs are performed with excellence

by the C20 and yet this instrument is easy to mount,

simple to operate, and is the finest in appearance.

ments the hum noises introduced by ordinary pre­amplifiers are eliminated by the C20.

Nearly all of the small resistor and capacitor com­ponent parts in the C20 are mounted on military-type
terminal boards. This specialized construction prevents
damaging mechanical strains. In addition it makes
servicing, if ever required, much easier and more
effective.

The mechanical layout of the C20 allows maximum

separation of the left and right channels. The terminal
board at the rear of the C20 is associated with the right

channel and the terminal board at the front with the

left channel. C20 terminal boards are constructed of

natural bakelite because of its low-volume resistivity.
This prevents the gradual increase of electrical noise
common to other terminal board materials after con­tinued use of the control center.

The C20 is the only control center that has an
illuminated front panel to provide convenient reading
under low-level lighting conditions. This feature is of
particular importance when a closet is adapted to house

stereo equipment.

Front panel tape input jacks are another exclu-

sive feature provided by the C20. With the start of
stereo FM broadcasting it will be important to tape
and preserve those special programs that will other-

wise never be heard again. This feature will be greatly

appreciated by you and your friends when recording
on portable tape machines.

Only extensive research and planning combined
with skilled engineering and manufacturing knowhow,
directed toward consumer needs, can produce such a
fine electronic instrument.

1

MECHANICAL SPECIFICATIONS

Chassis: 14-1/2 inches wide; 4-1/2 inches high; 12

DIMENSIONS

inches deep

Front Panel: 15-5/8 inches wide; 4-7/8 inches high

Front Panel Mounting Space Required: 16-3/8 inches

wide; 5 inches high

ELECTRICAL SPECIFICATIONS

POWER REQUIREMENTS

117 volts, 50/60 cycles, ac, 35 watts

INPUT SENSITIVITY AND INPUT IMPEDANCE

Auxiliary: 0.25 volt; 470,000 ohms
Tape: 0.25 volt; 470,000 ohms
Tuner 1: 0.25 volt; 470,000 ohms
Tuner 2: 0.25 volt; 470,000 ohms

Phono 1: 2 to 10 millivolts; 47,000 ohms for low-output

cartridge

10 to 70 millivolts; 47,000 ohms for high-output

cartridge

0.1 volt; 220 mmf in series with 47,000 ohms for

Xtal or ceramic cartridge
Phono 2: Same as phono 1
Tape Head 1: 2 to 10 millivolts; 47,000 ohms for low-

output head

10 to 70 millivolts; 267,000 ohms for high-output

head
Tape Head 2: Same as tape head 1
Tape Compare (Monitor): 0.25 volt; 115,000 ohms

FREQUENCY RESPONSE

±0.5db; 20 to 20,000 cycles

DISTORTION

Less than 0.2% at rated output, 20 to 20,000 cycles

HUM AND NOISE

High-level inputs: 85 db below rated output

Chassis: 18 pounds

WEIGHT

Shipping Weight: 28.5 pounds

FINISH

Anodized gold and black (front panel)

Low-level inputs: less than 2 microvolts at input ter-

minals

MAIN OUTPUT

2.5 volts with rated input
2 output jacks in parallel for each channel

TAPE OUTPUT

0.25 volt with rated input
1 output jack for each channel

LEFT PLUS RIGHT OUTPUT

2.5 volts from generator impedance of 23,000 ohms
VOLTAGE AMPLIFICATION

Low-level inputs to Main Output: 1,000 to 1 (60 db) at

1000 cycles

Low-level inputs to Tape Output: 100 to 1 (40 db) at

1000 cycles

High-level inputs to Main Output: 10 to 1 (20 db)

High-level inputs to Tape Output: 1 to 1 (0 db)

Tape Compare (Monitor) input to

Main Output: 10 to 1 (20 db)

A-C AUXILIARY OUTLETS

1 unswitched for tape machine or turntable,

colored Red

4 switched, colored Black

FRONT PANEL FACILITIES

Figure 1. C20 Front Panel

2

INPUT SELECTOR

TAPE

INPUT SELECTOR

S M

2

TUNER

1

AUX

PHONO 1

S

PHONO 2

M

1

TAPE HD

2

L . . . left channel to left speaker, no program to
right speaker.

R . . . right channel to right speaker, no program
to left speaker.

M LEFT . . . left channel only to both speakers.
M RIGHT . .. right channel only to both speakers.

RECORD COMPENSATOR

This ten-position program switch connects the

C20 as follows:

AUX ... any auxiliary service, such as a television
set, an extra tuner, or any other program source
requiring flat amplification.

TAPE . . . any self-contained tape machine (tape
machine having its own playback preamplifier).

TUNER 1 & 2 ... AM and FM outputs from a
stereo tuner or a pair of stereo tuners.

PHONO 1 & 2 — S & M ... stereo or monophonic

operation with the following types of phono car-

tridges.
a. Constant amplitude cartridges such as crystal,

ceramic, or frequency-modulated devices.

b. Magnetic cartridges having an output of 10mv

or more. Maximum input voltage for this output
is 70mv.

c. Magnetic cartridges having an output of less

than 10mv.

TAPE HEAD—1 & 2 ... a tape deck or a pair of
tape decks that do not contain their own playback
preamplifiers. Two types of tape head outputs can
be used in this position.

a. Tape heads having an output of less than 10mv.

b. Tape heads having an output of 10mv or more.

Maximum input voltage for this output is 70mv.

MODE SELECTOR

MODE SELECTOR

STEREO

L R

M LEFT

RECORD COMPENSATOR

BASS TREBLE

400 RIAA -10 -12

300

0

UP

TAPE TAPE

-5

RIAA

LP

These controls are used to correct for program

equalization introduced by the recording process.
Table 3 gives the recommended settings of the RECORD

COMPENSATOR controls of the C20 for monophonic

recordings.

BASS AND TREBLE

BASS

- ° +

TREBLE

- ° +

There are two tone controls on the C20—BASS

and TREBLE.

BASS . . . clockwise rotation increases bass loud­ness; counterclockwise rotation decreases bass
loud ness.

TREBLE . . . clockwise rotation increases treble
loudness; counterclockwise rotation decreases
treble loudness.

REV

M RIGHT

This six-position switch connects the C20 for
proper left-right distribution of sound for the program
source selected. Its positions are as follows:

REV . . . connects left channel to right speaker,
right channel to left speaker, corrects for left­right reversal of program.

STEREO . . . this is the normal stereo MODE
SELECTOR position; left channel to left speaker,

right channel to right speaker.

PHASE

PHASE

0° 180°

This two-position switch reverses the phase in the

left channel to correct for speaker or program phasing.

3

BALANCE

BALANCE

0

LEFT

RIGHT

This control balances the C20 for unequal pro-

gram sources.

LEFT . . . turning the control to the left slowly
reduces the right channel to no output and

accents the left channel.

RIGHT . . . turning the control to the right slowly
reduces the left channel to no output and accents

the right channel.

HF CUTOFF FILTER

HF CUTOFF FILTER

9KC

FLAT 5KC

This three-position switch minimizes surface noise
when reproducing old, badly worn recordings. Its posi­tions are as follows:

FLAT... filter disconnected.

9KC .. . rolls off response sharply starting at 9KC.
5KC . . . rolls off response sharply starting at 5KC.

With the RUMBLE FILTER button pushed to the

IN position, low-frequency rumble noise below 60 cps
created by a turntable or changer and undesirable
acoustically coupled feedback are reduced.

TAPE JACK

TAPE
JACK

IN-PLAYBACK

OUT-RECORD

This pushbutton controls the signal direction to

or from the front panel telephone jacks.

IN-PLAYBACK . . . TAPE input is switched from

rear tape jacks to front panel telephone jacks.

Any program originating from a portable machine
connected to the C20 can be heard by rotating
the INPUT selector to the TAPE position.

OUT-RECORD . . . connect signals from the rear
tape jacks to the INPUT SELECTOR TAPE posi­tion. Any program picked up by the C20 is con­nected to the portable machine for recording,

TAPE COMPARE

TAPE

COMPARE

LOUDNESS

L0UDNESS

2 3

I

FLAT

4

5

Music reproduced at very low volume loses its
bass and treble due to a selective shift in sensitivity
of human hearing. This effect is known as the Fletcher­Munson hearing characteristics. (See page 15, Figure

13.) The LOUDNESS control corrects for this effect.

RUMBLE FILTER

RUMBLE

FILTER

IN-ON

4

IN-TAPE

This pushbutton makes it possible to instantane-

ously compare the recorded material with the source

signal. TAPE COMPARE jacks are provided on the back
panel to accept a signal- from a tape recorder with a
monitor head and preamplifier. When the TAPE COM­PARE pushbutton is in the out position, the program
source is fed through the power amplifiers and the
loudspeakers. When the TAPE COMPARE pushbutton
is in the IN position (depressed), the signal source
becomes the monitored program from the recorded tape
and is fed through the power amplifiers and loud­speakers.

IMPORTANT

When the TAPE COMPARE pushbutton is de­pressed, signal from any other source will
not be heard from the loudspeakers. When
not in use, make sure the pushbutton is in

the out position.