THE MCINTOSH MC 2120 SOLID STATE STEREO POWER AMPLIFIER
Reading Time: 29 Minutes
Price $1.25
Your MC 2120 Stereo Power Amplifier
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: 607-723-3512
WARNING: TO PREVENT FIRE OR SHOCK
HAZARD, DO NOT EXPOSE THIS UNIT TO
RAIN OR MOISTURE.
Take Advantage of 3 years
of FREE Service ...
Fill in the Application NOW.
THREE YEAR SERVICE CONTRACT
An application for a FREE THREE YEAR SERVICE CONTRACT is included with this manual.
Contents
SERVICE . . . . 1
INSTALLATION . . . . 2
HOW TO CONNECT . . . . 4
FRONT PANEL INFORMATION . . . . 8
REAR PANEL INFORMATION . . . . 10
PERFORMANCE LIMITS AND RATINGS . . . . 11
PERFORMANCE CHARTS . . . . 12
TECHNICAL DESCRIPTION . . . . 14
BLOCK DIAGRAM . . . . 16
The terms of the contract are:
1. Mclntosh will provide all 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 ac-
company 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 mishandling is not covered by the SERV-
ICE CONTRACT.
4. The SERVICE CONTRACT is issued to you as
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 ® 1976 by Mclntosh Laboratory Inc.
1
INPUT
STEREO OR TWIN AMPLIFIER OPERATION
Use shielded cables to connect the signal from the preamplifier or signal source to the power amplifier input. To
minimize the possibility of hum the shielded cables should
be run parallel to each other or loosely twisted together.
Locate the cables away from speaker leads and AC power
cords. All connections are made on the back panel of the
MC
2120.
For stereo operation, the left output of the preamplifier
should be plugged into the Left input jack of the power
amplifier. The right output of the preamplifier should be
plugged into the Right (MONO) input jack of the power
amplifier.
available from the MC 2120, be sure to use large diameter
speaker leads. In all cases, the leads to and from the
speaker should be twin conductor or twisted together. Use
lamp cord, bell wire, or wire with similar type of insulation
to connect the speakers to the amplifier. When using 8 ohm
speakers and for the normally short distances of under 30
feet between the amplifier and speaker, #18 wire or
larger can be used. For distances over 30 feet between the
amplifier and speaker use larger diameter wire. Select the
correct size wire for the wire distance from the chart. It is
recommended that the DC resistance of the speaker leads
not be over 5% of the speaker impedance. Up to 10% can
be tolerated. Resistance of the leads should be computed for
the length of wire both to and from the speaker or speakers.
For twin amplifier operation a separate signal source
can be connected to each input. In stereo or twin amplifier
operation the MODE SWITCH must be in the STEREO position.
MONOPHONIC OR SINGLE CHANNEL OPERATION
A shielded cable from the signal source is plugged into
the Right (MONO) input lack of the MC 2120 only. The
MODE SWITCH on the back panel of the amplifier must be
placed in the MONO position. In the MONO position the
right channel input jack connects to both left and right
power amplifiers. The Left INPUT is disconnected. Only the
signal fed into the Right (MONO) input will be amplified.
Mono output is obtained by strapping both outputs in
parallel. Should the MODE SWITCH be left in the STEREO
position and the output transformers remain strapped for
a monophonic load, one channel will attempt to drive the
other which causes high circulating currents and over-
heating. Be certain that the MC 2120 is never operated in
the stereo mode with the outputs connected for monophonic operation.
OUTPUT
Be certain the loudspeakers connected to the MC 2120
are capable of handling the power output of the amplifier.
For multiple speaker operation, run separate leads from
each speaker to the amplifier. Because of the high power
Wire
Gauge
22
20
18
16
14
12
10
For 4
Feet
10
15
25
40
60
100
MAXIMUM
Ohm Load
Meters
6
1 83
3.05
4.57
7.62
12.19
18.29
30.48
WIRE
For 8
Feel
12
20
30
50
80
120
200
LENGTHS
Ohm Load
Meters
3.66
6.10
9 14
15.24
24.38
36.58
60.96
For 16
Feel
24
40
60
100
160
240
400
Ohm Load
Meters
7.32
12.20
18.30
30.50
48.80
73.20
122.00
Wire lengths above represent the wire resistance equal to
5% of the speakers impedance. Use of smaller wire sizes
will produce more resistance which causes more power
loss and less loudspeaker damping.
STEREO OR TWIN AMPLIFIER OPERATION
Check the impedance of the speaker which is usually
identified on the speaker itself or in the speaker owner's
manual. Connect a lead from the common terminal of the
left speaker to the amplifier LEFT OUTPUT terminal strip
COMmon screw Connect another lead from the other terminal of the left speaker to the screw with the number corresponding to the speaker impedance on the LEFT OUTPUT
terminal strip. The right channel speaker is connected in
the same manner on the RIGHT OUTPUT terminal strip. For
stereo or twin channel operation it is not necessary to use
the same impedance loudspeaker on each output. Connect
each channel for the impedance desired. Full power will
be delivered to each properly connected speaker.
4
When multiple speakers are to be connected to either or
both outputs, the combined load impedance must be calcu-
lated. The load must be connected to the appropriate im-
pedance tap. Use this table to aid in selecting the correct
impedance match:
Load Load
impedance impedance
in ohms Connect tor in ohms Connect for
1.6 to 3.2 2 ohm output 6.4 to 13 8 ohm output
3.2 to 6.4 4 ohm output 13 and up 16 ohm output
If a load impedance is used that is lower than the output
impedance tap, then reduced power and possible distortion
will result. If a load impedance is used that is higher than
the output impedance tap, then neither the signal nor the
amplifier will be harmed but the power available is limited.
FOR STEREO OR TWIN AMPLIFIER CONSTANT VOLTAGE
OPERATION:
For output
voltage of Connect for
25 volts 8 ohms
FOR MONOPHONIC CONSTANT VOLTAGE LINE
OPERATION;
For output
voltage of Connected for
25 volts 4 ohm output (mono)
For multiple speaker operation, run separate leads from
each speaker to the amplifier.
Should the MODE SWITCH be left in the STEREO position
and the output transformers be strapped for a monophonic
load, one channel will attempt to drive the other and cause
circulating currents and overheating. Be certain that the
MC 2120 is never operated in the stereo mode with the
outputs connected for monophonic operations.
AC POWER
The MC 2120 operates on 120 volts 50/60 Hz. The aux-
iliary AC OUTLET on the MC 2120 is not fused or switched.
MONOPHONIC OR SINGLE CHANNEL OPERATION
When the MC 2120 is used as a monophonic or single
channel power amplifier the two channels are combined to
produce output up to 240 watts. The outputs must be tied
together at the appropriate load impedance tap. In connecting a load to the MC 2120 for single channel operation
connect the common side of the load to the LEFT CHANNEL
OUTPUT terminal marked COM., the other lead is con-
nected:
If the speaker
or load
impedance is
1 ohm
2 ohms
4 ohms
8 ohms
Do not connect unequal impedance taps together
If the load impedance is between any of the above figures, select the best impedance match from;
Load Load
impedance impedance
in ohms Connect for in ohms Connect for
0.8 to 1.6 1 ohm output 3.2 to 6 4 4 ohm output
1.6 to 3.2 2 ohm output 6.4 and up 8 ohm output
The hot side
of the line is
connected to
Left 2
Left 4
Left 8
Left 16
Connect a jumper
wire between
Left 2 and Right 2
Left 4 and Right 4
Left 8 and Right 8
Left 16 and Right 16
5
Stereophonic
Connections
PROGRAM SOURCE
LEFT
SPEAKER
MUST BE IN STEREO
POSITION FOR STEREO
PROGRAMS
RIGHT
SPEAKER
STEREO
MODE
SWITCH
MONO
6
Monophonic
Connections
PROGRAM SOURCE
CONNECT FOR MONO USE
ONLY WHEN THE MODE
SWITCH IS IN MONOPOSITION
7
STEREO
MODE
SWITCH
MONO
Front Panel
Information
LEFT GAIN
Use the LEFT GAIN control to adjust the volume in the
left channel to the desired listening level. Turn the control
clockwise to increase the volume.
RIGHT/MONO GAIN
Use the RIGHT/MONO GAIN control to adjust the volume
in the right channel to the desired listening level. Turn the
control clockwise to increase the volume.
MONO
With the output of MC 2120 connected for monophonic
operation and the rear panel MODE SWITCH in MONO the
volume is controlled by the RIGHT/MONO GAIN control
only.
LIMIT/NORMAL
The NORMAL (green) and LIMIT (red) indicators are a
new Mclntosh development. As long as the amplifier
operates without overload the NORMAL indicator illu-
8
minates. A waveform comparator in the MC 2120 constantly compares the amplifier input and output waveform. Waveform difference of the output wave is converted to a voltage which is used as a control signal to
"turn
off"
the
NORMAL
LIMIT indicator. The LIMIT indicator turns on whenever the
waveform difference exceeds 0.5%. Generally, these
waveform differences are the result of applying voltages
at the input in excess of its rating. Because the limit indicator circuit can show an overload condition as short as
one cycle of a 20,000 Hz signal a holding circuit keeps the
red indicator on long enough for the eye to see.
HEADPHONES
The output of the front panel HEADPHONE jack has been
designed to feed low impedance dynamic stereo headphones. Electrostatic headphones generally require higher
power than dynamic headphones. Connect them to the LEFT
and RIGHT OUTPUT contacts on the back of the MC 2120.
indicator
and to
"turn
on" the
Plug dynamic headphones into the front panel HEADPHONE
jack. Adjust the front panel LEFT GAIN and RIGHT/MONO
GAIN control for comfortable headphone listening.
POWER
The power switch turns the MC 2120 ON or OFF. The
switch does not control the power outlet on the back panel.
If you wish to control the AC power from a preamplifier
control center leave the switch in the ON position. Be sure
the AC cord of the MC 2120 is plugged into the controlled
outlets on the rear of the preamplifier control center.
OFF: In the OFF position the AC power to the amplifier
is turned off.
9
Rear Panel Information
LEFT and RIGHT OUTPUT
For stereo operation, output impedances of 2, 4, 8 and
16 ohms have been provided on a secure, screw type barrier strip. For monophonic operation proper interconnection
provides, 1, 2, 4 and 8 ohms from the same barrier strips.
POWER GUARD
Amplifiers are capable of delivering large quantities of
power when they are driven to clipping. Clipping is
caused when the amplifier is asked to produce more power
output than it can deliver with low distortion. A clipped
amplifier can have more than 40% harmonic distortion.
The extra energy content of the clipped signal will damage most speakers. A new Mclntosh advancement helps
to protect your speaker from this kind of damage. The
MC 2120 has a built in "waveform comparator" that compares the wave shape of the input signal with the output
signal. If the non-linearity between the two signals exceeds
0.5% a front panel signal illuminates in red (LIMIT). With
the POWER GUARD switch in the NORMAL position, 1.0%
distortion will cause the POWER GUARD circuit to operate.
The POWER GUARD circuit limits the input dynamically so
that the amplifier cannot be overdriven. This eliminates
amplifier output clipping. Recommended operations is with
the POWER GUARD switch in the NORMAL position at all
times.
MODE SWITCH
The MC 2120 can be used in these different manners:
stereo (or as a twin amplifier) and mono. Because of the
excellent channel separation of the MC 2120, it can
all of these functions. When the instrument is properly con-
nected the MODE switch is used to select the method of
operation desired.
INPUT
In the stereo or twin amplifier mode of operation both
input jacks accept signal. In the mono mode of operation
the Left channel input jack is disconnected and only the
Right (MONO) channel input jack accepts signal.
INPUT LEVEL
The input sensitivity of the MC 2120 is 0.75 volts or
2.5 V depending on the position of the INPUT LEVEL switch.
With indicated voltage applied and the GAIN control clockwise, the amplifier will deliver its rated power. All Mclntosh
preamplifiers have been designed to deliver 2.5 volts output with rated input. For the best signal to noise ratio when
using Mclntosh source equipment, always have the INPUT
LEVEL switch in the 2.5 V position and the front panel LEFT
and RIGHT/MONO GAIN controls in the fully clockwise position. If more gain is desired the 0.75 V position may be
used. For source equipment other than Mclntosh set the
switch in the position nearest to the stated output rating of
the source equipment.
AC POWER
The MC 2120 is rated for 120 volts, 50/60 hertz. It uses
50 watts when there is no signal output and 460 watts
with both channels delivering rated power. An 8 ampere
fuse protects the MC 2120 electrically. An auxiliary AC
power outlet has been provided. The auxiliary outlet is
neither fused nor switched.
10
Performance Limits and Ratings
PERFORMANCE GUARANTEE
Performance Limits are the maximum deviation from perfection permitted for a Mclntosh instrument. We promise
you that the MC 2120 you buy must be capable of per-
formance at or exceeding these limits or you get your
money back. Mclntosh is the only manufacturer that makes
this guarantee.
PERFORMANCE
Mclntosh audio power ratings are in accordance with the
Federal Trade Commission Regulation of November 4, 1974
concerning power output claims for amplifiers used in home
entertainment products.
POWER OUTPUT
STEREO
120 watts minimum sine wave continuous
average power output per channel, both
channels operating into 2 ohms, 4 ohms, 8
ohms, or 16 ohms load impedance, which is:
15.5 volts RMS across 2 ohms
21.9 volts RMS across 4 ohms
31.0 volts RMS across 8 ohms
43.8 volts RMS across 16 ohms
MONO
240 watts minimum sine wave continuous
average power output into 1 ohm, 2
ohms, 4 ohms, or 8 ohms load impedance,
which is:
15.5 volts RMS across 1 ohm
21.9 volts RMS across 2 ohms
31.0 volts RMS across 4 ohms
43.8 volts RMS across 8 ohms
OUTPUT LOAD IMPEDANCE
STEREO
2 ohms, 4 ohms, 8 ohms, and 16 ohms;
separate terminals are provided for each
output
MONO
1 ohm, 2 ohms, 4 ohms, and 8 ohms; ob-
tained by connecting together the appro-
priate terminals of both channels
RATED POWER BAND
20 Hz to 20,000 Hz
TOTAL HARMONIC DISTORTION
STEREO
0.1% maximum harmonic distortion at any
power level from 250 milliwatts to 120
watts per channel from 20 Hz to 20,000
Hz, both channels operating
MONO
0.1% maximum harmonic distortion at any
power level from 250 milliwatts to 240
watts from 20 Hz to 20,000 Hz
INTERMODULATION DISTORTION
STEREO
0.1% maximum if instantaneous peak power output is 240
watts or less per channel with both channels operating for
any combination of frequencies, 20 Hz to 20,000 Hz
MONO
0.1% maximum if instantaneous peak power output is 480
watts or less for any combination of frequencies, 20 Hz to
20,000 Hz
FREQUENCY RESPONSE (at one watt output)
20 Hz to 20,000 Hz, +0 -0.25 dB
10 Hz to 100,000 Hz +0 -3.0 dB
NOISE AND HUM
95 dB below rated output
RATINGS
OUTPUT VOLTAGES
25 volts for distribution lines
DAMPING FACTOR
STEREO
16 at 2 ohms output, 50 at 4 ohms output, 20 at 8 ohms
output, 14 at 16 ohms output
MONO
16 at 1 ohm, 50 at 2 ohms, 20 at 4 ohms, and 14 at 8
ohms output
INPUT IMPEDANCE
100,000 ohms
INPUT SENSITIVITY
Switchable: 0.75 volt or 2.5 volts—Level control provided
for higher input voltages.
GENERAL INFORMATION
POWER REQUIREMENTS
120 volts 50/60 Hz, 50 watts at zero signal output, 460
watts at rated output
SEMICONDUCTOR COMPLEMENT
39 silicon transistors
24 silicon rectifiers and diodes
2 integrated circuits
MECHANICAL INFORMATION
SIZE
Front panel measures 16 inches wide (40.7 cm) by 5 7/32
inches high (13.2 cm). Chassis measures 14 5/8 inches
wide (37.2 cm) by 5 inches high (12.7 cm) by 13 inches
deep (33 cm), including connectors. Knob clearance required is 1 1/2 inches (3.8 cm) in front of mounting panel
FINISH
Front panel is anodized gold. Chassis is chrome and black
WEIGHT
57 pounds (25.9 kg) net, 70 pounds (31.8 kg) in shipping
carton
11
Performance Charts
Per Channel Both Channels Operating
HARMONIC DISTORTION AT RATED OUTPUT
FREQUENCY RESPONSE AT 1 WATT OUTPUT
POWER BANDWIDTH AT 0.1% THD
12
OUTPUT SIGNAL WAVEFORM SHOWING ACTION OF POWER
GUARD TO ELIMINATE OUTPUT SIGNAL CLIPPING. AMPLIFIER
INPUT IS OVERDRIVEN BY 20 dB FOR BOTH OSCILLOGRAM
TRACES.
INTERMODULATION DISTORTION VS. POWER OUTPUT
HARMONIC DISTORTION VS. POWER OUTPUT
13
Technical
Description
INPUT AMPLIFIER
Separate input amplifiers are used for the right and
left channels. Each input amplifier is a two transistor feedback amplifier which has unity gain. They are used to pro-
vide high input impedance and low output impedance for
driving the power amplifier sections. In addition, impe-
dance matching is provided for the POWER GUARD system
input attenuator.
When the MODE selector is switched to MONO, only the
right channel input, GAIN control, POWER GUARD attenua-
tor, and input amplifier is used to drive both power ampli-
fiers.
POWER AMPLIFIER
There are two identical power amplifier sections. At the
input to each power amplifier is a monolithic differential
transistor selected for low noise. The monolithic design
yields closely matched transistors for low distortion. The
input and the feedback signals are applied to this differen-
tial amplifier. The second stage is a class A voltage ampli-
fier using a single transistor with feedback.
The predriver, driver, and output sections follow and are
fully complimentary with high efficiency and negligible dis-
tortion. The predriver is a complimentary transistor pair
biased to nearly class A operation for low crossover distor-
tion. Bias is supplied by a transistor which serves as a tem-
perature tracking circuit so the predriver is properly biased
regardless of temperature. The predriver has a generous
amount of emitter degeneration for low distortion.
Another complimentary pair of power transistors biased
for class AB operation make up the output driver stages.
Their bias is developed across a thermistor which holds the
bias current stable regardless of temperature.
The output stage consists of 4 rugged complimentary
power transistors connected in single ended push-pull
parallel. This stage is connected as a balanced emitter follower which allows great stability and low distortion. Due
to a unique arrangement of the output bias network, the
output transistors are operated class B free of crossover
distortion. Heat is not produced by these transistors when
there is no output. The bias network for the output stage
is temperature compensated assuring class B operation
at any temperature or power level.
The output transistors and drivers are mounted on gener-
ous sized black anodized aluminum heat sinks. The free
flow of room temperature air that passes through the oversized heat sinks provides the cooling necessary fur the
long life of components.
The MC 2120 has transient free turn on and turn off characteristics. This is because the output of the amplifier is
switched by a heavy duty relay to the output autotransformer. The relay is driven by a transistor switch. The con-
trol signal to this transistor is derived from a long time con-
stant capacitor charging network. The switch turns on the
relay approximately two seconds after the MC 2120 is
turned on. The same circuit has a short turn-off time constant which turns off the relay before the amplifier's main
power supply has had a chance to drop. The output signal
is fed through a matching network to the front panel
HEADPHONE jack. The HEADPHONE output is designed to
feed low impedance dynamic stereo headphones.
The amplifier output signal is fed to the output terminals
through the autotransformer. The Mclntosh designed inter-
leaved multifilar wound autotransformer is used to properly
match the amplifier to any output impedance tap. The MC
2120 will deliver full power over the entire audio frequency
range at any of these impedances. The MC 2120 will not
overheat when driving low impedance loads (2 ohms or
4 ohms). The autotransformer also protects speakers from
damage in the event of amplifier failure. Should a direct
current component appear in the output it is shunted by the
autotransformer and cannot damage the speaker.
The Mclntosh patented Sentry Monitoring circuit constantly monitors the output signal and instantly reacts to
prevent overload of the output transistors. At signal levels
up to rated output this circuit has high impedance and has
no effect upon the output. If the power output exceeds
design maximum, the Sentry Monitoring circuit operates
to limit the signal to the output transistors. In the event
of a short circuit across the amplifier output or severe im-
pedance mismatch the Sentry Monitoring circuit will
protect the output transistors from failure. The positive and
negative halves of the output signal are monitored inde-
pendently.
LIMIT INDICATOR AND POWER GUARD
The front panel NORMAL (green) and LIMIT (red) indi-
cators are activated by a newly designed Mclntosh circuit.
The MC 2120 has a built in "waveform comparator." The
waveform comparator electrically compares the amplifier's
output waveform with the input waveform. Should the
waveform differences reach 0.5%, the red LIMIT indicator
on the front panel is turned on and the NORMAL indicator
is turned off. If the differences increase to 1% the POWER
GUARD circuit is activated. POWER GUARD provides an
unusual margin of safety for loudspeakers by the prevention of amplifier clipping yet permits the amplifier to deliver designed maximum power.
14
The input waveform and the output waveform are compared in an integrated circuit differential amplifier. Any
differences, due to distortion of the output waveform, are
converted to a control voltage. This control voltage is applied
differentially to a high gam operational amplifier where it
is amplified about 100 times. The amplified control voltage is detected by a full wave bridge rectifier then enters a
Schmidt trigger which is programmed to sequence the
front panel indicators from NORMAL (green) to LIMIT (red]
at a preset distortion level. Also built into the trigger circuit
is a timing capacitor which holds the LIMIT indicator on
long enough to be visible even when the duration of the
overload is much shorter. The LIMIT indicator will be illuminated for an overload condition as short as one cycle of a
signal at 20,000 Hz.
The same control voltage used to activate the LIMIT
indicator also actuates the POWER GUARD system. If the
control voltage is greater than is needed to illuminate the
LIMIT indication then the POWER GUARD begins operating.
The control voltage is used to operate a light emitting
diode/light dependent resistor (LDR) network. The LDR is
part of an attenuator between the volume control and the
input amplifier. The input to the power amplifier can be
attenuated to as little as one tenth, if necessary, in order
to maintain low distortion in the event of excessive input
signal.
The POWER GUARD circuit is activated when the rear
panel switch is in the NORMAL position. POWER GUARD
protection is disabled m the OUT position. The LIMIT indi-
cator operates regardless of the position of the POWER
GUARD switch.
15
16
MCINTOSH LABORATORY INC.
2 CHAMBERS ST., BINGHAMTON, N. Y. 13903
607-723-3512
Design subject to change without notice.
Printed in U.S.A.
038-921
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