STEREO POWER
AMPLIFIER
MC225
TABLE OF CONTENTS
GENERAL DESCRIPTION
1
TECHNICAL DESCRIPTION
Electrical Specifications
Mechanical Specifications
1
2
3
PANEL FACILITIES
Input
4
4
Output
Line Output
Fuse
AC Outlet
Line Voltage
4
4
4
4
4
INSTALLATION
5
CONNECTIONS
Input—Stereo
Input—Twin Amp
Input—Mono
5
5
5
6
Output—Stereo or Twin Amp
Output —Mono
Line Output—Stereo or Twin Amp
Line Output—Mono
AC Power
6
6
6
7
7
OPERATING THE MC225
7
ADJUSTMENTS
Input Switch-Stereo
Input Switch-Mono
7
7
7
GUARANTEE
8
FREE 3-YEAR FACTORY SERVICE
CONTRACT APPLICATION
cover pocket
SCHEMATIC DIAGRAM
cover pocket
PARTS LIST
cover pocket
RESISTANCE AND VOLTAGE CHART
cover pocket
MC225 POWER AMPLIFIER
GENERAL
Over one million watts of amplifier output
power capacity have been manufactured by
Mclntosh since 1947. In this 1,000,000 watts
of audio power there are less than 10 watts of
distortion capacity and less than 1/100 watt
of noise capacity!
The dramatic difference in the quality of
music reproduction when you listen through
Mclntosh instruments is directly due to low
distortion performance. Careful, devoted
research is a way of life at Mclntosh. The
world's finest amplifier is the creation of persevering, resourceful Mclntosh engineers.
Mclntosh Laboratory is the only manufacturer in the entire industry to guarantee the
lowest distortion at all audio frequencies, at
full power. The U.S. Patent Office has recognized the advanced technology of the
Mclntosh circuit by granting 6 patents. 1
DESCRIPTION
Long life, flexibility, highest quality construction are characteristic designs in every
Mclntosh instrument. Wide electrical and
thermal margins of safety for all components
and tubes, advanced engineering, and cool
operating design add to the long life built
into every Mclntosh product. Reliability prolongs your investment without expensive
maintenance costs.
The MC225 has on one chassis two 25
watt power amplifiers. In addition to use as a
stereo amplifier, the flexibility of the MC225
permits it to be used as a monophonic
amplifier that delivers 50 watts, or as two
separate 25 watt amplifiers with each channel amplifying completely separate programs, or as two amplifiers for use with an
electronic crossover network. Such flexibility permits maximum use for greatest
return from your investment.
TECHNICAL DESCRIPTION
The patented Mclntosh Unity Coupled
circuit and output transformer have established Mclntosh amplifiers as the unchallenged leaders in the audio field.
Before 1947, low distortion at high power
and high efficiency was impossible. A completely new engineering approach resulted in
an amplifier that for the first time permitted
high power with distortion below 1%. That
new engineering produced the Mclntosh
Unity coupled circuit and the Mclntosh bifilar
wound output transformer. With the introduction of the Mclntosh amplifier new standards for distortion-free performance were
established.
P R I M A R Y NO
1
turn of primary number one is next to the
same turn of primary number two. There is
almost complete magnetic coupling between
the two wires. The magnetic coupling is reinforced by the capacitance between the
two wires.
INPUT
OUTPUT
P R I M A R Y NO. 2
Fig. 1 —Representation of bifilar winding.
The Mclntosh output t r a n s f o r m e r is
unique. It has two primary windings which are
wound bifilarly. In the bifilar technique both
primary wires are wound side by side. Each
Fig. 2-Representation of Mclntosh output circuit.
'U.S. Patent No's: 2,477,074; 2,545,788; 2,646,467; 2,654,058; 2,860,192; 2,929,028.
1
In the Mclntosh Unity Coupled circuit one
of the bifilar primary windings is connected
through the power supply to the plate and
cathode of one of the output tubes. The other
bifilar primary winding is similarly connected
to the other tube.
All low distortion high power amplifiers
use push-pull output circuits known as Class
AB 1 , AB 2 or B. Two tubes are arranged in a
balanced circuit. This permits each tube to
operate alternately somewhat over half the
time. Compared to full time operation of the
tubes, the push-pull method reduces heating
and permits more power from a given type of
tube. Despite this advantage of the conventional push-pull circuit one problem in particular remained to be solved. When current
in each tube is cut off to begin the idle period
distortion is produced at the instant of cut
off and again at the instant when current
flows. This form of distortion is known as
Notch Distortion and was well illustrated by
Mr. Pen Tung Sah in the "Proceedings of the
I.R.E." Volume 24, pp 1522-1541 in 1936.
Imperfect coupling between the primary
windings found in all conventional output
transformers produces the notch distortion.
Trying to improve coupling in a conventional transformer decreases the power response
at both low and high frequencies, heating
the output tubes and lowering the available
power output.
The Mclntosh Unity Coupled output circuit and bifilar transformer is the first commercial breakthrough that eliminates notch
distortion by coupling both output tubes
almost to perfection. In the Mclntosh trans-
ELECTRICAL
Power Output:
Stereo or twin amplifier: 25 watts continuous per channel
Monophonic: 50 watts continuous
Harmonic Distortion:
Less than 0.5% at rated output or less,
2
former the extremely close coupling of the
bifilar windings removes the condition which
permits notch distortion. Furthermore the
two output tubes are arranged as partial
cathode followers. Half of the output circuit
is in the cathode and half in the plate of each
tube. The output tubes now are operating in
a local feedback loop which, reduces their
distortion, reduces their internal generator
resistance, and reduces their balance
requirements. The Mclntosh circuit in
reality perfects push-pull high efficiency
output circuits.
Good voltage regulation in the power
supply permits overloads without overshoot
or blocking, provides good transient response, and complete stability. To improve
regulation a silicon rectifier power supply is
used in the MC225. In addition to better
voltage regulation, the silicon rectifier allows
even higher operating efficiency, cooler
operation, and longer amplifier life.
To greatly extend tube and component
life a thermistor in the MC225 limits current
surges produced when the equipment is
turned on. The thermistor is a special type of
resistor. Its resistance depends on its temperature. When the amplifier is off the thermistor has a high resistance value (about 79
ohms). Just after the amplifier is turned on
the current which flows through the thermistor heats it and causes its resistance to
decrease to a low value (less than .7 ohms).
Current is thus limited when the MC225 is
first turned on but is not limited as the unit
warms.
SPECIFICATIONS
20 cycles through 20,000 cycles. Typical
performance is 0.3% or less at 20 cycles
and 20,000 cycles at full power. Typical
performance in the mid frequencies is
0.1% or less at full power.
Intermodulation Distortion:
Less than 0.5% for any combination of
frequencies from 20 cycles through
20,000 cycles if instantaneous peak
power is below twice the rated power.
Phase Shift:
Less than ±15° 20 cycles through 20,000
cycles
Frequency Range:
At rated output:
+0, -0.1 db 18 cycles through 30,000
cycles
+0, -0.5 db 18 cycles through 60,000
cycles
At one-half rated output:
+0, -1 db 14 cycles through 100,000
cycles
Noise and Hum:
90 db or more below rated output
Output Impedance:
Stereo or Twin Amp:
(Center tap internally grounded) 600 ohms
(One side internally grounded) 4 ohms,
8 ohms, 16 ohms, 150 ohms, 200 ohms
Mono:
(Center tap internally grounded) 300 ohms
(One side internally grounded) 75 ohms,
100 ohms
Output Voltages: One side grounded
Stereo (or twin amplifier):
25 and 70.7 volts
Mono (single channel):
25 and 70.7 volts
Internal Impedance:
Less than 10% of rated load impedance
Input:
Input impedance: 250,000 ohms
Input sensitivity: 0.5 volts to 30 volts
through gain control. 2.0 volts with gain
control turned to dot indicator.
Power Requirements:
117 volts AC, 50/60 cps
Uses 200 watts at rated output; 85 watts
at zero signal output.
Fuse
2.0 Amp Slo Bio
Tube Complement:
Voltage Amplifier—1ea. 12AX7
Phase Inverter—2 ea. 12AU7
Drive Amplifier—2 ea. 12BH7
Output—4 ea. 7591
MECHANICAL SPECIFICATIONS
Dimensions:
Chassis dimensions: 14½" long x 91/8"
wide x 7" high. (See INSTALLATION on
Page 5 for minimum mounting space.)
Weight:
Chassis only—34 pounds. In shipping
carton—39 pounds.
Finish:
Chrome and Black
Accessories Supplied:
Instruction Manual
Mounting Template
Mounting Flanges
Octal output plug with cap
3
PANEL
FACILITIES
Fig. 3—End panel showing all input and output facilities.
INPUT
The input of the MC225 has a two position lever switch to permit the amplifier to be
used in one of two ways:
(1) As a monophonic 50 watt amplifier
(2) As twin 25 watt amplifiers used with an
electronic crossover network, or two
completely separate amplifiers, or in
stereo applications.
With the input switch in the "Stereo"
position the input marked "L" and the input
marked "R" both have sensitivities of 0.5
volts when their gain controls are set fully
clockwise. The input impedance of each is
250,000 ohms. Each gain control allows
signal sources with output from 0.5 volts to
30 volts to be connected without overloading the input to the amplifier. The controls
are set for 2.0 volts sensitivity by turning
them to the dot indicator.
With the input switch in the "Mono"
position the control marked "Right Mono
Gain" sets the input sensitivity at the input
marked "R."
The MONO position of the input switch
parallels the inputs to both amplifiers. When
the outputs are properly connected the
MC225 becomes a 50 watt monophonic
amplifier. The MONO input GAIN control
permits connecting signal sources up to 30
volts without overloading the amplifier's
input: The input impedance is 250,000 ohms.
OUTPUT
The two barrier terminal strips marked
OUTPUT provide stereo connections for the
normal speaker impedances of 4 ohms, 8
ohms, and 16 ohms. For monophonic operation connections for 2 ohms, 4 ohms, 8 ohms,
4
and 16 ohms are provided. The terminal
strips may also be connected for a constant
voltage output of 25 volts in either stereo
or mono.
As supplied, the secondary of the MC225
is grounded.
The barrier strip output connections are
fed from the secondary of the output transformer.
LINE OUTPUT (See also pages 5, 6, and 7)
The octal socket marked LINE OUTPUT
has connections for 25 volts, 70.7 volts, and
600 ohms for stereo operation and 300 ohms
for monophonic operation.
The 600 ohm winding and the 300 ohm
winding are balanced to ground. Pin 1 of the
octal socket is the center tap for these windings and is grounded.
The connections at the octal socket are
fed from taps on the cathode winding in the
primary of the output transformer.
FUSE
The MC225 uses a 2.0 ampere slo-blo
type fuse. The auxiliary AC socket is not
fused.
AC OUTLET
The auxiliary AC outlet can be used to
supply power to other equipment in the system. The outlet will provide a maximum of
400 watts of power. The AC outlet is not
fused.
LINE VOLTAGE
The MC225 operates on line voltages
between 105 volts to 130 volts, 50 cycles to
60 cycles.
INSTALLATION
Adequate ventilation should always be
provided in the installation of electronic
instruments. The trouble free life of these
instruments can be extended by adequate
ventilation. It is generally found that each 10°
centigrade (18°F) rise in temperature reduces the life of electrical insulation by one
half. Adequate ventilation is an inexpensive
and effective means of preventing insulation breakdown because of unnecessarily
high operating temperatures. The direct
benefit of adequate ventilation is longer,
trouble free life.
The suggested minimum space for mounting the MC225 is 16" long x 10" wide x 10"
high. Always supply ventilation holes in the
bottom of the amplifier mounting space and
a means for the warm air to escape at the top.
The MC225 can be mounted in any position except upside down. If the amplifier is
to be installed on a vertical surface it is
recommended that the transformers be on
the down side. The advantage of this position is that the flow of heat from the tubes
rises vertically and does not tend to heat
the transformers.
if the MC225 is to be permanently mounted
use the two mounting flanges supplied with
the amplifier. The flanges are shipped
separately and must be attached to the
amplifier bottom at each end.
Turn the MC225 over with the transformers down and place it on a piece of cloth
or cardboard to prevent scratches. The
amplifier is shipped with 4 plastic feet on the
bottom cover. Remove the 4 plastic feet.
Remove the three self tapping screws at one
end of the amplifier that hold the bottom
cover to the chrome chassis. Place the
square edge of the mounting flange with the
three round holes positioned over the three
holes in the bottom cover. The three holes in
the mounting flange will line up over the
three holes in the chassis. Replace the three
self tapping screws through the mounting
flange and bottom cover. The mounting
flange is now firmly attached to the amplifier. Repeat the same procedure to install
the mounting flange on the other end of
the amplifier.
Place the MC225 mounting template on
the area where the amplifier is to be mounted.
Center punch the 4 slotted holes in the
mounting flanges shown on the template.
Remove the template and install four #10
round head screws in the center punch
marks. Do not tighten these screws. Place
the amplifier over the screw heads and slide
the amplifier to either side in the slotted
holes in the mounting flanges. The #10
wood screws can then be tightened securely.
Fig. 4 Bottom of MC225 with mounting flanges attached.
CONNECTING THE MC225
INPUT-STEREO OR TWIN AMPLIFIER
The shielded cable from the left output
of the Mclntosh preamplifier is plugged into
the jack marked INPUT-L. The shielded cable
from the right output of the McIntosh preamplifier is plugged into the jack marked
STEREO INPUT-R. The input switch is moved
to "Stereo."
5
INPUT-MONO
The shielded cable from the program
source is plugged into the jack marked
INPUT-R and the input switch moved to
"Mono."
OUTPUT-STEREO OR TWIN AMPLIFIERS
For stereo or twin channel operation it is
not necessary to use the same value of load
impedance on each output. Simply connect
each output for the impedance desired.
Warning: Do not parallel the amplifier outputs when using the amplifier as a stereo
amplifier or twin channel amplifier. Damage
to the output tubes may result if parallel
operation is attempted when the output voltages are not the same phase and amplitude.
Speakers are connected at the barrier
strips marked OUTPUT on the left end of
the panel.
In compliance with the National Electrical
Code, Class II wiring can be used between
the speaker and the amplifier at the 4 ohm,
8 ohm, or 16 ohm connection. Class II wiring
is lamp cord, bell wire, or other wire with
this type of insulation. For the normally short
distances of under 100 feet between the
amplifier and speaker, #18 wire or larger
can be used. For distances over 100 feet
between the amplifier and speaker use
larger wire.
The loudspeaker impedance is usually
identified on the loudspeaker itself. Connect
one of the leads from the left loudspeaker to
the screw marked COM on the LEFT barrier
strip. Connect the other lead from the left
loudspeaker to the screw marked with the
number corresponding to the speaker impedance on the LEFT barrier strip.
Connect one left speaker
Connect one right speaker
If the speaker lead to the screw marked
lead to the screw marked
impedance is:
LEFT-COM and the
RIGHT-COM and the other to:
other to:
4 ohms
LEFT-4
RIGHT-4
8 ohrns
LEFT-8
RIGHT-8
16 ohms
LEFT-16
RIGHT-16
The only adverse effect on the operation
of a Mclntosh amplifier when it is improperly
matched is a reduction in the amount of
distortion-free power available to the loudspeaker. Close impedance matching is de6
sirable for maximum distortion-free power.
Use this table to determine proper speaker
connections:
Connect the
If the speaker impedance speaker leads
is between:
between COM and:
3.2 to 6.5 ohms
4 ohms
6.5 to 13 ohms
8 ohms
13 to 26 ohms
16 ohms
OUTPUT-MONOPHONIC
When the MC225 is to operate as a 50
watt monophonic amplifier, the outputs of
the two channels combine to produce a
single 50 watt output. This chart lists the
proper connections and interconnections for
monophonic operation.
If the Connect one speaker
speaker
lead to the screw
impedance marked LEFT-COM
Connect a
and the other to:
wire between:
is:
LEFT-COM and
LEFT-4
2 ohms
RIGHT-COM
Connect another
wire between:
4 ohrns
LEFT-8
LEFT-COM and
RIGHT-COM
LEFT-4 and
RIGHT-4
LEFT-8 and
RIGHT-8
8 ohms
LEFT-16
LEFT-COM and
RIGHT-COM
LEFT-16 and
RIGHT-16
When connected as outlined the MC225
operates as a 50 watt monophonic amplifier.
LINE OUTPUT-STEREO or TWIN AMPLIFIERS
The octal socket marked LINE OUTPUT
has connections for 70.7 volts, 200 ohms and
600 ohms. The 25 volt line is fed from the
barrier strips.
For 25 volt line operation connect one of
the left leads to the screw marked COM on
the LEFT barrier strip. The other left lead is
connected to the screw marked 16 on the
LEFT barrier strip. Connect the right leads
in the same manner on the RIGHT barrier
strip.
The 200 ohm and 70.7 volt connections are
the same. To connect for 200 ohm or 70.7
volt operation the octal plug supplied in the
hardware bag is used. Solder one of the left
leads to pin #7 and the other to pin #1.
Solder one of the right leads to pin #8 and
the other to pin #1. Pin #1 is the ground
side.
The 600 ohm connections for the left side
are made between pin #2 and pin #3. The
600 ohm connections for the right side are
made between pin #4 and pin #5. The 600
ohm winding is balanced to ground. Pin #1
on the octal socket is the center tap of the
600 winding. Pin # 1 is grounded.
For all applications using 600 ohms use
Class I wiring if the installation must meet
the requirements of the National Electrical
Code.
LINE OUTPUT-MONOPHONIC
The octal socket marked LINE OUTPUT
has connections for operating the MC225
monophonically at 70.7 volts, 100 ohms or
300 ohms. For operating at 25 volts connections are made at the barrier strips.
To feed a 25 volt line, connect one side to
the screw marked COM on the LEFT barrier
strip. Connect the other side to the screw
marked 16 on the LEFT barrier strip. Then
connect with a wire the screw marked COM
on the LEFT barrier strip to the screw marked
COM on the RIGHT barrier strip. Connect,
with another wire, the screw marked 16 on
the LEFT barrier strip to the screw marked 16
on the RIGHT barrier strip.
The 100 ohm and the 70.7 volt connections are the same. To connect for 100 ohms
or 70.7 volts solder one of the leads to pin
#1 in the octal socket. The other lead is
soldered to pin #7. A wire is connected between pin #7 and pin #8.
To connect for 300 ohms one lead is
soldered to pin #2. The other lead is soldered to pin #3. Connect pin #2 to pin #4
with a wire. With another wire connect pin
#3 to pin #5. Pin #1 is the center tap of the
300 ohm winding. The 300 ohm winding is
balanced to ground.
AC POWER
The MC225 operates on 105 volt to 130
volt, 50 to 60 cycles power. The amplifier will
be turned on and off if its power cord is
plugged in one of the auxiliary AC outlets on
the program source.
OPERATING THE MC225
Before turning the MC225 on check all
connections and plugs to see that they are
firmly and correctly connected. Check to
make sure that the tubes are firmly seated
in the proper sockets. After the following
adjustments have been completed the MC225 will operate without any further attention.
ADJUSTMENTS
INPUT SWITCH-STEREO
With the input switch in the left position
marked STEREO the input of the MC225 is
connected to the two jacks marked "L" and
"R" INPUT located below the GAIN controls. The amplifier has an input sensitivity
in this position of 0.5 to 30 volts.
The two center control knobs marked
GAIN should be adjusted to match the input
sensitivity of the MC225 to the output voltage
of the program source equipment.
To balance a stereo system using the
STEREO input position use a monophonic
signal from the program source. A monophonic signal supplies the same voltage to
both channels. Turn the volume control on
the source equipment to the 12 o'clock position (half rotation). Turn up the GAIN control on the LEFT input until the loudness in
the room is not quite as loud as you like to
listen. Then turn the GAIN control on the
RIGHT input until it is equally as loud as the
left channel. The system is now balanced for
loudness and provides the most convenient
amount of loudness change when using the
volume control on the source equipment.
INPUT SWITCH-MONO
With the switch in the right position
marked MONO the inputs of the MC225 are
internally connected in parallel and to the
jack marked "R" INPUT. This jack is located
below the right control knob marked "RIGHT
MONO GAIN." The input sensitivity in this
position is 0.5 volt. The GAIN control should
be adjusted to match the input voltage sensitivity of the MC225 to the output voltage of
the program source.
For maximum flexibility set the GAIN
control as follows: Turn the volume control
7
on the program source equipment (i.e. tape
recorder, tuner, preamplifier, etc.) to the
12 o'clock (half rotation) position. Turn the
"RIGHT MONO GAIN" control on the MC225
until the loudness of the sound from the
speaker is just a little louder than you normally like to listen. The volume control on
the program source equipment now has the
most convenient amount of loudness change
in either direction from the 12 o'clock position.
IMPORTANT
The excellent performance that is inherent in all Mclntosh amplifiers does not
depend on the critical adjustment of bias or
balance controls in the output circuit. The
patented Mclntosh circuit delivers its advertised specifications without any need for
these controls and is not dependent on carefully balanced tubes for its performance.
With Mclntosh you can install the amplifier
and forget it.
The MC225 you have purchased will give
you years of pleasant and satisfactory performance. If you have any questions concerning the operation or maintenance of this
amplifier please contact:
Customer Service
Mclntosh Laboratory, Inc.
2 Chambers Street
Binghamton, New York
Our telephone number is 723-5491.
The direct dial area code is 607.
GUARANTEE
Mclntosh Laboratory Incorporated guarantees this equipment to perform as advertised. We also guarantee the mechanical and
electrical workmanship and components of
this equipment to be free of defects for a
period of 90 days from date of purchase.
This guarantee does not extend to components damaged by improper use nor does it
extend to transportation to and from the
factory.
3-YEAR FACTORY SERVICE CONTRACT
An application for a FREE 3-YEAR FACTORY SERVICE CONTRACT is included in the
pocket in the back cover of this manual. The
FREE 3-YEAR FACTORY SERVICE CONTRACT
will be issued by Mclntosh Laboratory upon
receipt of the completely filled out application form. The term of this contract is defined
in the 3-year factory service contract. If the
application is not mailed to Mclntosh Laboratory only the services offered under the
standard 90-day guarantee will apply on this
equipment. TAKE ADVANTAGE OF 3 YEARS
OF FREE FACTORY SERVICE BY FILLING
IN THE APPLICATION NOW.
In Canada: Manufactured under license by:
McCurdy Radio Industries, Ltd.
22 Front Street West
Toronto, Canada
Design subject to change without notice.
8
LABORATORY
INC.
2 CHAMBERS STREET, BINGHAMTON, N. Y.
Made in U.S.A.
Phone-Area Code 607-723-5491
Form No. 225-HP-761-2M
AA-126-161
Printed in U.S.A.