McIntosh MC-2505 User Manual
STEREO
MC2505
PRICE $1.25
POWER AMPLIFIER
MC25O5
SOLID
STATE
The Mclntosh "will to perfection" requires that we probe
constantly into the unknown to bring the performance of our
electronic equipment closer to perfection than ever before.
This requires a constant and relentless search for low noise,
broad band conservative design with an ever lower distortion
factor. This is not required of ordinary equipment of average
designs. It is, for us, a costly but worthwhile scientific and
engineering effort. Our continuing research benefits our customers with the almost complete lack of obsolescence and
the most reliable equipment ever made. It also means the
lowest long-range cost to you. Nearly all of the Mclntosh
equipment ever made is still useable, or in use, though it
may have been made twenty years ago.
MC2505
CONTENTS
GENERAL DESCRIPTION 2
TECHNICAL DESCRIPTION 2,3
LOCKING METERS 4
PERFORMANCE CHARTS 5,6
CHARTS 8
SPECIFICATIONS 9
IN A HURRY 10
INSTALLATION 11
TYPICAL HOOKUP 12
CONNECTING 13
BLOCK DIAGRAM 14
OPERATING 15
GUARANTEE 16
STEREO
POWER AMPLIFIER
Your purchase of a Mclntosh instrument
shows that you are a careful discriminat-
ing buyer. One who is interested in quality
performance, quality engineering, quality
manufacturing, and long trouble-free
equipment life. You can protect your investment by spending a few minutes reading this owner's manual.
When you bought a Mclntosh, you bought
countless hours of musical pleasure and
superior performance. Enjoy it!
MC25O5
1
SOLID
STATE
GENERAL DESCRIPTION
TECHNICAL DESCRIPTION
The dramatic difference in the quality of music reproduced through
a Mclntosh instrument is due to low distortion. The distortion of your
MC2505 is guaranteed to be less than 0.25% at any frequency from
20 Hz to 20 kHz with both channels operating at 50 watts RMS.
Distortion is measured at full rated power output with both channels
operating. At less than rated power, distortion becomes so small it
can be measured only by the most sophisticated laboratory instruments. Only Mclntosh gives you this kind of performance.
Your MC2505 passed more than 75 tests before it was ready for
you. Each connection, wire, resistor, capacitor is checked and rechecked. All specifications are checked. Mclntosh testing takes time.
The extra investment in thorough testing assures you of greater
musical enjoyment.
The performance of your Mclntosh MC2505 is backed by a money
back guarantee. Only Mclntosh gives you a money back guarantee
of performance. Your MC2505 must be capable of meeting its pub-
lished specifications or you get a refund of your purchase price.
Mclntosh promises performance. We either meet our promise or you
get your money back.
Your MC2505 can be protected by a free three year factory service
contract. Take advantage of this service. Fill in the application card
found in the owner's packet. The free three year factory service
contract covers parts and labor. If anything goes wrong just bring
your MC2505 to a factory service station, or return your MC2505
to Mclntosh. All parts and labor necessary to repair your MC2505
will be supplied free of charge. Fill in the service contract application found in the owner's packet now.
A two stage preamplifier with three transistors in each channel
increases the input voltage 16 dB.
There are 13 transistors in each power amplifier section. The two
stage preamplifier is fed to a pair of matched transistors arranged
as an emitter coupled amplifier with two inputs and one output.
The signal from the preamplifier section connects to one of these
inputs. Both AC and DC negative feedback are applied to the other
input. This large quantity of feedback is used to reduce noise and
distortion. The signal is then fed to a voltage amplifier. The voltage amplifier is followed by two driver transistors.
The output section is arranged as a series push-pull amplifier.
The power transistors used in the output section of your MC2505
are selected for their high power dissipation capability, wide
frequency response, and large "safe operating area." In addition,
each power transistor is given four separate tests before it is put
in your MC 2505. This additional testing makes sure your MC 2505
will deliver its rated power from 20 to 20 kHz with low distortion
and complete reliability.
The power transistors are mounted on oversized anodized heat
sinks. The heat sinks assure that under normal operation the
transistors will operate at a low temperature. If temperatures increase due to a shorted speaker, or restricted ventilation, an automatic temperature sensing device turns off the MC2505. The
device operates automatically at a preset temperature. The MC
2505 will turn on again when the temperature has returned to
normal limits. This additional feature gives your MC2505 complete reliability under the most extreme operating conditions.
2
The output stages are matched to the load by the Mclntosh autoformer. The Mclntosh autoformer is carefully wound using Mclntosh trifilar winding and interleaving techniques. Trifilar winding
and interleaving gives the transformers exceptional bandwidth.
The autoformers properly match the power transistors to 4, 8, and
16 ohm loads at all audio frequencies.
The use of the Mclntosh designed trifilar autoformer makes the
Mclntosh solid state amplifiers the only amplifiers that deliver
FULL POWER AT ALL SPEAKER IMPEDANCES. You have not
been power penalized for your choice of loudspeakers when using
the Mclntosh MC2505.
Another of the advantages of the autoformers is the 25 volt output
for a constant voltage distribution system. With the MC2505 several sets of speakers can be operated independently throughout
your home.
POWER SUPPLY SECTION
To further insure reliability a special power output SENTRY MONITORING CIRCUIT prevents failure of the power output transistors
due to excessive mismatch of the output. When your MC2505
operates normally the SENTRY MONITORING CIRCUIT has no
effect on signals passing through the power amplifier. If the
power dissipation should rise above normal operation, the SENTRY MONITORING CIRCUIT restricts the drive to the output
transistors. The SENTRY MONITORING CIRCUIT acts instantaneously for any input signal or load combination. This arrangement
assures complete circuit reliability. Only Mclntosh gives you this
degree of protection.
There are three separate power supply sections. One positive and
one negative high current supply is used for the output stages.
The other positive supply is used for the driving amplifier stages.
All supplies are full wave and use silicon rectifiers. Adequate filtering is used to assure an absolute minimum of hum. The power output stage filter capacitors have very high capacity, which allows
full power output below 20 Hz. The power transformer is generous
in size and runs cool, even under heavy use.
3
DYNAMIC PEAK
LOCKING METERS
Ordinary meters lack the capability of indicating
the short interval power in a sound wave. The
mass of the meter movement is too great to respond to instantaneous changes in music program material. Mclntosh superior engineering
has developed new circuitry that permits the
meters on the MC2505 to respond to the short
interval power in a sound wave to an accuracy of
98% of the true value. This is another Mclntosh
development that represents a major step forward in the use of power level meters.
There are two circuits that give these meters
the indicating capability of the short interval
power in a sound wave. The first circuit is an
accelerating circuit that compensates for the
inertia characteristics of the meter movement.
Because the short interval power fluctuation is
so rapid, the eye might not perceive the instantaneous power reading. This caused the development of the second circuit, which is a "time
stretching" circuit. The time stretching circuit
delays the movement of the meter needle at
peak reading for a few milliseconds.
With the aid of the CBS test record STR100, the
frequency response of your phono cartridge can
be measured. The graph on page 5 shows the
ideal RIAA curve using the CBS record STR100.
Follow these steps to plot the performance of
your phonograph cartridge.
1. Set the "METER RANGE SWITCH" to the
—20
position.
2. Play the 1000 Hz test tone recorded on
the CBS Test Record STR100 on your
phonograph.
3. Turn the "LEFT GAIN" control until the
left meter indicates "0."
4. Turn the "RIGHT GAIN" control until the
right meter indicates "O."
5. Write down the meter indication at each
frequency as the record plays.
6. Transfer the readings by frequency to
the graph.
7. The graph shows the ideal RIAA re-
sponse curve using the CBS #STR100 test
record. Compare your curve with the curve
on the graph. A deviation of 3 dB from the
ideal is acceptable. By making this check
at regular intervals, (for instance, every 6
months) any deterioration in the cartridge
or system will be quickly detected.
A tape recorder can be checked in the same
fashion.
1. Use a standard frequency response tape
as the signal source.
2. Complete all steps outlined for phono
cartridges.
3. You now have a graph of the playback
characteristics of your tape recorder.
To find the record characteristics of the tape
recorder follow this procedure:
1. Record the CBS Test Record #STR100
on your tape recorder. Adjust the record
volume only on the 1000 Hz signal for
proper recording level. DO NOT ADJUST
THE RECORD VOLUME CONTROL DURING THE RECORDING.
2. Play back the tape just recorded. Complete all steps outlined for tape playback
characteristics.
3. A comparison of the two curves will give
the recording characteristics of your tape
recorder. A deviation of 3 dB is acceptable.
Similar checks can be made on all program
sources in your stereo system. Follow the same
general procedure for any program source for
which a standard reference is available.
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