Input Amplifier
Each channel input
contains a complete
seven transistor low power amplifier.
A differential transistor pair
provides high input impedance
and low noise.
The differential signals are
combined in a cur-rent
mirror circuit which
drives a class
A amplifier stage.
The following output
stage is a
complementary pair of
transistors with class
AB biasing. The
output signal drives
the metering circuit,
headphone jack, and the high
power output amplifier. This discrete
transistor amplifier design was
selected for low noise, low distortion, adequate power output
capability to drive headphones, and
freedom from turn
on and turn
off transients.
The Input
Level selector and
Gain controls are
passive attenuators which precede the input amplifier circuitry.
Therefore, the input
system to the amplifier
cannot be overloaded
when the controls
are correctly set.
In the
Mono mode of
operation the input
signal feeds only the right input
amplifier via the Right/Mono Gain control.
The output of
the right input amplifier feeds both output power
amplifier sections. When
Mono Bridge Mode is selected
the left channel input amplifier is used as a phase inverter
before the left output amplifier.
The output channels are therefore 180° out of phase with
respect to each
other which is
the correct arrangement
for bridge output
connections. When Mono Parallel
Mode is
selected the channels
operate in phase. The
channels, of course,
also operate in phase
for the Stereo Mode
Output Power
Amplifier
The power
amplifier inputs are
coupled to the
input amplifier outputs
through an electronic
switch. The switch
eliminates turn-on and
turn-off transients and is used
for speaker switching.
A junction FET
and LED/light dependent resistor
network make up the
switch. This combination allows the lowest possible distortion when the amplifier is on and
high isolation when
the output power
amplifier is off.
The control signal to the switch is
held off during
the turn-on delay
period. Therefore, transients
that may come
into the amplifier
from source equipment
will not be
amplified or reach the
loudspeaker. Since the headphone output and meters
are powered by the input
amplifier, their operation is
not affected by the Speaker switch
or turn on
delay system. The first
stage of the
output power amplifier
is a differential
transistor pair biased
for best linearity.
The offset to
the differential pair
is adjustable. Correct
adjustment allows the lowest possible distortion
at low frequencies.
A current mirror circuit combines the
differential outputs into
one signal which
is then amplified by a following class A voltage amplifier. Both the differential transistors
and the voltage amplifier are supplied
by active current sources. The results
are lower distortion and cleaner turn on
characteristics.
The driver
stage consisting of a
complimentary pair of
power transistors biased
class AB follow
the voltage amplifier.
Next, six complimentary
pairs of rugged
power transistors make
up the power
output stage. All power
transistors are mounted
on conservatively sized anodized
aluminum heat sinks. Because of a unique
connection of the bias
network, the output transistors operate class B
and exhibit no crossover
distortion often associated
with class B
operation. The heat sinks, therefore remain cool when there is no output. During heavy demands,
temperature controlled fans keep the heat sinks cool.
The amplifier output signal
is fed to the
output terminals through the
output autotransformer. The Mclntosh designed interleaved multifilar wound
autotransformer is used to properly match the amplifier to stereo
output load taps
for 1, 2,
4 and 8
ohms. The MC
2500 will deliver
full power over
the entire audio frequency
range at any of these impedances. 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 DC cannot
damage the speaker.
A 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 impedance mismatch
the Sentry Monitoring
circuit will protect the output
transistors from failure. Both positive
and negative halves
of the output signal
are monitored and
protected independently
Power Guard Protection
Circuit
The Mclntosh
patented Power Guard circuit eliminates
amplifier clipping due to overdrive.
The circuit also illuminates red
Limit indicator lamps when the amplifier
is driven beyond its maximum
output capacity. Power
Guard prevents loudspeaker
damage and eliminates harsh output distortion caused by
amplifier clipping.
The Power
Guard circuit consists
of a waveform
comparator which monitors
the wave shape
of the amplifier input
and output signals.
Normally there is
no disparity between these signals
and the comparator produces
no output. When
the amplifier is
driven beyond its maximum
power capacity a
difference will developed.
If the disparity
exceeds 0.5% (equivalent to 0.5% total harmonic distortion)
the comparator output
causes the red
Limit indicators to
light. If there
is a further
increase in the
disparity the comparator output
controls an electronic attenuator at the amplifier input to reduce the
amplifier gain, thus
holding the amplifier
output to its maximum undistorted value regardless of
the degree of overdrive
to the amplifier.
The amplifier may
be overdriven by 20
dB before the output
distortion exceeds 2% .
The comparator is an
especially compensated operational
amplifier integrated circuit.
Its output is
detected by a
full wave bridge
that feeds signals
to the control
circuitry for the
Limit and Normal
indicators and to the electronic attenuator at the amplifier input. The
attenuator is a light emitting
diode/light dependent resistor network selected especially for
its low distortion
and time constant
characteristics
Meter Circuit
The meter circuit
has three basic
sections: a logarithmic amplifier, a full wave
rectifier, and a DC amplifier. In the
Watts ranges, the
logarithmic amplifier is used. In
the DECIBEL ranges, the signal
bypasses this amplifier and
goes directly to
the full wave
rectifiers through an
attenuator which is
controlled by the
Meter Range switch.
The logarithmic
amplifier consists of a high
gain operational amplifier
with a biopolar
connected silicon diode
pair as feedback
elements. These diodes
have a uniform
logarithmic characteristic over an
80 dB range. Only 60
dB of this
logarithmic range is
used in the
MC 2500.
The full
wave rectifier circuit
uses an operational
amplifier with silicon diode
feedback networks. This amplified diode
circuit has nearly
perfect rectification characteristics. One
rectifier detects only
positive signals. The other responds only to negative signals and
produces a positive
output. The outputs of the rectifiers are combined at the
operational amplifier output, so the highest signal, either positive or
negative, is the
one that is
indicated by the
meters. Gate diodes
are used to
charge a low
leakage capacitor which
attains and holds
a charge during
signal peaks. The
operational amplifier provides
a large amount
of current so
the capacitor can charge
suddenly. The charge
on the peak
holding capacitor is amplified in
a two transistor DC amplifier which
is used to
drive the meter.
From the output
of this amplifier
there is a
DC feedback network
that connects back
to the detector
to assure excellent overall linearity and
frequency response. The current drive to the meters has a peaking
capacitor to accelerate
the upscale response
of the meter
needle. The meters
also have a
parallel shunt resistor to correctly damp their action. In the
Watts mode the discharge
of the peak holding
capacitor is controlled
by a resistor
current source. In Watts Hold, the resistor is disconnected
so the peak reading is retained.
The rate of
decay is about
6 dB per
minute.
Power Supply
The power supply is
a conventional full wave bridge rectifier arrangement providing plus and
minus 50
volts DC. Electronic
regulators step down and
regulate plus and
minus 15 volt
sources for low
level circuits. Thermistors are used in the power transformer primary
circuit to limit
the turn-on current.
Specifications
Power Output
Stereo : 500
watts minimum sine
wave continuous average
power output, per
channel, both channels operating
into 1, 2,
4 Ω , or 8 Ω load
impedance, which is:
22.4 volts RMS
across 1 Ω
31.6 volts RMS
across 2 Ω
44.7 volts RMS
across 4 Ω
63.2 volts RMS
across 8 Ω
Mono : 1000 watts
minimum sine wave
continuous average power output into 0.5; 1; 2; 4; 8; or 16 Ω load impedance,
which is:
22.4 volts RMS
across 0.5 Ω
31.6 volts RMS
across 1 Ω
44.7 volts RMS
across 2 Ω
63.2 volts RMS
across 4 Ω
89.4 volts RMS
across 8 Ω
126.5 volts RMS
across 16 Ω
Output Load
Impedance
Stereo :
1 Ω
, 2 Ω , 4 Ω , and 8 Ω ;
separate terminals are
provided for each
output.
Mono-Parallel :
0.5 Ω , 1 Ω , 2 Ω , and 4 Ω ;
obtained by connecting
in parallel the
appropriate terminals of
both channels.
Mono-Bridged :
2 Ω ,
4 Ω ,
8 Ω ,
or 16 Ω ;
obtained by connecting
to the output terminals
of both channels.
The bridged output
is balanced to
ground. Neither side
is grounded.
Rated Power
Band: 20 Hz
to 20,000 Hz
Total Harmonic
Distortion
Stereo : 0.02% maximum
harmonic distortion at any power level
from 250 milliwatts to 500
watts per channel
from 20 Hz to 20,000 Hz,
both channels operating.
Mono : 0.02% maximum
harmonic distortion at any power
level from 250 milliwatts
to 1000 watts from 20 Hz to 20,000
Hz.
Intermodulation
Distortion
Stereo : 0.02% maximum if instantaneous
peak power output
is 1000 watts or
less per channel with
both channels operating
for any combination
of frequencies, 20 Hz
to 20,000 Hz.
Mono : 0.02% maximum if instantaneous
peak power output
is 2000 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 +
0.25 - 1 dB.
Noise and Hum: 95
dB below rated
output
Damping Factor: Greater
than 30
Input Impedance: 50,000 Ω
Input Sensitivity
(Switchable): 0.75 volt
or 2.5 volts, Level control
provided for higher
input voltages.
Power Guard: THD not
to exceed 2% with up to 20 dB overdrive at 1 kHz.
Power Requirements:
120 Volts 50/60
Hz
Power
Consumption: 0.7 to 22 amps.,
15 amps UL/CSA
Semiconductors: 91 silicon
transistors; 35 silicon rectifiers
and diodes; 6 integrated circuits
Dimensions
Front Panel (W x H): 19" x 10½" (48,26 x 26.67 cm).
Chassis (w H x D)
: 17" x 10" x 17" (43,18
x 25,4 x 43,18 cm), including
connectors.
Clearance in front
of mounting panel including knobs
2" (5.08 cm)
Finish: Front
panel is anodized
gold and black.
Chassis is black
baked enamel.
Mounting: Standard
19" (48.26 cm)
rack mounting
Weight: 129
pounds (58.5 kg)
net, 144 pounds (65.3
kg) in shipping
