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
Nessun commento:
Posta un commento