The MR 500
Digital FM Tuner complements the performance and appearance of all Mclntosh
products. It is specifically designed to match the industry's leading compacts
— the Mclntosh MC 502 Power Amplifier and the Mclntosh C 504 Preamplifier.
Separate, flexibile Mclntosh quality components that are particularly useful
where space is limited.
On the front
panel of the MR 500 there are: volume control with tuner on/off AC power
switch, 6 momentary feather touch push buttons for selecting preset stations, a
jack for headphone listening, 1 on/off push button for between station noise
muting, 4 momentary tuning assistance feather touch push buttons, a digital
frequency display, 6 preset station indicators, an all solid state tuning
indicator and the manual tuning knob and indicator.
TUNING
SYSTEM
Todays FM
electronic tuning systems can generally be classified as either a Phase Locked
Loop (PLL) Synthesizer System or Voltage Synthesizer, Signal Locked Loop
System.
Mclntosh has
selected the Voltage Synthesizer, a Signal Locked Loop (SLL) System which locks
the MR 500 to the incoming signal of the FM station. With the tuner tuned to
the station frequency and the correction voltage being generated by the
detector "zero" crossing, all parts of the tuner, mixer, local osc.,
IF, etc. are inside the tuning loop and accurate tuning is assured. As long as
the station transmits within the tuning range of the MR 500, regardless of
frequency, the SLL system will lock on to the station and remain center channel
tuned.
In contrast,
in the PLL system, the local oscillator frequency is compared to a reference
within the tuner, usually a crystal controlled oscillator. A comparison circuit
generates a correction voltage to hold the local oscillator to an exact
multiple of the reference oscillator. The frequency of the reference oscillator
must be equal to the station channel spacing (200 kHz in the United States) in
order to cover all the FM channels. Tuning is accomplished in discrete steps
which makes it impossible to tune to frequencies between channels. In areas
where FM programs are distributed by cable or non-standard frequencies, and in
other parts of the world where stations may be assigned to frequencies other
than those standard in the United States, proper reception is not possible.
Another limit of the PLL system is that the system can generate harmonics and
spurious responses in the divider chain and frequency/ phase comparator. These
unwanted signals will add noise interference to the received station.
The
superiority of the Signal Locked Loop System made it the obvious choice for the
demanding performance required of the MR 500.
FREQUENCY
COUNTER
The
frequency counter consists of two IC's. One is an Emitter Coupled Logic (ECL)
buffer circuit and divide by 100 prescaler. The other is a C-MOS LSI IC that
contains the reference oscillator, various dividers, latches, and LED segment
drivers for the display. The LED display does not generate RF interference
because it is static, not multiplexed. An adjustable IF offset is provided so
that the display reads the tuned frequency accurately for the full span of IF
filter tolerances.
FM TUNER
Antenna
connections for 300 ohm twin lead and 75 ohm coaxial cable are provided on the
rear panel. The normal input impedance of the FM-RF section is 75 ohms. A
Mclntosh designed and manufactured internal balun matching transformer provides
a perfect match for a 300 ohm antenna. Connections for the twin lead are made
with push type terminals and a type F connector is provided for the coaxial
connection.
Following
the antenna matching circuit is a RF tuner of exceptional performance. It uses
a DMOSFET RF amplifier, a double balanced mixer, and circuits that are tuned by
a series parallel connection of four matched varactor diodes, all of which
produce high spurious rejection and great sensitivity. This circuitry and the
high tuning voltage (5-25V) eliminates RF intermodulation distortion caused by
diode non-linearities.
The FM IF
section uses 3 transistors, 3 linear phase piezoelectric filters, and one
integrated circuit. They combine to provide over 120dB of gain and a
selectivity greater than 65 dB. Limiting, muting, signal strength drive, and FM
detection are all functions of the integrated circuit.
A phase
locked loop (PLL) stereo decoder integrated circuit is the heart of the
multiplex section. It has a high signal to noise ratio, low distortion, high
channel separation, and high SCA rejection. The PLL MPX IC eliminates inductors
to minimize drift, provides integral lamp driving capability to indicate the
presence of the 19 kHz pilot, and has transient free mono/stereo switching.
Following
the PLL MPX decoder is a J-FET switched filter circuit that reduces 'out of
phase noise' when the MR 500 is tuned to a weak stereo station. A LC tuned
notch filter is used to prevent tape recorder bias interference. The LC filter
is driven from the MPX output amplifier and is terminated by the following
fixed output operational amplifier. This provides the necessary filter input
and output impedances for proper phase response. A low power amplifier with
less than .01% harmonic distortion drives headphones output and the variable
output jacks.
The MR 500
has transient free turn on/off characteristics. A light dependent resistor,
whose light source is time controlled, connects the tuner outputs approximately
two seconds after the power is turned on to allow all circuits to stabilize
before sound can be heard.
POWER SUPPLY
The + 15, -
15, and + 5 volt supplies use IC three terminal regulators, while the +33 volt
supply uses a zener diode regulator. This insures proper operation of the tuner
even during periods of low AC line voltages.
Specifications
Tuning
Range: 88 MHz to 108 MHz
Tuning
Methods: Manual tuning. Push buttons to
select any one of six stations stored in the electronic memory. Scan tuning up
or down; scan-up can be controlled remotely.
Usable
Sensitivity: 13 dBF, which is 2.5 µV across 300 Ω or 1.25 μV across 75 Ω
50 dB
Quieting Sensitivity:
Mono
: 16.8 dBF, which is 3.8 μV across 300 Ω or 1.9 μV across 75 Ω
Stereo
: 37.3 dBF, which is 40 μV across 300 Ω or 20 μV across 75 Ω
Frequency
Response:
Mono : 20 Hz to 15 kHz ±1 dB
Stereo: 20 Hz to 15 kHz ± 1 dB
Harmonic
Distortion:
Mono
0.08% at 100
Hz
0.08% at 1
kHz
0.1% at 10
kHz
Stereo
0.18% at 100
Hz
0.18% at 1
kHz
0.25% at 10
kHz
Spurious
Response: 100 dB
Image
Response: 100 dB
AM
Suppression: 60 dB
Stereo
Separation:
40 dB at 100
Hz
50 dB at 1
kHz
35 dB at 10
kHz
Signal-to-Noise
Ratio:
Mono : 80 dB
Stereo
: 75 dB
Intermodulation
Distortion:
Mono : 0.08%
Stereo
: 0.18%
Alternate
Channel Selectivity: 70 dB
SCA
Rejection: 60 dB
Audio Output
Levels
Fixed Output
: 1V
Variable
Output and Headphones : 0 to 2.5V
Antenna
Inputs: 300 Ω balanced and 75 Ω unbalanced
Power
Requirements: 120V, 50/60 Hz, 15W
Semiconductors: 33 Transistors; 15 Integrated Circuits; 8
Varactors; 20 Light Emitting Diodes (LED's); 34 Diodes
Dimensions
(W x H x D): 406 x 92 x 368 mm (16" x
3-5/8" x 14-½"),
including connectors.
Knob
clearance required : 32 mm
(1-1/4") in front of mounting panel.
Finish: Front panel is anodized gold and black with
special gold/teal nomenclature illumination. Chassis is black.
Mounting: Exclusive Mclntosh developed professional
PANLOC
Weight: 8,2 kg (18 pounds) net, 13,6 kg (30 pounds) in shipping carton.
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