McIntosh MR 500

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.