McIntosh MR500 Tuner Review

The McIntosh MR 500 FM tuner has the distinctive styling and high-quality construction that are typical of McIntosh products. While it is called a digital tuner and has a numerical frequency readout and much digital circuitry, the MR 500’s actual tuning system differs from that ordinarily used in digital-synthesis tuners and receivers. Described as a “Signal Locked Loop” (SLL) system, the tuning circuitry locks onto the actual received signal frequency instead of locking the local oscillator to the frequency of an internal quartz-crystal oscillator. Stations can be tuned in initially either manually or with an automatic scan function that uses a time-varying control voltage. Six station frequencies can be entered into the memory for automatic pushbutton recall.

The tuner’s black front panel has two knob controls, one at the left for power on/off and volume, another at the right for manual tuning. Between them is a large framed control/display area with eleven pushbuttons, a headphone jack, and the readout panel. When the unit is turned on, the volume-control markings and the McIntosh logo are back-lit in blue-green while the tuned frequency appears in bright-red 5/8-inch-high digits. If a preset station has been selected, the preset number appears below the frequency display. On the right of the display panel is a column of nine LED’s that show relative signal strength. Above this column is a stereo indicator light (marked mpx), and below it are three arrow-shaped LED’s used as tuning aids. The arrows to the left and right light up to show that the station is tuned in either below or above the center of the channel; the central arrow lights when the station is correctly tuned in.

Pushing one of the two scan buttons, marked with arrows pointing up or down, causes the tuner to scan the FM band automatically in the desired direction, with the audio output muted, until a signal is received. The scanning then stops, and the oscillator is “pulled” by an error voltage derived from the FM detector until the tuner is set exactly to the center of the broadcast channel. The muting is then released and the program is heard. To store a frequency for recall, the enter button is held in while one of the six preset buttons is pressed. Preset stations are held in the memory indefinitely by an internal battery. The frequency readout has a 0.1-MHz resolution and always shows the actual frequency to which the tuner is set; in the automatic tuning modes (scanning and preset) it is possible to see the final frequency correction, typically a change of 0.1 to 0.2 MHz, taking place over a couple of seconds.

Momentarily pressing the manual button on the panel transfers control from the scan and preset buttons to the tuning knob. The knob adjusts a control voltage that replaces the scanning voltage and simultaneously tunes the unit’s local oscillator, r.f. amplifier, and mixer circuits. In the manual mode it is necessary to press the mute button to silence interstation noise and the sound of stations whose signals are too weak to be tuned in clearly. The tuner automatically unmutes after the final frequency correction is made and the channel-center tuning light comes on.

McIntosh points out some of the advantages of the SLL tuning system over the usual phase-locked-loop (PLL) digital-synthesis tuning, including the ability to tune to any frequency in the FM band (not just an exact multiple of 0.1 or 0.2 MHz) and automatic adjustment to the actual station frequency (unlike PLL systems in which the tuned frequency is ultimately determined by an internal oscillator rather than by the station’s broadcast frequency). Another advantage of the SLL approach is that it does not generate the noise and spurious signals often associated with PLL synthesizers.

The actual tuner circuitry of the MR 500 differs from that of most tuners. Following its MOSFET r.f. amplifier, it has a passive (diode) double-balanced mixer, which provides superior rejection of cross modulation due to overload by strong local signals and reduces the amount of local-oscillator signal entering the i.f. amplifier. Following three stages of ceramic i.f. filtering, the tuner has an integrated-circuit limiter/detector and a PLL IC multiplex demodulator as well as a number of auxiliary circuits involved in the SLL tuning system.

The MR 500’s internal audio amplifier is capable of driving either low- or medium-impedance headphones to a good listening level. In the rear of the tuner are two sets of audio-output jacks, one at a fixed level (nominally 1 volt) and the other controlled by the front-panel volume knob (as is the headphone output). Other rear-apron features include output jacks, marked vertical and horizontal, for connection to an external oscilloscope multipath-distortion indicator. Insulated spring clips accept the ends of 300-ohm antenna-feed wires, and there is a coaxial jack for a 75-ohm input. A slide switch enables the Automatic Frequency Lock (AFL) to be disabled for manual tuning if that is desired (it is always on during automatic or preset operation). A remote scan jack accepts the plug of the 24-foot remote-control cable supplied with the tuner. Pressing the button at the far end of the cable is equivalent to pressing the up scan button on the tuner’s front panel. Upon reaching the high end of the band, the tuner jumps back to the low end and continues its upward scanning. The last rear-apron feature is a single switched a.c. convenience outlet.

The entire exterior of the McIntosh MR 500 is finished in black, with pale gold accents on the front panel. As with other McIntosh components, the tuner’s key performance specifications appear on a plate on the front of the top cover and it is equipped with the patented McIntosh “Panloc” system, which can be used to lock the unit securely in an optional wooden cabinet ($69); it can be released and slid out from the cabinet for access to the rear apron by pressing two buttons on the front panel. The MR 500 is 16 inches wide, 14-1/2 inches deep, and 2-3/8 inches high, and it weighs 18 pounds. Price: $1,499.

Laboratory Measurements

The mono usable sensitivity of the MR 500 was 13.5 dB (2.6 microvolts, or μV). Its stereo threshold was 23.8 dBf (8.5 μV). The 50-dB quieting sensitivity was 15 dBf (3 μV) in mono, and in stereo it was 31.6 dBf (21 μV). These sensitivity measurements, especially the last, are not only very good but also agree closely with the tuner’s ratings.

With a 65-dBf (1,000-μV) input the tuner’s distortion was 0.12 per cent in mono and 0.18 per cent in stereo, and the respective noise levels were —75 and — 67 dB. The maximum output from the variable outputs was 6.6 volts at 100 per cent modulation, and the listening volume using medium-impedance headphones was excellent (no external amplifier is needed for comfortable listening to this tuner with headphones). The frequency response was flat within 0.5 dB from 30 to 6,000 Hz, rising slightly to a maximum of + 1.4 dB at 15,000 Hz. Despite this rise, the tuner’s multiplex filter reduced the 19- and 38-kHz leakage in the audio to a very low —68 dB. The tuner hum was an almost unmeasurable —76 dB at 120 Hz. The stereo channel separation was about 42 dB from 30 to 15,000 Hz except for a slight decrease to 40 dB at 10,000 Hz.

Because of the AFC-like “pulling” action of the MR 500’s SLL tuning system, some of our double-signal measurements could not be made accurately. The capture ratio in particular was almost un-measurably low, less than 1 dB. The alternate-channel selectivity measured 65 dB (rated 70 dB), and adjacent-channel selectivity measured 7.6 dB. Image rejection was an excellent 84 dB. The AM rejection was very good, measuring 75 dB. The muting threshold was close to the stereo threshold at 25 dBf (10 μV). The signal-strength indicators are useful rather than just ornamental: the lowest light came on with an input of only 15 dBf (3 μV), the others at intervals of 4 to 6 dB.

Comment

Much of the appeal of the rather expensive McIntosh components lies in their no-compromise engineering and construction standards and in the company’s long-term back-up policies. To these we can add tasteful yet distinctive styling and a typical smoothness and general feel of precision both in the control movements and in the quality of the sound.

The MR 500 follows in that tradition. It does just about anything one might expect from a modern, full-featured FM tuner, and it does it supremely well. The smoothness and silence of the automatic tuning are exemplary, and the manual tuning indicators are very effective. We especially appreciated the conservatism of the ratings, which are in marked contrast to the all too common practice of citing performance specifications that can be verified, if at all, only with special and often unavailable test equipment. It is always difficult to duplicate measurements on an FM tuner that were made elsewhere, but we came closer to doing so with the MR 500 than we have with many other tuners.

There are only two aspects of this tuner with which I could quibble. Perhaps I simply have an aversion to cables trailing across the floor, but I would have preferred a wireless remote control. The MR 500 also has no way of being switched manually into mono. While the tuner’s very good 50-dB stereo quieting sensitivity permits quiet stereo reception of weak stations, there are times when it is advantageous to listen in mono even to powerful stations, as when a station is broadcasting a mono program but doesn’t turn off its stereo encoding circuits.

The MR 500 is definitely an expensive tuner, and it may be debatable whether any $1,500 FM tuner can sound better than many less-expensive models given the limitations inherent in the broadcast medium and how it is being used at present. No, the appeal of this tuner (and it is most appealing) lies in its superb handling, obvious manufacturing quality, and almost certain longevity. These are the true values, we believe, of any premium product, whether it be a fine car or a tuner like the McIntosh MR 500.

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