THE Bang & Olufsen MMC 2 phono cartridge, one of a new series from the Danish manufacturer, represents a further refinement of the "Moving Micro-Cross" design that has long been a feature of B&O cartridges (it predates stereo by several years). The MMC 2 is a moving-iron cartridge in which the flux from a fixed magnet is divided between four pole pieces in accordance with the motion of a tiny X shaped magnetic armature. The signal voltages are generated in two pairs of push-pull coils surrounding the pole pieces.
This basic construction is essentially similar to that of other B&O cartridges. The special properties of the MMC 2 derive from the materials and processes used to re duce its effective tip mass and total weight.
---------
In the graph at left, the upper curve represents the frequency response of the cartridge. The distance (measured in decibels) between it and the lower curve is the average separation between the two channels (anything above 15 dB is adequate). The inset oscilloscope photo shows the cartridge's response to a recorded 1,000-Hz square wave, which indicates resonances and overall frequency response (see text). At right is the cartridge's response to the intermodulation-distortion (IM) and 10.8-kHz tone-burst test bands of the TTR-102 and TTR-103 test records. These high velocities provide a severe test of a phono cartridge's performance.
The intermodulation-distortion readings for any given cartridge can vary widely, depending on the particular test record used. The actual distortion figure measured is not as important as the maximum velocity tracked before a sudden increase in distortion occurs.
-------------------------
The stylus is a line-contact diamond (a derivative of the Pramanik stylus shape introduced by B&O some years ago for playing CD-4 quadraphonic records) whose shank, only 0.12 millimeter square, is nude-mounted on a single-crystal sapphire tube cantilever. The pivoted end of the cantilever moves a miniaturized Micro-Cross armature, whose small size also contributes to the very low effective tip mass, specified as 0.3 milligram.
The low overall weight of each of the new MMC series cartridges, as well as their small size, has been achieved by using a powerful samarium-cobalt magnet. The cartridge body is a tiny tapered rectangular tube whose four integral pins plug into a special mount that can be installed in any standard tone arm with 1/2-inch mounting centers. The cartridge body weighs only 1.6 grams, and the combined weight of the MMC 2 cartridge and its mounting bracket is only 4.6 grams (about 2 grams lighter than most other magnetic cartridges). An additional weight is supplied to increase the total weight to 6.3 grams for use in arms that cannot be balanced at the lower weight.
The MMC 2 is fitted with an integral hinged stylus guard to protect the stylus during installation or removal of the cartridge. The stylus assembly is not replaceable by the user, but it is actually easier to insert or remove the cartridge body from its mount than it is to install most replacement styli. A small stylus-cleaning brush is furnished with the cartridge.
The B&O MMC 2 is designed to use a 1-gram vertical tracking force and is rated to deliver an output of at least 2.12 milli volts for a recorded velocity of 3.54 cm/sec.
The channel levels are matched within 1.5 dB. The rated frequency response is 20 to 20,000 Hz ± 1.5 dB, with a channel separation of at least 25 dB at 1,000 Hz and 20 dB or better over the full 50- to 15,000-Hz range. Each cartridge is supplied with a computer printout of its output voltage, balance, separation, tracking ability, and 20-kHz output level. The MMC 2 does not have critical loading requirements, and a resistive load of at least 47,000 ohms, shunted by no more than 400 picofarads of capacitance, is satisfactory.
The MMC series currently includes five models, from the MMC 1 to the MMC 5, having identical dimensional and electrical specifications but differing in their stylus-tip shapes and compliance specifications, and therefore in their overall frequency-response uniformity. The price of the B&O MMC 2 is $290.
Laboratory Measurements. We used the B&O MMC 2 in the low-mass tonearm of a Dual 741Q record player. Initial response measurements with different load capacitances showed a negligible difference (less than 1 dB) for load values between 200 and 435 picofarads, and the lower value was used for subsequent tests. The frequency response with the CBS STR 100 test record showed a gentle drop above 500 Hz to about -2 or -3 dB from 5,000 to 16,000 Hz.
The output rose slightly at 20,000 Hz, resulting in an overall response variation of 3 dB over the 40- to 20,000-Hz range of the test record. The average channel separation was measured as 20 to 25 dB in the mid range, 28 to 30 dB at 10,000 Hz, and 15 dB at 20,000 Hz.
As expected, other test records yielded different frequency-response curves. The best was obtained with the JVC 1007, which produced a response flat within ±0.5 dB from 1,000 to 20,000 Hz. The B&K 2009 record, on the other hand, showed a rising response above 7,000 Hz. The square-wave response with the CBS STR 112 test record was excellent, with a single damped overshoot and very low-level ringing visible on the square-wave output. The cartridge's output was 2.85 millivolts, with a 0.9-dB channel imbalance. In the Dual arm, the total effective mass of the arm and cartridge was 14 grams, and the low-frequency resonance was about 8 to 9 Hz. The vertical tracking angle was measured as 24 degrees.
At its rated 1-gram tracking force, the MMC 2 tracked our high-level low- and middle-frequency records easily, and it was able to play the 80-micrometer level of the German HiFi:2 record. Its intermodulation distortion with the Shure TTR-102 test record rose smoothly from less than 1 percent at 7 cm/sec to 6 percent at 27 cm/sec. At no time was any obvious mistracking visible on the output waveform. High-frequency tracking, measured with the 10.8-kHz tone bursts of the Shure TTR-103 test record, was also very good, as evidenced by a virtually constant (and very low) distortion of about 0.8 percent over the 15- to 30-cm/sec range of the record.
Comment. The subjective tracking performance of the B&O MMC 2 lived up to the promise of its bench tests. The Shure ERA IV "Audio Obstacle Course" record could be played without audible distortion except at the highest level of the flute section, and the ERA V record was tracked successfully at all levels. The sound character of this cartridge was much like that of earlier B&O cartridges we have used. Com pared with some other cartridges having exaggerated high-frequency response, this one might seem unexciting, but we would prefer to describe its sound as "neutral." Its frequency response is undeniably smooth (the actual output variation, as our tests show, is largely a function of the particular test record used for the measurement).
Installing the MMC 2 is very easy since the mounting bracket is installed without the cartridge, eliminating any risk to the delicate stylus. One must be careful to seat the cartridge firmly in its socket, however, since we noted a tendency (on our test sample, at least) for the cartridge to work loose from the socket, especially when its stylus was being brushed free of dust or lint. The open design of the stylus protector enables the entire stylus assembly to be seen when cueing the pickup or playing a record, a feature we greatly appreciated.
The B&O MMC 2 impresses us as being a logical evolutionary development, almost certainly better in its record-playing performance than its predecessors and with its bulk and weight trimmed substantially to match the capabilities and requirements of modern tonearms.
-Julian D. Hirsch
[Equipment Test Reports Hirsch-Houck Laboratories: Julian D. Hirsch and Craig Stark]
----------------
Also see: