At first glance, Kenwood's DP-M7750 seems conventional-just another CD
changer with a six-disc magazine and a single-disc tray-but conventional
it is not. It is the first computer-controlled 6 + 1 changer that's crossed
my bench, and for me that sets it apart from the crowd. In fact, it's
fair to say computer control is the DP-M7750's most distinguishing feature.
Yet I found it surprising that, unless a salesperson explained the feature
to you (or you had mighty sharp eyes), you'd be hard pressed to know
it exists. The only reference I found in the owner's manual appears on the
back-panel line drawing on page 8, where a callout for the DB-25 RS 232C
jack simply states: "For the U.S.A. only. This connector is for use
in connection with an IBM PC (or compatible) using the software floppy
disk FD-M7750 avail able as an optional accessory."
I expect that the disk referred to is Ken wood's MoodMaker Version 2.0, a software package to catalog and manage your music library and, if you have the DP-M7750, control the player from a PC. To use the software, you'll need an IBM-compatible computer running Windows 3.1 under MS-DOS 5.0 (or higher) and have installed the standard line of Windows TrueType fonts. Kenwood recommends that the soft ware run on at least a 20-MHz 386SX plat form with 4 megabytes of RAM; I've yet to find software that didn't benefit from more power, speed, and RAM. You'll also need a bus or serial mouse (a bus mouse is recommended), a VGA or SVGA monitor, and at least 4 megabytes of room on your hard disk. If you use MoodMaker to operate the DP-M7750, you'll also need a free serial port (or a switch box to share a port with whatever's using it now) and a serial inter face cable to lash the two together. Mood Maker is provided on a 3 1/2-inch floppy (no indication of it being available on 5 1/2), so you'll also need a compatible drive in the computer to load it initially.
Since MoodMaker is designed to be used with the DP-M7750 (even though it can be used simply as a music-oriented relational database), it catalogs CDs in six disc clusters. The file for each disc is organized on the basis of disc title, artist, and composer as well as on the basis of track title and artist. You also can append notes to categorize CDs by type of music and can "rate" each CD on a scale of 1 to 5.
As a relational database, MoodMaker searches for specific words that appear in the title, artist, and composer fields. The title and track databases are scanned separately, as requested, when initiating a search. (The search is case-sensitive; you won't find "love" if you ask for "Love.") If you search on the basis of CD title, the magazine numbers and CD titles that match the criteria appear on the screen. If you search the track data, the magazine numbers, CD titles, and track names will appear.
MoodMaker also facilitates compiling play lists of similar music based on the search criteria you've given it. The preprogrammed list can be sent directly to the DP-M7750, but, of course, you will have to have loaded the appropriate magazine, since the database will presumably contain many more discs than the player can handle at once. This reinforces the desirability of placing similar CDs in the same magazine. MoodMaker's control screen also permits you to operate the DP-M7750 from your computer (all functions can be con trolled via the serial port), and it displays the CD and track title that's being played on the computer screen. You can even store cover art or graphics for display, provided they're available as .BMP or .DIB files.
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SPECS
Frequency Response: 4 Hz to 20 kHz, ±1 dB.
S/N: Greater than 96 dB.
Dynamic Range: Greater than 94 dB.
THD: Less than 0.005% at 1 kHz.
Channel Separation: Greater than 90 dB at 1 kHz.
Wow & Flutter: Unmeasurable.
Output Levels and Impedances:
Fixed, 2.0 V/3.3 kilohms; headphone, 20 mW maximum/16 ohms.
D/A Conversion: One-bit.
Oversampling: Eight-times.
Power Consumption: 15 watts.
Dimensions: 17 3/8 in. W x 5 1/4 in. H x 14 1/2 in. D (44 cm x 12.8 cm x 36.8 cm).
Weight: 11.9 lbs. (5.4 kg).
Price: $399.
Company Address: 2201 East Dominguez St., Long Beach, Cal. 90810.
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Control Layout
But enough about MoodMaker. On to the DP-M7750 itself, which has enough panel buttons to keep the busiest centipede happy. On the upper left is a jumbo "Power" button and, beneath it, a "Phones Lev el" control and a gold-plated "Phones" jack. At the lower right are the main trans port controls: "Stop," "Play/Pause," and conventional up/down skip and search keys (all of which have dual functions) and "Function" and "Display" buttons (which I'll describe later).
Immediately beneath the display and just to the right of the single-disc tray are seven equally spaced buttons that select the disc for playback: "P" for the one in the single disc tray and six buttons further to the right for those in the magazine. Slightly to the left is the magazine "Eject" button and, below it, the tray's "Open/Close." To its right is an array of 20 keys to be used for track selection.
The "Function" button mentioned previously permits magazines and discs to be named and permits programs to be stored in, and recalled from, the DP-M7750's internal memory. The names and programs of up to 42 magazines, including the names of 255 CDs, can be registered in the DP M7750 and displayed on its panel. There's a limit of eight characters per designation.
Characters are entered via the seven disc-select buttons and the first 19 numeric keys, which together cover the 26-character English alphabet. Certain typographical symbols can be entered with the track search and skip keys.
The nine buttons above the transport controls serve a variety of purposes. Some involve the DP-M7750's programming and editing features; others, however, strike me as logically unrelated. For example, although I can come up with a rationale for having the "Random" but ton in the cluster (the DP-M7750 can randomize playback of any disc, magazine, or program group), the button that activates the player's peak-search function (for tape recording) also lies here. All but two of these buttons ("Repeat" and "Clear") have dual purposes, which further complicates usage. What it all comes down to is that Kenwood has loaded this player with so many features that selection becomes difficult-and for that very reason, they probably will not be used.
Thank you, I'll gladly forgo the mixed benefits of Kenwood's 47-button main panel (86 buttons if you count those with dual purposes twice) for the blessed simplicity of its 31-button remote. Even 31 keys sound like a lot for simple-minded folk like me, until you figure that 17 of them are numeric pads for disc and track selection. Another two open/close and select the single-disc tray, and six more operate the transport-all second nature to every audiophile. That leaves just a half dozen buttons to contend with: "Random," "Repeat," "Time Disp.," "Check," "Clear," and "P.Mode." The first three are pretty conventional; the latter three are used for programming.
Measurements
Kenwood claims to employ dual D/A converters in the DP-M7750, and based on the interchannel phase measurement I made, I have no reason to dispute the issue, since phase error was less than 1° from 20 Hz to 20 kHz. Kenwood also claims that the converters use eight-times-oversampled filters and one-bit conversion, but I suspect that, compared with high-end one-bit systems, Kenwood's digital filters have relatively few taps. As you can see in the frequency response of Fig. 1, there's measurable high-end ripple and a modest treble boost. Nevertheless, the response is within -0.04, +0.33 dB across the band which isn't all that bad, and far better curves than Kenwood's ±1 dB tolerance. And the two channels are extremely well balanced, within ±0.005 dB.
With 0-dB recordings, THD + N was virtually identical on both channels; only the left is plotted in Fig. 2. The distortion and noise are shown in dB versus frequency rather than as a percentage. Converted to percentages, the numbers come out to be 0.005% from 20 to about 200 Hz, above which the residual contamination falls to a minimum of 0.0025% at 1 kHz, only half of Kenwood's 0.005% specification. At higher frequencies, distortion rises fairly rapidly and reaches 0.039% at 10 kHz and 0.48% at 20 kHz. These figures are rather higher than I'd like to see and may be caused by the analog output stages rather than by the converter itself.
Fig. 1-Frequency response.
Fig. 2-THD + N vs. frequency.
Fig. 3-linearity error.
Fig. 4-Fade-to-noise test.
Linearity error (Fig. 3) is a bit less on the right channel than on the left but is essentially nonexistent down to the-60 dB recorded level on both channels. At -70 dB, the error is -0.27 dB on the left channel (-0.11 dB on the right) and increases to -1.33 and -1.00 dB (left and right, respectively) at-80 dB. At -90 dB, left-channel error is -4.58 dB without dither (-5.22 dB with), while the right channel is -3.22 dB without dither (-3.80 dB with). Not long ago, these would have been excellent results; in today's "one-bit" world, they're not. The fade-to-noise linearity error curves (Fig. 4) extend down to -120 dB and tell a similar tale (right channel some what better than the left), but now we can see that the linearity error doesn't get much worse at lower levels than it is for signals at-90 dB.
With the possible exception of high-frequency distortion (assuming that the rather poor figures measured in that area were due to the analog circuitry), the DP M7750's analog section is exemplary. Channel separation (which is affected more by the layout of the analog circuitry than anything else) was better than 93 dB from right channel to left across the entire test band and 72.8 dB or better from left to right. At 1 kHz (Kenwood's test point), separation in either direction was 92.5 dB or better, which easily clears spec.
The A-weighted signal-to-noise ratio (which also really measures analog performance, since the converter is not exercised) was a spectacular 128 dB referenced to a 0-dB recording. A spectral analysis of the noise indicated specific components existed only at the power-supply ripple frequency (120 Hz) and its second harmonic. Even these were exceedingly low: -116 dB at 120 Hz and -128 dB at 240 Hz, referenced to the 0-dB level. Quantization noise (which does include converter effects) averaged only 86 dB below the 0-dB reference, so it's clear that the converters, not the analog stages, set the noise floor of this system.
Dynamic range takes converter distortion and noise into account. When measured on an A-weighted basis in accordance with EIAJ standards, it clocked in at 95.8 dB on the left channel and 97.2 dB on the right, both better than Kenwood's 94-dB claim. Unweighted, the numbers became 92.3 and 92.6 dB, respectively.
Output at the rear-panel RCA jacks was a higher-than-typical 2.56 V (0-dB recording at 1 kHz). Output impedance measured a relatively high 3 kilohms, suggesting that you should not use connecting cables longer than necessary. With the "Phones Level" control at maximum, 2.49 V (open circuit) were available at the headphone jack with a source impedance of 69 ohms. You can expect more than adequate drive for the aver age consumer headset (about 22 mW into 50 ohms) and a reasonable 8.3 mW into 600-ohm ("high-impedance") head phones.
Use and Listening Tests
If you're the type who slams the doors of a new car in the showroom hoping to be reassured that the construction is solid, you may not cotton to the Kenwood DP-M7750. Like most other magazine changers that are based on a mostly plastic trans port, this one is rather noisy when loading or changing discs. The thin sheet-metal housing doesn't do much to add to a feeling of solidity either. And I found the skip and search buttons less than positive in action. Ultimately, they always did work, but sometimes I had to press twice to get the desired result, and sometimes I'd get a double skip on what I thought had been a single tap. I'm not sure what kind of switches Kenwood uses-there's a circuit board right up against the control panel but considering the number of buttons and the proximity of the circuit board, I expect they use either membrane switches or even contacts made directly to circuit-board traces. Such is the price of packing lots of features into a relatively inexpensive component.
When it comes to listening, let's face it, most CD players sound pretty good. So does the Kenwood DP-M7750. But it didn't approach the sound quality of my one-bit reference player. On piano, the bass was rather tubby and somewhat indistinct, the tenor a bit zingy, and the treble brittle. Violins tended to be wiry, piccolos overly shrill. On the positive side, the DP-M7750 struck me as being admirably quiet, especially on some older "audiophile" CDs that may have been recorded without dither. (Undithered recordings are a mixed blessing; they tend to be quiet, but ambience of ten is lost.)
Overall, I consider the Kenwood DP-M7750 to be an average CD changer with wads of toys, not least of which is computerized operation. Computer hackers turned audiophiles should love it. So will non computer audiophiles who like to spend their leisure hours punching in their disc library character by character. If you fit into neither category, you'll probably want a more-standard single-disc CD player.
-Edward J. Foster
(Audio magazine, Jan. 1995)
Also see:
Kenwood DP-11008 Compact Disc Player (Equip. Profile; Jan. 1985)
Kenwood Spectrum 70 (One Brand System) (Jul. 1982)
Magnavox CDB650 CD player (Mar. 1987)
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