The Mod Squad Phono Drive phono preamplifier Page 2
According to Steve McCormack, designing the MC amplifier involved a tradeoff between minimizing the noise and getting the best sound. After trying FETs, he settled on a matched differential pair of bipolarsthough obscured by their plastic "oven" covers, these appear to be something like "Supermatch" National LM394sfollowed by a p-channel FET source follower.
A front-panel rotary knob, connected to a board-mounted switch by a long coupler, selects the MC-stage input impedance, from 10 to 1000 ohms. A sixth position selects a user-definable value, a pair of gold-plated sockets per channel on the board allowing the user to insert his or her preferred-value resistors. Without any additional resistors, this loads the MC cartridge with 47k ohms, which is recommended by many users (though not this one). The MM stage has its own pair of input jacksall sockets are gold-plated Tiffanysand is selected by another front-panel switch; I measured its impedance setting as 45k ohms. Here Steve has gone for a differential J-FET input stage, with a bipolar emitter-follower stage. This then feeds an output driver using a pair of complementary MOSFETs per channel, these heavily biased into class-A and sporting porcupine-like TO5 heatsinks. The output of what in effect is a baby power amplifier is then taken via Wonder Wire links to a pair of RCA sockets on the rear panel labeled "Direct Out."
The phono stages are direct-coupled throughout; though the MC amplifier is "as is," as it were, the MM stage contains a DC servo circuit to minimize voltage offsets at the "Direct Out" sockets.
The Phono Drive also contains a line-level section, and here the user is given an option. As supplied, the phono and line-level sections are completely separate. By repositioning a pair of gold-plated circuit-board jacks, however, the phono-stage output can be connected directly to the line stage, giving, in effect, a dedicated LP replay preamplifier, complete with balance and volume controls and capable of driving long cables. The line stage appears to use a pair of complementary bipolar transistors per channel and is direct-coupled; no DC servo circuit is used, nor is there any offset protection, so users are advised to check their ancillary source components. (Such good housekeeping is always a good ideaI once destroyed a review pair of Celestion SL600s by attempting to drive the power amplifier with a defective preamplifier having 15V of DC offset on its outputs.)
The Phono Drive was clearly more musical than the PS Audio 4.6's phono stage, the latter, though excellent, having a slightly "electronic" nature to its treble in comparison. I therefore carried out detailed comparisons primarily with the Vendetta Research SCP-2C phono amplifier, which is, its absence of a line stage apart, the Phono Drive's nearest equivalent product. (Levels at 1kHz were matched as carefully as possible between all the preamplifiers by using a test record, and both were connected to the Line Drive with 9" lengths of MIT 330.)
The main audible difference between the two phono preamplifiers was caused by noise. My listening room has, at least when the neighbors are not splitting logs for their fire, a very quiet noise background, well under 40dBA. The John Curl design was as quiet as the grave, even with maximum system gain. The Phono Drive, driven by the Linn Troika and with the Line Drive gain set for average listening levels in the mid 90s, could be heard to have a very slight "rushing noise" in the background. A degree of 60Hz hum could also be faintly heard submerged in this background at these spls, which puzzled me given the double-regulated nature of the Phono Drive power supply. I determined this hum to be a property of the interaction of the Phono Drive with the VTL power amps rather than the Phono Drive itself (see later), and it was not at all audible once music was playing. The noise on the MM input was inaudible at the listening seat, even with the Line Drive gain at maximum.
Compared with the Vendetta, the Phono Drive appeared to have a more solid bass, extending lower and having more weight. The Vendetta's low frequencies were slightly more "rounded," which in themselves was not displeasing, particularly with the Thiel CS1.2s, but the Mod Squad unit's sounded more natural and gave a better foundation to the music with the more tightly controlled Celestion SL700s. Up the range, the Phono Drive was a little more forward in its presentation of the midband than the Vendetta, being more akin to the KRS2. High frequencies were very similar from the two phono units, but the Vendetta had slightly more of music's top octave apparent than either the Krell or the Phono Drive.
Both Phono Drive and Vendetta Research presented more detail than the KRS2, but differed in the way detail was differentiated within the overall sound. The Phono Drive seemed to excel in the way it allowed the listener to hear the tonal nature of individual instruments. It was obvious, for example, on the Ry Cooderproduced and naturally recorded Bobby King and Terry Evans album (Live and Let Live!, Rounder 2089), that drummer Jim Keltner was using a snare drum with quite a deep shell. In fact, all percussion instruments were presented with their characters beautifully delineated. The Vendetta Research, however, was just that bit better at presenting the spaces between the instruments in the soundstage (as is the Mark Levinson No.26).
As I said earlier, it was the ability of a high-end system to present a vivid and believable soundstage which had a formative influence on my listening, and that soundstage needn't have any real analog. A favorite recording of mine of a Bacharach and David song is Sylvester's "I Took My Strength From You" on the overweight androgyne's 1978 album Step II (Fantasy FT9556) (footnote 1). Here is Mr. S wishing he was Gladys Knight, amply supported by the Two Tons o' Fun (the no-less-ample Izora Rhodes and Martha Walsh). Leslie Drayton's arrangement, though a little on the Nutrasweet side, is still a virtuosic piece of scoring. The singers weave an intricate Staple Singers vocal weave on the chorus, accompanied by duetting flutes, tastefully Leslied organ, Spanish guitar lacework, mariachi trumpets, a spee-onk bass guitar part, and aetherially ascending violin lines. Mawkish, but made more than bearable by the way producers Sylvester and Harvey Fuqua have carefully created an artificial but no less believable sense of space.
The Phono Drive's presentation of this track was excellent by any standard, the music communicating effectively, but the Vendetta's spatial presentation was more precise, the careful layering of the instruments being more delicately reproduced. The Vendetta Research soundstage also reproduced with noticeable height information; why this should be I have no idea, but it was a consistent feature through many different recordings (nearly all multi-miked, multi-mono).
This was also the case with such purist-miked classical recordings as James Boyk's muscular live performance of Beethoven's C-minor piano sonata, Op.111 (Performance Recordings PR-1). The sound of the piano had more weight, and sounded more "real" via the Phono Drive, but the image of the instrument in the acoustic of Caltech's Dabney Lounge was more accurate via the Vendetta phono amplifier. Similarly with the 1982 Hyperion collection of songs by Stanford, Trottin' to the Fair (A66049), where the music was equally enjoyably robust through either preamplifier, but the image of the singer placed within the piano's recess was more delicately and precisely presented with the John Curl design.
Whether this aspect of the Phono Drive is connected with its higher noise floor, I'm not sure, but it was a consistent feature of my auditioning. Certainly the ambience and reverberant field could be heard to be extracted from the grooves, but the presentation of that information was less coherent, less well correlated with the direct sound of the instruments or singers picked up by the microphones than when played back over the Vendetta Research. I wouldn't have cared so much, though, if the fundamental nature of the Phono Drive's sound was not so damned musical.
I did less listening to the Phono Drive's Line Stage as its gain was superfluous in my particular system. In a large room, however, with more powerful amplifiers and long runs of cable, it will prove useful. I felt that, overall, it sounded a little brash compared with the intrinsic quality of the Phono Drive's disc-amplifier circuitry and the incredible transparency of the Deluxe Line Drive. Low frequencies were powerful, but the soundstage shrank somewhat both in width and depth.
Beautifully constructed, the Phono Drive is a true high-end product which, considering its versatility and the fact that it contains a single-input, line-level control preamplifier, is actually cheap for the level of performance it has to offer. The Phono Drive, however, gave me a harder time reaching an ultimate value judgment, perhaps because putting together a musically satisfying LP player and preamplifier combination is a much more personal affair. The higher degree of synergy involved, as well as the wider spread of what is regarded as good sound, mean that every audiophile ends up with, if not a unique, then at least an intensely personal setup. Dropping a different component into that matched system will probably knock it away from its optimum position for that particular listener.
Taking that as a given, the Phono Drive excels at presenting the music engraved within the grooves of the disc. It is one of those rare products where one record inexorably leads to another and the listening sessions tend to last into the wee hours of the morning. Excellent at retrieving information from the disc and reproducing the true sounds of instruments, it is similar to the Krell KRS2, however, in that it is less good at preserving the exact relationships between different aspects of that information. This is mainly at the expense of the soundstage, which becomes a little untidy compared with, say, that presented by the Vendetta Research SCP-2 or the Mark Levinson No.26. Tempted to recommend it in Stereophile's Class A category, I must temper my enthusiasm: the noise floor on its MC input is disappointingly high, at least with the Linn Troika. This will be less of a drawback with MCs like the Koetsus that are possessed of a hefty output, however, and I suggest that you audition it with your own set-up to determine whether its noise will be significant in your system. If not, it is heartily recommended.
Footnote 1: Long-deleted, I suspect, this album features two mammoth disco hits, "You Make Me Feel (Mighty Real)" and "Dance (Disco Heat)," the latter one of the highest-energy pieces of dance music ever committed to vinyl.