The Mod Squad Line Drive passive preamplifier Page 2

John Atkinson reviewed The Mod Squad Deluxe Line Drive AGT in January 1989 (Vol.12 No.1):

The Mod Squad's standard Line Drive, the one that's become the passive control preamplifier, costs a mere $500; Steve McCormack is the first to admit that it is a value-oriented design. His ultimate goal, however, was to construct a passive control system that would be sonically identical to plugging a source component directly into the power amplifier. The Deluxe AGT version represents Steve's best shot yet at that goal: a slam-dunk of a shot in which nothing has been spared to wring that nth degree of transparency from the recalcitrant circuitry—or the lack of it.

It may be thought that $900 is a lot of money for a component that doesn't feature one iota of active circuitry. Yet when I opened up the case of the Deluxe Line Drive, my immediate reaction was to think that it might even be too inexpensive, such is the labor-intensive quality of the workmanship and parts (footnote 1). This is a handmade item, in the best sense of the word. Starting at the logical place, three of the inputs have Tiffany sockets while two, "CD" and "AUX2," the latter intended for connection to a phono preamplifier, have the expensive WBT sockets. The two premium input channels have their signals taken to the source selector switch with the conductor used by MIT for their 330 interconnect, while the other three use the uninsulated solid-core Wonder Wire used throughout the conventional Line Drive. (As these are uninsulated, they are routed via what looks like a patch of grooved carpet tile to hold them apart. This also minimizes microphony.)

From the selector switch, MIT conductor takes the signal to the tape selector, which also has a "Mute" position, then to the balance control (this having a central detented position and sourced from Noble), and finally to the volume control. This is the piece de resistance, a conductive-plastic attenuator from Penny & Giles compared with the conventional Line Drive's Noble component. This component, used in such thoroughbreds as the Krell KRS2, alone costs more than the total parts for a typical inexpensive preamplifier and has a sexy, silky feel that, once experienced, makes all other pots feel unsubtle. (If Noble, Bourns, and ALPs are the Rolexes of the attenuator world, then Penny & Giles are the Blancpain.) MIT conductor then takes the signals from its wipers to the output sockets, one pair WBT, the other Tiffany.

Two tape loops are provided, the appropriate in- and output sockets connected to the selector switch with uninsulated solid-core wire, though it doesn't appear to be possible to dub from one deck to another unless the source deck is plugged into one of the regular inputs. The tape outputs are buffered with series 2200 ohm resistors. Conforming to audiophile philosophies concerning magnetic distortion, the chassis is nonferrous, being made from aluminum, as is the black-anodized front panel.

Considerable attention has been paid to the grounding arrangement of the Deluxe Line Drive (AGT stands for "Advanced Ground Topology"), Steve feeling that the way it is achieved affects the transparency at high frequencies. Every ground connection, whether it be the body of each socket or the ground references for the balance and volume controls, is taken individually to a single "star grounding" point at the center of the chassis and soldered to it. (Wonder Solder is used throughout.) The user then has the option of whether to electrically connect the chassis to the star ground point for maximum shielding, or to leave it floating for maximum transparency. The chassis is grounded when a rear-panel switch is in the "up" position, which is advised in the owner's manual only if hum or buzzing is otherwise apparent.

Sound quality
Looking at and listening to the Deluxe Line Drive with CD and open-reel tape sources, I had expected this component to be very similar to both the PAS-01 passive preamplifier and to the PS Audio 4.6 preamplifier in its passive "Straightwire" mode, which had become my line-level reference following its appearance in the preamplifier review last December.

I was wrong.

Compared with the PS Audio in its passive mode, the Line Drive was just that much more open. It allowed the sound to breathe just that much more easily. Everything sounded just that much more like the real thing. And the ambience . . .

Let me tell you about the ambience.

The venerable Holt (in whom we have trust) has a tendency to put down my love of soundstaging. Indeed, he dismisses my quest for perfection in this area as some queer kind of quirk, brought on by too much rain and the lack of airconditioning featured in my typically British upbringing. Yet it was the realization that stereo playback had the intrinsic ability not only to reproduce the sounds of instruments and voices, "absolutely" if you care to use that particular adverb, but also to present a two- or even three-dimensional space between and behind the plane of the loudspeakers, that fired my interest in high fidelity. Prior to that damascene moment, as a practicing musician—I'm still practicing—I had dismissed records as a convenient consumer product that usefully added to a musician's meager income. But when I realized that a system can give the listener the illusion that he or she is listening into the concert-hall acoustic from what might be the best seat in the house—the listener as ultimate voyeur—the recorded medium for me sprang to life.

One of my standard test tracks for soundstaging is the Chopin Waltz recording on the HFN/RR Test CD. I recorded the piano with a Calrec Soundfield mic set to its crossed figure-eights pattern, and had to work hard to find the best position in the hall—which had, probably still has, a beautiful reverberation signature—to place it in order to get the best ratio between the direct sound of the instrument and the reverberant soundfield. In the control room (actually it was a small dressing room around 100' from the stage), using BBC LS3/5As driven by a Quad 405 handling the direct output of the microphone preamps, the soundstage was awesome, stretching from the tip of my nose to infinity. I made two sets of recordings from the mic feed, one on my Revox A77 and the other on a Sony PCM-F1, of which the analog gets closer to reproducing the sense of space I heard in the control room and the digital gets closer to the actual tonal quality of the Steinway. The CD track was cut from the PCM-F1 recording, so when I listen to it, I am acutely aware of what it tends to lack when compared with what I thought I was recording when listening to the live mic feed. In particular, the hall tends to sound smaller and the instrument closer.

Feeding the CD version straight into the power amplifiers gave a soundstage indistinguishable from when the Deluxe Line Drive was in circuit. The soundstage remained wide and deep. I wouldn't go so far as to say that it approached that from the Revox (though the latter has its own sonic problems), but you could "hear the walls"—the decay of the ambience was presented with sufficient coherence that the recorded acoustic was laid bare.

Of the active preamplifiers available to me, only the Krell KRS2's line stage came close to the Mod Squad's midrange transparency, but was still identifiable when set to unity gain and switched in the Line Drive Deluxe's tape loop. The opposite was much harder, and in fact I have a suspicion that the sonic effect of whatever cables you use to perform such a bypass test is greater than that of the Line Drive itself.

Faults there were few, and I think were more due to the nature of the interaction of the source components and the total load of two sets of interconnects and the Line Drive between them and the power amplifiers. Dynamics occasionally seemed a little suppressed and the bass was very slightly softer than when compared with that from the Krell, particularly when the LiveWire Lapis was replaced with MIT 330 CVT or Monster Cable M1000.

Conclusion
The standard Line Drive is rated as Class C in Stereophile's "Recommended Components," the original reviewer, Anthony H. Cordesman, ultimately thinking back in early 1987 that an active preamp better presents dynamic contrasts. I actually think that his apparent problem with dynamics was one of interfacing, as a passive control center throws all responsibility for preserving this aspect of the music back on the output stage of whatever source component is being used. For the same reason, coupled with the highish output impedance of the Line Drive, the sonic signatures of the cables used will also have a proportionately larger effect on the ultimate sound quality. Its own faults will be subtractive in nature, in my opinion, affecting more the presentation of the soundstage and the overall transparency.

From my auditioning, I would consider the Deluxe Line Drive AGT to be at least a Class B component in its intrinsic effect on the music, and with the best source components and cables it will consistently produce Class A sound quality. It will not suit every system, particularly those involving insensitive loudspeakers, power amplifiers that need a lot of voltage swing to drive them to full output, and long cable runs. But in a carefully matched system, it is the least expensive way to obtain the most transparent, most musically satisfying sound.

I bought the review sample to be my reference!—John Atkinson



Footnote 1: As this issue was being sent to the printer, Steve McCormack informed me that a price hike on the Deluxe Line Drive was imminent, although he was unable to say how much it would be.—John Atkinson
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Starman's picture

Still use my WBT version every day.

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