McCormack Line Drive TLC-1 preamplifier

A truly great preamplifier lets everything through, both music and distortion, but with such generosity that cramped and narrow.Larry Archibald(footnote 1)

Finding a truly great, full-function preamplifier is a quest at which I have had only limited success. For a long time, I used an Audio Research SP-10 that I purchased back in the summer of 1984, but gave up on that fabulous device because, while its phono stage was (is) one of the greatest, it was too colored for CD use, and tube-replacement costs became unrealistic. The SP-10 gave way to a passive Mod Squad Line Drive Deluxe, which I enjoyed immensely, but which in turn was replaced by the inexpensive Melos SHA-1 headphone amplifier. The Melos offers powerful bass reproduction, a wide, deep soundstage, and a very musically satisfying sound, at the expense of some upper-midrange forwardness and a lack of top-octave air; but its basic design means plugging and unplugging leads every time I want to tape a CD or make a DAT dub from an open-reel original. It also has much more gain than I need, meaning that its volume control has to be set at 10 o'clock or lower, where its inter-channel tracking is less good.

Such practical frustration led to me to write, a year or so back (footnote 2), that "The preamplifier is the heart of any system: every source signal passes through it, it has to provide all the source switching flexibility, control of signal level, and the ability to drive an arbitrary length of perhaps severely reactive interconnect cable. Yet compare any preamplifier (set to unity gain) with a length of wire and you will find that it has its own sonic character which interferes with the music to a greater or lesser extent." I further developed my theme: "It seems there is an inverse relationship with the number of functions a preamplifier performs and the accuracy with which it passes on the music unscathed...My wish is simple: Give me a full-function preamplifier that affects the music as little as a piece of wire. Oh, and design it to have a sensible amount of gain."

The McCormack Line Drive TLC-1 doesn't offer [any] gain, being basically a "buffered passive preamplifier" of the type made famous by Corey Greenberg a couple of years back (footnote 3). It is fundamentally related to Steve McCormack's classic Mod Squad Line Drive Deluxe in that it's a conceptually simple control unit designed around a high-quality, two-channel potentiometer, the outputs of which are brought out to rear-panel jacks for direct connection to a power amplifier. (Don't use cabling with more than 1000pF of capacitance per channel.)

Where it primarily differs from the older unit, apart from its attractive gray-anodized front panel and control knobs and a lower price (!), is that it offers a second pair of output jacks, where the signals taken from the potentiometer wipers are taken to the gates of a pair of FETs per channel. These act as complementary unity-gain source followers to buffer the pot from the vagaries of the outside world.

Looking inside the gray-painted aluminum chassis, the circuitry is carried on a large, double-sided printed circuit board that runs the width of the rear. Short, solid-core jumpers carry the input and output signals from the gold-plated RCA jacks to the board. The input signals are taken by short pcb tracks to two five-position, silver-contact switches located on the board and connected to the front-panel knobs by shaft extenders. These offer source and Source/Tape/Left-Mono/Right-Mono selection. (As supplied, the unwanted channel is muted; internal jumpers allow the selected channel to be sent to both outputs; optional resistors allow summed-mono operation.)

The selected signal is then routed to two pairs of tape outputs, these buffered by an Analog Devices AD712 dual-op-amp chip that can be muted by a front-panel switch, then to the balance control (a stepped attenuator, with symmetrical -0.5dB, -1dB, -1.5dB, -2dB, and -3dB settings) and to the high-quality, conductive-plastic volume control (no detents). Both these controls, again, are mounted on the pcb with shaft extenders connecting them to the front-panel knobs.

Footnote 1: "The Final Word," April 1988, Vol.11 No.4, p.210.

Footnote 2: "As We See It," February 1993, Vol.16 No.2, p.5.

Footnote 3: See Vol.14 No.11, p.91, and Vol.15 No.11, p.100.

2733 Merrilee Drive
Fairfax, VA 22031
(703) 573-9665