Summit Audio has enjoyed a longstanding reputation for being a manufacturer of premium tube gear for studio recording. Its most recent offering — a tube direct-injection box that includes unbalanced high-impedance, balanced line-level, mic-level XLR, and headphone outputs — is equally at home onstage, at home, or in any professional or personal studio. The TD-100 tube DI and instrument preamp ($495) is a direct box with a difference — many important differences, actually.
Outside the Box
There are several useful features on the TD-100's exterior that are far from standard equipment. Most noteworthy is the continuously variable impedance control, which lets the user adjust how the TD-100 affects pickup loading and related tonal changes when it is used with guitars and basses. At the fully clockwise 2 M• setting, the device presents its highest possible input impedance, minimizing loading and typically allowing the clearest, brightest sound from most pickups. At the fully counterclockwise setting (10 k•), input impedance is relatively low and timbral changes caused by pickup loading, usually characterized as a dulling or warming of the sound, are easy to hear.
Intermediate impedance settings can produce subtle to drastic modifications of the instrument's tone, depending on the type of pickup and electronics, among other factors. For example, when I tested the TD-100 with a Fender guitar and bass, I heard little difference in tone between the 11 o'clock position (just below 1 M•) and the maximum 2 M• setting. Around the 10 k• extreme, however, both instruments' highs decreased dramatically, as they might when plugged straight in to a mixing board without the benefit of a direct box.
Other pluses include a Polarity switch for adjusting the pickup signal's phase; a Ground-Lift switch; and an unbalanced ¼-inch high-impedance direct output for connection to an amplifier or stompbox. The direct out (which is unaffected by the TD-100 circuitry) lets the unit interface with an onstage guitar or bass rig as a standard DI would while contributing its tube character to a house sound system through a standard mic-level XLR output.
A balanced ¼-inch line-level output is provided for studio applications, and a headphone out enables additional monitoring for practice sessions or limited studio setups. The levels of the rearmounted mic, line, and headphone outputs are controlled by the Output Gain knob, which ranges from 0 to +24 dB in stepped 4 dB increments. (There is no separate control for headphone volume.) Two indicator LEDs, located above the output gain pot, light up to show average signal presence and overload or near-clipping status.
On the inside, a 12AX7A/ECC83 vacuum tube imparts its glow to the signal path while the output-amplifier stage is discrete solid state. An internal power supply is a welcome feature that means a wall wart is unnecessary. A standard detachable IEC cable provides the AC connection. All switches, indicator lights, and controls are mounted on the front panel, as are the high-impedance in and out. The TD-100 is one rack unit high and half a rackspace wide, making it eligible for mounting in a number of commercially available rack trays. Mounting screws are included.
In a round of studio testing, the TD-100 received high marks. For direct guitar, the Summit was clean and airy, and on bass it compared favorably to a rack full of tube DIs and preamps with headroom to spare. In addition, the TD-100 imparted a convincing vintage punch to a sampled Rhodes electric-piano patch, making it a strong contender for processing a wide range of sample-based sounds. Overall, the build quality, sonics, and specifications of Summit Audio's first DI and preamplifier are impressive, especially considering its reasonable price and three-year manufacturer's warranty.