Threshold T2 preamplifier

Threshold is one of the longest-surviving high-end audio companies. Founded in the 1970s by Nelson Pass and René Besne, it was acquired by a large, publicly traded corporation in 1988. This had both positive and negative results in that Threshold was then able to expand its activities, adding the cost-effective Forté line of products, but energies were drained away from cutting-edge design. Besne left the company in 1991, while Pass resigned in 1992 to pursue other interests. (These blossomed into the Pass Aleph 0 amplifier reviewed by DO in March '95, Vol.18 No.3.)

Threshold's T2 remote-control preamplifier, along with the other T-series products (like the T200 amplifier reviewed by Robert Deutsch in April '95, Vol.18 No.4), are Threshold's new owner Randy Patton's answer to the question "What happened to Threshold since Nelson Pass left?"

The T2 preamp is Swedish engineer Michael Bladelius's creation, with assistance from Mike Jaynes, Wayne Colburn, and Larry Roedenbeck. (Michael designed much of the Forté amplifier line.) The design goals for the T2 were wide, open-loop bandwidth; low, open-loop distortion; short signal paths; thermal stability with low offset drift; complementary symmetry in all active stages; full DC coupling; low noise floor; fully balanced topology; and minimal use of feedback.

Bladelius, Jaynes, and Co. started with a differential FET (field effect transistor) input stage using eight matched N- and P-channel low-noise JFETs in a cascoded circuit. The signal gets its voltage gain from a common-base stage, which serves as the third element in the cascode. The preamp's current gain is achieved with four power MOSFETs, also cascoded in a complementary output arrangement. With a final output impedance of 0.1 ohm, the T2 should be able to drive very long runs of capacitive cable without ill effects.

The T2 also features a fully balanced, differential circuit topology. Rather than convert a balanced signal source to single-ended signal, amplify it, then convert it back to a balanced output, the T2 is balanced throughout its signal path. This means it requires identical gain-stages for both positive- and negative-phase signals. This doesn't come cheap. Each input circuit is therefore mirrored for hot and cold signal phases, and replicated so that there are two gain-stages per channel. Each channel of the T2 has its own circuit board consisting of these two gain stages.

The T2's power transformer is rated at 150VA. Each circuit board is supplied by separate positive and negative regulated power supplies to minimize channel interactions. The T2 features multiple high-current regulators for each stage as well as pre-regulation for the analog stages in the external power supply, using a 15A pass transistor filtered with 10,000µF capacitors, which are in turn high-frequency–bypassed with film capacitors. The digital section of the T2 (which controls the volume) has a separate winding in the power transformer and isolated grounding to eliminate any interference between the analog and digital sections of the preamp.

The T2's volume control is worthy of special note. In order to maintain the highest common-mode rejection ratio, a balanced control's tracking accuracy has to be ridiculously tight. Currently available rotary-gang potentiometers just didn't have the accuracy needed to do the job, so the Threshold T2 employs a 12-bit, modified-R2R resistive ladder network (similar to what you'd find in a DAC) switched by high-reliability relays. With this device, tracking-control accuracy is better than 0.01dB. The digital attenuator has 4096 discrete steps available—a few too many for audio purposes, so two custom-programmed Programmable Logic Devices are used to select 60 decibel steps, shown on the front-panel fluorescent display.

The smallest incremental volume change available is 1dB. While some users—Tom Norton, for instance—prefer a finer delineation, such as the 0.1dB offered on the Rowland Consummate or Mark Levinson No.38, I found 1dB increments adequate for all but the most critical gain-matching situations. However, you'll need a light touch; I often found the volume jumping 3dB after a brief touch of the button.

The T2 has six single-ended RCA and two balanced XLR line-level inputs, labeled Bal.1, Bal.2, Video, CD, and HL (high-level) 1–4. As an analog holdout, I feel somewhat discriminated against: CD got a named input; LP deserves one too. The T2 also lacks a polarity-inversion switch, which is no loss for most CD folk, since most high-class D/As have their own phase switches. As most phono preamps don't, however, again as an analog user, I feel a bit cheated.

Neither does the T2 have any dedicated tape inputs, so if you select the same source for tape output that's been selected for source input (a tape machine or surround-sound processor), you can easily get feedback. Naughty, naughty—and potentially dangerous to the health of your speakers (and ears).

The T2 features two balanced XLR outputs, two single-ended RCA outputs, and a pair of tape outs. I never ran out of input or output connectors—not even in my Home Theater installation. I did, however, quickly learn to always mute before changing input sources and tape-out sources to avoid feedback howls.

The T2's front panel is quite spartan. Next to the blue fluorescent display are a button for selecting record-out source, a button for adjusting the display's level of brightness, and two rotary knobs—one for source selection, the other for output level. The real power is in the remote, which has Volume controls, Balance controls, Mute, Standby, Mono, Display level, Tape Output, and Source Select controls.

The remote, which resembles a high-tech boomerang, sports a silver-satin top plate that has faux-suede sides and back. It's big enough that it'll be hard to lose between your seat cushions, but not so big that you'll need to pump up your muscles to hold it. As long as I wasn't at too oblique an angle to the faceplate, the remote worked pretty well. However, whenever I was more than 40° off-axis, it got balky.

Threshold Audio, Inc.
PO Box 41736
Houston, TX 77241
(713) 466-1411