Tube Preamps

A complete set of measurements was made on the Sonic Frontiers Line 2 in its unbalanced mode, with selected measurements repeated in the balanced configuration. Unless otherwise stated, results shown here are for unbalanced operation.

The Line 2's unbalanced main output impedance measured 118 ohms in the left channel, 121 ohms in the right (164 ohms left, balanced). Its unbalanced input impedance measured 9.8k ohms left and 10.4k ohms right (20.8k ohms left, balanced). Regardless of the source impedance, the impedance at the tape outputs was very low—around 1 ohm—and differed only slightly with the two measurement methods used to check it. This indicates buffered outputs.

The DC offset at the main outputs measured 0mV in the left channel, 0.8mV in the right. The preamp is noninverting from its unbalanced outputs, with pin 2 positive at the balanced outputs. Maximum voltage gain measured 11.9dB unbalanced, 17.6dB balanced. Unbalanced unity gain was at a volume-control setting of 83.5 (78 balanced). Signal/Noise Ratio (reference 1V) measured 93.3dB from 22Hz to 22kHz, 71.7dB from 10Hz to 500kHz, and 95.6dB A-weighted (87.7dB, 75.4dB, and 90dB, respectively, for the same measurements in balanced mode). Volume-control tracking was excellent.

The Line 2's frequency response—a virtual flat line from 10Hz to 50kHz in all modes—is not shown. Fig.1 shows the crosstalk, better in balanced mode but excellent even at its worst.

Fig.1 Sonic Frontiers Line 2, crosstalk (from bottom to top at 10kHz): R-L, L-R, unbalanced; L-R, R-L, balanced (10dB/vertical div.).

The THD+noise percentage vs frequency plot is shown in fig.2; again, the balanced result is considerably lower, but primarily because of the input level chosen. As with the Line 1 reviewed in the November 1997 Stereophile, the Line 2's minimum THD+noise in its unbalanced mode occurs at an output level of under 200mV—not enough to drive most amplifiers to maximum output. For these measurements, therefore (as well as for the crosstalk, above), I used an output of 2V. Though this results in a higher distortion reading in the unbalanced mode, it's enough for the maximum output of virtually any amplifier.

Fig.2 Sonic Frontiers Line 2, THD+noise vs frequency at 2V into 100k ohms (from top to bottom): unbalanced, balanced (right channel dashed).

Fig.3 Sonic Frontiers Line 2, distortion (%) vs output voltage into 100k ohms (from top to bottom at 5V): unbalanced, balanced.

Spectral analysis of a 50Hz input at a high output level (not shown) indicates that the primary distortion components are third and second harmonic, in that order.

The Line 2's test-bench results were very similar to those for the Line 1 in most significant ways—which is to say, very respectable indeed.—Thomas J. Norton