Tube Preamp Reviews

The VTL TL-6.5 Reference's maximum voltage gain was 19.6dB for balanced signals from its balanced outputs, 13.6dB for unbalanced signals from its unbalanced outputs. The preamplifier preserved absolute polarity— ie, was non-inverting—from both outputs with the red "Phase" LED not illuminated. (The XLRs are wired with pin 2 hot.) The input impedance didn't change with the setting of the volume control; across the audioband, it was a uniform 14k ohms unbalanced and 32k ohms balanced. This is lower than the specified 50k ohms, but should still be sufficiently high to keep bass frequencies from sounding lean with sources other than CD players with a tubed output stage.

The output impedance at high and midrange frequencies was a low 29 ohms unbalanced, 53 ohms balanced. Though this did rise at 20Hz to 150 ohms unbalanced and 363 ohms balanced, these figures are still low enough to give a full low-frequency response with the power amplifiers with which this preamp will typically be used. VTL's S-400, for example, has an unbalanced input impedance that ranges from 56k ohms at the frequency extremes to 74.4k ohms at 1kHz, well above the range where there will be audible modification of the frequency response due to the impedance interaction.

The TL-6.5 offered very wide bandwidth from both sets of jacks, its output being just a fraction of a dB down at 200kHz (fig.1, top pair of traces). This was with the volume control set to its maximum; turning down the volume reduced the ultrasonic bandwidth, as shown by the bottom pair of traces in fig.1, which were taken with the control set to unity gain in unbalanced mode ("79" of a possible "95"). The output is now down 1dB at 144kHz, but is still impressively flat in the audioband. The ultrasonic rolloff was a little more pronounced with the preamp operating in full balanced mode, to –3dB at 160kHz (fig.2), but the response was still flat to almost 20kHz.

Fig.1 VTL TL-6.5, unbalanced frequency response at 1V into 100k ohms with volume control set to maximum (top) and unity gain (bottom). (0.5dB/vertical div., right channel dashed)

Fig.2 VTL TL-6.5, balanced frequency response at 1V into 100k ohms with volume control set to maximum (top) and unity gain (bottom). (0.5dB/vertical div., right channel dashed)

Channel separation was excellent, at better than 105dB in the midrange and treble in unbalanced mode, but a little less so in balanced mode (not shown). The signal/noise ratio was good rather than great at 76dB, unweighted wideband, unbalanced ref. 1V, with the input shorted and the volume control set to full. This was due to the presence of some very-low-frequency noise and a low-level hum component at 180Hz in unbalanced mode that I couldn't eliminate by experimenting with the grounding. A-weighting the S/N ratio improved it to a fine 98dB, however.

The VTL preamp could swing very high voltages with very low distortion. Figs.3 and 4 show how the percentage of THD+noise varies into a wide range of impedances for the unbalanced and balanced outputs, respectively. Into loads of 10k ohms or higher, the TL-6.5 doesn't clip (1% THD) until the output level reaches an extraordinary 32V (unbalanced) or 66V (balanced)! Even into 600 ohms, the VTL puts out several volts before clipping, well above what will be required to drive a power amplifier into clipping. The upward slope of the traces in these graphs below 1V (unbalanced) and 2V (balanced) indicates that the distortion is below the noise floor at these levels. Though the distortion begins to emerge from the noise above these output voltages, the fact that the traces show minima between 1V and 2V indicates that the preamp's gain architecture has been sensibly managed: higher output levels where its own distortion might start to matter will overload the partnering power amplifier.

Fig.3 VTL TL-6.5, distortion (%)vs 1kHz unbalanced output level into (from bottom to top at 2V): 100k, 10k, 1000, 600 ohms.

Fig.4 VTL TL-6.5, distortion (%)vs 1kHz balanced output level into (from bottom to top at 100W): 100k, 600 ohms.

Though the distortion rises with decreasing load impedance, it doesn't change with frequency (fig.5), and remains well below 0.1%, even at 1V unbalanced into 600 ohms. Balanced operation is even more linear, due to the usual cancellation of even-order harmonics (fig.6). For unbalanced operation into a moderately low 8k ohms, the highest-level harmonic is the subjectively benign second, at a low –79dB left and –80dB right (0.01%, fig.7). Higher harmonics were all below –100dB. With the balanced output driven at 3V into 600 ohms, about the highest level the preamp will see in practice, the third harmonic rose to –70dB (0.03%, fig.8), which is still relatively low.

Fig.5 VTL TL-6.5, THD+N (%)vs frequency at 1V unbalanced into (from bottom to top): 100k, 10k, 1000, 600 ohms.

Fig.6 VTL TL-6.5, THD+N (%)vs frequency at 1V balanced into (from bottom to top): 100k, 10k, 1000, 600 ohms.

Fig.7 VTL TL-6.5, spectrum of 1kHz sinewave, DC–10kHz, at 1V unbalanced into 8k ohms (linear frequency scale).

Fig.8 VTL TL-6.5, spectrum of 50Hz sinewave, DC–10kHz, at 3V balanced into 600 ohms (linear frequency scale).

Finally, intermodulation distortion was low (fig.9), with the second-order difference component resulting from a full-scale mix of 19kHz and 20kHz tones lying at –77dB (0.014%).

Fig.9 VTL TL-6.5, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 1V peak into 8k ohms (linear frequency scale).

This is overall excellent measured performance, giving no clue to the pair of 12AU7 tubes hiding within the TL-6.5's hefty chassis.—John Atkinson