Tube Preamp Reviews

I measured the VTL TL6.5 Series II Signature's electrical performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Most of the measurements were taken with balanced input and output signals; I repeated some tests with unbalanced signals. The volume control's unity-gain setting was "78" on the front-panel display and the control appears to operate in accurate 0.75dB steps. At the control's maximum setting of "95," the voltage gain at 1kHz, measured at the balanced output jacks, was 13.2dB for both balanced and unbalanced input signals. The gain at the unbalanced output was the expected 6dB lower than this, 7.2dB, for an unbalanced input. The preamplifier preserved absolute polarity (ie, was non-inverting) for all input and output configurations.

The balanced input impedance was 34k ohms at low and middle frequencies, dropping inconsequentially to 33k ohms at the top of the audioband. The unbalanced input impedance was half these values, as expected. The balanced output impedance was 358 ohms at 20Hz, dropping to a low 32 ohms at 1kHz and above. The unbalanced output impedance was half the balanced figure, again as expected.

With the volume control set to "95," the TL6.5 II's frequency response into 100k ohms was flat almost up the 200kHz limit of my measurement system, with superbly close channel matching (fig.1, blue and red traces). Reducing the load impedance to a demanding 600 ohms rolled off the low frequencies, due to the increased output impedance in this region, the response reaching –3dB at 25Hz (fig.1, cyan and magenta traces). However, it is very unlikely that the TL6.5 will encounter this low a load impedance in practice. The audioband response was still flat at lower settings of the volume control, but the ultrasonic extension was reduced. At unity gain, for example, the output was down by 1.4dB at 200kHz. The picture was similar for unbalanced operation, even at the volume control's maximum setting.

Channel separation (not shown) was superb, at >120dB in both directions below 10kHz. The unweighted wideband signal/noise ratio, measured with the input shorted to ground but the volume control set to "95," was very good, at 80.6dB (average of both channels) ref. 1V output. Restricting the measurement bandwidth to the audioband increased the ratios to 95dB left and 92dB right; an A-weighting filter further increased the ratios, to 98.3dB left and 95.9dB right. Spectral analysis of the output signal (fig.2) revealed that the level of random noise was very low, but with low-level spuriae present at the AC power-line frequency of 60Hz and its harmonics. These are most likely due to magnetic interference from the power transformer being picked up by ferrous circuit elements, perhaps the tubes' steel pins. Nevertheless, these spuriae are very low in absolute level and will not affect sound quality.

The TL6.5 II offers enormous dynamic range, the preamplifier's balanced output not clipping (ie, when the THD+noise reaches 1%) until 27V into 100k ohms (fig.3). Even into 600 ohms, the preamp didn't clip until >9V. The unbalanced output clipped at 18V into 100k ohms. Note the downward slope of the trace below 600mV in fig.3, which suggests that the actual distortion lies under the noise floor below this level, and is very low in absolute terms. I therefore measured how the THD+N varied with frequency (fig.4) at a level, 2V, where I could be sure I was looking at actual distortion. Even so, the percentage remains constant at all audio frequencies, though it's a bit higher in the left channel (blue and cyan traces) than in the right (red, magenta).

With nonlinear distortion, what matters more than the absolute level is the harmonic signature. Fig.5 reveals that the VTL's dominant harmonic was the subjectively benign second, though this was still at a low level: –89dB in the left channel, –96dB in the right (0.004% and 0.0015%, respectively). The third harmonic lies at –100dB (0.001%) in both channels, around the same level as the third harmonic of the powerline frequency. The second harmonic didn't change into the low 600 ohm load, though the third harmonic rose to –96dB (fig.6). There was a higher level of second harmonic with unbalanced drive, at –70dB (0.03%). Intermodulation distortion was also very low (fig.7).

The TL6.5 Series II Signature is a well-engineered preamplifier—as I have come to expect from VTL.—John Atkinson