Vacuum Tube Logic TL-7.5 Reference line preamplifier Measurements

Sidebar 3: Measurements

The two-chassis TL-7.5 was a delight to test, both ergonomically and because it proved oblivious of every grounding scheme I tried between it and my test equipment. Someone has done their homework with respect to arranging the preamp circuit's grounding arrangement. Maximum voltage gain with balanced input to balanced output measured 25.25dB; unbalanced to unbalanced was 18.4dB, both figures slightly but inconsequentially lower than specified. Unity gain on the stepped volume control was "60" balanced, "69" unbalanced.

Unless the Phase button was pushed, the TL-7.5 preserved absolute polarity for both balanced and unbalanced signals. The balanced input impedance was 30k ohms across the audioband, the unbalanced figure 48k ohms—both are reasonably high. The balanced output impedance was 44 ohms over most of the audioband, this rising to 167 ohms at 20Hz. The corresponding unbalanced source impedances were 25 and 69 ohms, respectively. All these figures are low.

The TL-7.5's frequency response was dead flat in the audioband into high impedances (fig.1, top pair of traces), and was the same at all volume-control settings and for both balanced and unbalanced inputs and outputs. The ultrasonic rolloff was well above the audioband, at -0.5dB at 75kHz and -3dB at 150kHz. At the other end of the spectrum, the bass extended to below 10Hz into high impedances. The low frequencies rolled off earlier into a 600 ohm load (fig.1, lower traces), reaching 0.5dB down at 40Hz. But as 600 ohms is well below what the TL-7.5 would see in practice, this is academic.

Fig.1 VTL TL-7.5, balanced, frequency response at 1V into 100k ohms (top) and 600 ohms (bottom), both with volume control at unity gain (0.5dB/vertical div., right channel dashed).

The channel separation (not shown) was better than 110dB in the audioband, which is superb. The signal/noise ratio (ref. 2V balanced output) was good rather than great; the unweighted, wideband figure measured 79dB, this improving to 88dB when the measurement bandwidth was reduced to the audioband. A-weighting gave a further improvement of 3dB.

Fig.2 shows how the balanced output's percentage of distortion+noise changes with output voltage. The downward slope of the traces below 1.75V indicates that the measurement is dominated by noise in this region. The actual distortion harmonics rise above the noise floor only above 2V, which reveals a sensibly arranged gain architecture, 2V being about the maximum the TL-7.5 will be required to deliver into a real-world power amplifier. The lowest trace in this graph was taken with the preamp driving the high 100k ohm load. Note that the 1% clipping point is reached at an astonishing 69V! Even into the punishing 600 ohm load (top trace), no fewer than 24V are available at 1% THD. As expected, half this voltage was available from the single-ended jacks—still an order of magnitude higher than that required to drive any power amplifier into clipping.

Fig.2 VTL TL-7.5, balanced, distortion (%) vs output voltage at 1kHz into (from bottom to top): 100k, 600 ohms.

I measured the way the distortion percentage changed with frequency at 2V output—the level, as noted above, where the harmonics begin to rise out of the noise (fig.3). Even so, the THD level was uniformly low and not significantly affected by the load. The spectrum of the distortion was predominantly the subjectively benign second harmonic, though at a low level (fig.4). The third harmonic rose into low impedances but still remained below the second (not shown). Intermodulation distortion was also very low in level (fig.5), and did not increase significantly into low impedances. For fun I drove the demanding mix of 19kHz and 20kHz tones at 10V balanced into 600 ohms. The 1kHz component rose, but only to a still low -73dB (0.02%). Extraordinary!

Fig.3 VTL TL-7.5, balanced, THD+N (%) vs frequency at 2V into 100k ohms (bottom) and 600 ohms (top). (Right channel dashed.)

Fig.4 VTL TL-7.5, unbalanced, spectrum of 1kHz sinewave, DC-10kHz, at 1V into 8k ohms (linear frequency scale).

Fig.5 VTL TL-7.5, unbalanced, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 1V into 8k ohms (linear frequency scale).

VTL's TL-7.5 demonstrates virtually bombproof measured performance, with no clue—other than its sound quality—that it is a tubed preamplifier. With its enormous dynamic-range potential, in real-life systems this preamp will be idling almost all the time. I look forward to auditioning it in my own system.—John Atkinson