Vacuum Tube Logic TL-5.5 line preamplifier Measurements
The VTL preamp offered a maximum voltage gain of 20.5dB into 100k ohms and was noninverting from both its balanced and unbalanced outputs with the Phase LED green. The input impedance at 1kHz was 12k ohms, while the unbalanced output impedance was 184 ohms across most of the audioband, rising to a still low 409 ohms at 20Hz.
As a result of the increase in source impedance at low frequencies, the low-frequency response rolled off to reach -3dB at 16Hz into the punishing 600 ohm load (fig.1, bottom pair of traces below 200Hz). However, into 100k ohms the TL-5.5's infrasonic response was well-extended (fig.1, top pair of traces below 200Hz). The ultrasonic response is also a little less extended into the low load impedance. However, the bandwidth changes with the setting of the volume control: at its maximum position, the HF response is -3dB at 200kHz; at unity gain (1:30), the -3dB frequency decreased to 105kHz. However, as there was no significant change in the audioband rolloff, this behavior is probably insignificant.
Fig.1 VTL TL-5.5, unbalanced frequency response (from top to bottom at 150kHz) into 100k ohms with volume control at maximum and unity gain, and into 600 ohms with volume control at maximum (0.5dB/vertical div.).
Channel separation was excellent in the low treble, at 100dB L-R and 90dB R-L (fig.2), but this worsened at higher frequencies due to capacitive coupling, and at lower frequencies due (presumably) to increasing power-supply impedance. The A-weighted signal/noise ratio ref. 1V output with the volume control at its maximum and the input jack short-circuited was a high 89.3dB, this worsening to 73.5dB with an unweighted wideband measurement.
Fig.2 VTL TL-5.5, unbalanced channel separation (R-L dashed, 10dB/vertical div.).
The VTL preamp was both very linear and capable of swinging very high voltages into real-world loads. Fig.3 plots the percentage of THD+noise in the TL-5.5's output against output voltage into 100k, 10k, and 1k ohms. The preamp clips (1% THD) at around 16V into the higher impedances, and will still drive 7V into 1k. The downward slopes of the traces at the left of this graph indicate that, below 600mV or so, the reading is dominated by noise. The true distortion level is a hair over 0.02% (-74dB). Fig.4 plots the THD+N against frequency at 1V into both 100k and 600 ohms. A slight but inconsequential rise in distortion is evident at the frequency extremes into the lower impedance, but the right channel can be seen to be even more linear than the left.
Fig.3 VTL TL-5.5, distortion (%) vs unbalanced output voltage into (from right to left at 10% THD): 100k, 10k, and 1k ohms.
Fig.4 VTL TL-5.5, unbalanced THD+noise vs frequency at 1V into 100k ohms (bottom) and 600 ohms (top) (right channel dashed).
The spectrum of the TL-5.5's distortion into 8k ohms, which is about the lowest impedance the VTL will be called on to drive, is shown in fig.5. The second harmonic is the highest in level, but will be subjectively innocuous at -78dB (0.013%). Similarly, the level of intermodulation distortion products (fig.6), even into the demanding 1k ohm load, is low. The difference product at 1kHz is the highest in level, but at -76dB (0.015%) will be insignificant.
Fig.5 VTL TL-5.5, unbalanced spectrum of 1kHz sinewave, DC-10kHz, at 1V into 8k ohms (linear frequency scale).
Fig.6 VTL TL-5.5, unbalanced HF intermodulation spectrum, DC-24kHz, 19+20kHz at 1V into 1k ohms (linear frequency scale).
All in all, the VTL TL-5.5 seems to be a well-engineered preamp, with nothing to indicate that its use of tubes has compromised its performance.—John Atkinson