Tube Power Amp Reviews

The VTL's input impedance at 1kHz was a hair over 100k ohms, while its sensitivity was high, at just 71.6mV for 1W into 8 ohms, both factors making it suitable for use with passive preamps. The lefthand amplifier was 0.5dB less sensitive, however, as can be seen from the Tiny Triode's frequency response at 1W into 8 ohms (fig.1). This showed a slight rolloff in the extreme treble, reaching –0.4dB at 20kHz. The limited HF bandwidth can also be seen in the shape of a 10kHz squarewave (fig.2), which features a mild droop to the leading edges.

Fig.1 VTL Tiny Triode, frequency response at 2.83V into 8 ohms (0.5dB/vertical div., right channel dashed).

Fig.2 VTL Tiny Triode, small-signal 10kHz squarewave into 8 ohms.

Unweighted wideband noise was low in level, lying some 67dB below 1W into 8 ohms. Signal polarity was non-inverting while the amplifier's output impedance was typical for a tube design in being quite high across the audio band, at 0.85 ohms (left) and 0.67 ohms (right). This will give some modification of a loudspeaker's frequency response, as well as leading to a lack of control of the low-frequency box resonance.

Distortion levels were moderate, however. Fig.3 shows THD+noise plotted against frequency at 2.83V into 8 ohms (equivalent to 1W, lower two traces) and 4 ohms (2W, upper two traces). The Tiny Triode definitely has to work hard into lower impedances, particularly at higher frequencies, though the added distortion harmonics here will be above human hearing. The midband and bass distortion is predominantly third harmonic, as can be seen from fig.4—which shows a 1kHz waveform and the distortion waveform left after the fundamental has been notched out, and fig.5, which shows the spectrum of harmonics and AC mains components imposed on a 50Hz sinewave at a level of 12.5W into 4 ohms. Again, the cute little Tiny Triode is having to work hard in the bass, with THD reaching –44.7dB, or 0.6%. Even though this is mainly the relatively subjectively benign third harmonic, this will lend low frequencies a "furry," poorly defined character.

Fig.3 VTL Tiny Triode, THD+N (%) vs frequency at 2.83V into (from bottom to top), 8 and 4 ohms (right channel dashed).

Fig.4 VTL Tiny Triode, 1kHz waveform at 12.5W into 4 ohms (top), 0.6% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 VTL Tiny Triode, spectrum of 50Hz sinewave, DC–1kHz, at 12.5W into 4 ohms (logarithmic frequency scale).

High-frequency intermodulation was fairly good considering the rising THD in this region. A mixture of 19 and 20kHz tones at a p-p voltage of 30V into 8 ohms produced a 1kHz product at –58.4dB, or just over 0.1% (fig.6).

Fig.6 VTL Tiny Triode, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 30V peak–peak into 8 ohms (linear frequency scale).

Regarding maximum output power, fig.7 shows the way in which distortion rises against output power into 8 ohms (lower trace) and 4 ohms (upper trace). (The line voltage was 117V for these measurements.) The Tiny Triode is definitely tiny: defining clipping as the 1% THD point, the TT raised 21.9W/22.2W L/R into 8 ohms (13.4dBW/13.5dBW), and just 21.2W/17.4W (10.3dBW/9.4dBW) into 4 ohms. But as can be seen from fig.7, the rise in distortion with level is not nearly so on/off as with a typical solid-state amplifier. Although it might be expected that the Tiny Triode will behave more like a Bantamweight Triode, with spectrally pure sounds such as piano, the added distortion is audible. Why did CG find the TT's clipping to be objectionable? Probably because it doesn't take much more drive signal to take the little amp into full clipping.

Fig.7 VTL Tiny Triode, distortion (%)vs 1kHz continuous output power into (from bottom to top at 10W): 8 and 4 ohms.

Looking at these measurements in total, the Tiny Triode does surprisingly well, compromised only by its restricted output power and the high levels of distortion in the low bass, the sonic consequences of which were well-described by Corey. Tell you what. The Tiny Triode would make a great headphone amplifier!—John Atkinson