Smart Devices 2X150VT power amplifier Measurements
When I opened up the chassis of the Smart Devices 2X150VT—the input tube had come loose in transit—I flashed back to the early 1980s. Other than Sovtek 6922 tube mounted behind the window in the front panel, I appeared to be holding in my hands a Hafler DH-200, right down to the bird's nest of components wired to the output MOSFETs. Ivor Humphreys, my friend and colleague at Hi-Fi News back in the early '80s, had built and modified many DH-200s, and I had got well familiar with this high-value design.
Following about 30 minutes of preconditioning at one-third power into 8 ohms—the most thermally stressful condition for an amplifier with a class-B or -AB output stage—the 2X150VT's heatsinks were too hot to touch, but things otherwise seemed okay. The input impedance at 1kHz measured around 42k ohms, with slight differences at different volume-control settings. The maximum voltage gain (into 8 ohms) was slightly on the low side at 25.5dB, and the amplifier didn't invert absolute polarity.
The 2X150VT's output impedance was a very low 0.06 ohm at low and middle frequencies, this rising inconsequentially to 0.13 ohm at 20kHz. As a result, there was very little modification of the amplifier's response into our simulated loudspeaker load, or with different resistive loads (fig.1). Of more concern, however, was a 0.4dB imbalance between the two channels. The 2X150VT's ultrasonic response is sensibly rolled off by 3dB at 160kHz, resulting in a slight slowing of a 10kHz squarewave's risetime (fig.2). Channel separation was good over most of the audioband, with slight reductions apparent at very high and very low frequencies (fig.3).
Fig.1 Smart 2X150VT, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (right channel dashed, 0.5dB/vertical div.).
Fig.2 Smart 2X150VT, small-signal 10kHz squarewave into 8 ohms.
Fig.3 Smart 2X150VT, channel separation (10dB/vertical div., R-L dashed).
Small-signal distortion levels remained just above 0.01% across the band, as long as the load didn't drop below 4 ohms. Into 2 ohms, a rise in THD became apparent above 8kHz (fig.4), though this was not to a level of any significance. Fig.5 reveals that the predominant distortion harmonic is the subjectively innocuous second, though I did note that the trace on the 'scope screen bumped around a little, due to the presence of very-low-frequency noise.
Fig.4 Smart 2X150VT, THD+noise (%) vs frequency at (from top to bottom at 10kHz): 2.83V into simulated loudspeaker load, 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms (right channel dashed).
Fig.5 Smart 2X150VT, 1kHz waveform at 22W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).