Vincent TubeLine SV-236MK integrated amplifier Measurements

Sidebar 3: Measurements

I used the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and to measure the Vincent SV-236MK; for some tests, I also used an Audio Precision System One Dual Domain.

As usual, before performing any tests, I ran the Vincent for 60 minutes at one-third its specified power into 8 ohms, which is thermally the worst case for an amplifier with a class-B or -AB output stage. The measured distortion+noise was 0.021% with the amplifier cold; at the end of the hour's preconditioning, it had risen inconsequentially, to 0.022% and the heatsinks were around 60°; ie, just too hot to touch. Unlike a number of amplifiers I have measured recently, the Vincent has enough heatsink capacity for its specified output power.

The maximum voltage gain measured at the speaker terminals into 8 ohms was a high 48.66dB. Measured at the preamplifier output jacks, the maximum gain was 18.1dB, which is also somewhat on the high side. The amplifier preserved absolute polarity, ie, was non-inverting, from both sets of outputs. The input impedance was lower than the specified 47k ohms but still fairly high at 41k ohms at low and middle frequencies. It dropped slightly but inconsequentially to 36k ohms at 20kHz. The source impedance from the preamplifier output jacks was a high 2.3k ohms at high and midrange frequencies, rising to an even higher 4.1k ohms at 20Hz. However, the source impedance from the speaker terminals was a low 0.09 ohms over almost all the audioband, rising slightly to 0.12 ohms at 20kHz. As a result, the modification of the amplifier's frequency response with our standard simulated loudspeaker remained within ±0.1dB limits (fig.1, gray trace).

Fig.1 was taken with the volume control at its maximum, and the channel matching can be seen to be superb. However, at lower settings of the control, an imbalance of up to 0.4dB appeared, in favor of the left channel. The Vincent amplifier has a wide small-signal bandwidth, its output not dropping by 3dB until 120kHz. As a result, its reproduction of a 10kHz squarewave featured very short risetimes, as well as a complete lack of overshoot or ringing (fig.2). These measurements were taken with the tone controls and Loudness defeated. The top and bottom pairs of traces in fig.3 show the effect on the SV-236MK's small-signal response of the Bass and Treble controls set to their maximum and minimum positions, respectively. The boosts and cuts are usefully moderate, with the exception of the treble cut, which is a bit too aggressive. The central pair of traces in this graph show the Vincent's response with the tone controls in-circuit but set to their central detented positions. Both the bass and treble regions suffer some boost in this condition. The effect of the Loudness button is shown in fig.4. The effect will vary with the volume control setting—it was set to –20dB or 1:00 for this measurement—but the treble boost does seem excessive compared with the low-frequency boost.

Fig.1 Vincent SV-236MK, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (1dB/vertical div.)

Fig.2 Vincent SV-236MK, small-signal 10kHz squarewave into 8 ohms.

Fig.3 Vincent SV-236MK, response with, from top to bottom: treble and bass controls set to maximum, detented zero, and minimum, with volume control set to –20dB (left channel blue, right red).

Fig.4 Vincent SV-236MK, loudness contour with volume control set to –20dB (left channel blue, right red).

Channel separation (not shown) was good rather than great, at 70dB below 1kHz in both directions. Perhaps as a result of its high gain, the Vincent also offered modest signal/noise ratios. With the input shorted but the volume control at its maximum (the worst case), the unweighted wideband ratio ref. 2.83V into 8 ohms (1W) was just 63.5dB. This improved to 68.4dB when the measurement bandwidth was restricted to the audioband, and to 70.6dB when A-weighted. I suspect, therefore, that the specified "80dB" was referenced to the amplifier's maximum power rather than to the usual 1W.

Fig.5 plots the THD+noise percentage in the Vincent's output power against output power into 8, 4, and 2 ohms. With both channels driven, the SV-236MK meets its specified maximum power of 150Wpc into 8 ohms (21.75dBW) at clipping (defined as 1% THD). However, it falls a little short into lower impedances, delivering 231Wpc into 4 ohms rather than the specified 250Wpc (20.65dBW rather than 21dBW). With one channel driven into 2 ohms, the amplifier clipped at 362W (19.6dBW), but didn't otherwise misbehave into this low impedance.

Fig.5 Vincent SV-236MK, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4, 2 ohms.

The traces in fig.6 show how the THD+N percentage changes with frequency. I chose a fairly high level—20V RMS—to perform this set of measurements, in order to be sure I was at a level where the actual distortion would not be buried within the noise. There is only a slight increase in THD at the top of the audioband, which suggests that the Vincent has quite a wide open-loop bandwidth. The distortion approximately doubles as the load impedance halves, which is what I expected from fig.5. But you can also see that the right channel (red and magenta traces) is significantly more linear than the left channel (blue, cyan, and green traces).

Fig.6 Vincent SV-236MK, THD+N (%) vs frequency at 16V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).

The spectral content of the Vincent SV-236MK's distortion is predominantly the subjectively innocuous second-harmonic in nature (fig.7), though higher-order harmonics appear as higher powers (fig.8) and output currents (fig.9). This last graph also shows that the less-good linearity of the left channel compared with the right is almost entirely due to an increase in the amount of third harmonic. Note also in this graph the presence of AC power-supply components at 120Hz, 180Hz, 240Hz, 420Hz, etc. Though these all lie beneath –90dB, they do indicate that the amplifier is working close to the edge at these power levels. These spuriae can also be seen as the rise in the noisefloor to the left of the spectrum in fig.10, which analyzes the Vincent's output as it drives an equal combination of 19kHz and 20kHz tones into 4 ohms, at a level just below visible clipping on the oscilloscope screen. Actual intermodulation is fairly low, however, with a kHz difference component reaching –70dB (0.03%) and all the higher-order components lying at or beneath –84dB (0.006%).

Fig.7 Vincent SV-236MK, 1kHz waveform at 48W into 8 ohms (top), 0.03% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.8 Vincent SV-236MK, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (left channel blue, right red; linear frequency scale).

Fig.9 Vincent SV-236MK, spectrum of 50Hz sinewave, DC–1kHz, at 190W into 4 ohms (left channel blue, right red; linear frequency scale).

Fig.10 Vincent SV-236MK, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 190W peak into 4 ohms (linear frequency scale).

The Vincent SV-236MK measures well considering its $1995 price.—John Atkinson

Vincent T.A.C.
US distributor: WS Distributing
3427 Kraft Ave. SE
Grand Rapids, MI 49512
(866) 984-0677