Integrated Amp Reviews

I performed a full set of measurements on the Musical Fidelity Nu-Vista 800.21 using my Audio Precision SYS2722 system. As the amplifier is specified as having a maximum output power of 330Wpc into 8 ohms, I preconditioned it before the measurements by following the CEA's recommendation of running it at one-eighth that power into 8 ohms for 30 minutes. Following that period, the top panel was warm, at 100.1°F (37.9°C), and the side-mounted heatsinks hotter, at 130.1°F (54.5°C).

The Nu-Vista 800.2's volume control operated in accurate 0.5dB steps. At the maximum volume control setting of "7.0dB," the voltage gain at 1kHz into 8 ohms was relatively low for an integrated amplifier, at 30.6dB for the unbalanced "CD" input and 30.8dB for the balanced input. The Nu-Vista 800.2 preserved absolute polarity for both balanced and unbalanced inputs, its XLRs being wired with pin 2 hot. The input impedance is specified as 40k ohms. I measured 46k ohms at 20Hz, 41k ohms at 1kHz, and 17k ohms at 20kHz for the unbalanced inputs. The balanced input impedance was the same as I found with the original Nu-Vista 800, at 18k ohms across the audioband.

The output impedance, including 6' of spaced-pair speaker cable, was relatively low in the bass and midrange, at 0.118 ohm, rising to 0.165 ohm at the top of the audioband. Consequently, the modulation of the Nu-Vista 800.2's frequency response due to the Ohm's law interaction between this impedance and the impedance of our standard simulated loudspeaker was also low (fig.1, gray trace). The amplifier's response into resistive loads was flat in the audioband, with its output into 8 ohms down by 0.15dB at 20kHz and by 3dB at 85kHz (blue and red traces). This graph was taken with the balanced inputs and the volume control set to its maximum. Commendably, both the very close channel balance and the overall response were preserved at lower settings of the volume control and with the unbalanced inputs. The Nu-Vista 800.2 accurately reproduced a 10kHz square wave (fig.2), with no overshoot or ringing.

Channel separation below 2kHz was superb, at >100dB R–L, >90dB L–R, and still 77B in both directions at 20kHz. The wideband, unweighted signal/noise ratio, taken with the unbalanced CD input shorted and the volume control set to its maximum, was superior to that of the original NuVista 800, at 69.5dB in both channels, this ratio ref. 2.83V, which is equivalent to 1W into 8 ohms. This ratio improved to 77.6dB when the measurement bandwidth was restricted to the audioband, and to 80.2dB when A-weighted. Musical Fidelity specifies the amplifier's A-weighted S/N ratio as >107dB. I assume that this is ref. full power into 8 ohms, as my 80.2dBA ref. 1W into 8 ohms is equivalent to 105.2dB ref. 330W into the same impedance. The blue and red traces in fig.3 show the amplifier's low-frequency noisefloor at 1Wpc into 8 ohms with the volume control set to its maximum. Reducing the volume by 20dB and increasing the input signal by the same 20dB so that the output power remains at 1W drops the random noisefloor by 9dB (green and gray traces) and unmasks some odd-order harmonics of the AC supply frequency. These will be due to magnetic interference from the two massive power transformers, but are all very low in level.

Figs.4 and 5 respectively plot how the THD+noise percentage varies with output power with both channels driven into 8 ohms and 4 ohms. The downward slope below 30Wpc into 8 ohms and 60Wpc into 4 ohms indicates that the distortion lies below the noise up to these powers, but it remains low until the actual onset of clipping. At our usual definition of clipping, which is when the THD+N reaches 1%, the Nu-Vista 800.2 didn't quite meet its specified output power of 330W into 8 ohms (25.2dBW). Fig.4 indicates that the amplifier clips at 290Wpc into 8 ohms (24.6dBW), though it's fair to note that the supply voltage had dropped from 119V AC with the amplifier quiescent to 116.5V AC when both channels were clipping into 8 ohms. (I don't have a sufficiently powerful Variac to hold the wall voltage constant in my amplifier testing.) Into 4 ohms with both channels driven, the Nu-Vista 800.2 clipped at 470Wpc (23.7dBW, fig.5) compared with the specified 500W (24dBW). However, the wall voltage had dropped to 115.9V AC with the amplifier clipping into 4 ohms. I didn't test the Musical Fidelity's clipping power into 2 ohms, as the amplifier isn't specified as being able to deliver full power into 2 ohms except on peaks (footnote 1).

Fig.6 shows how the THD+N percentage changed at 20V, which is equivalent to 50W into 8 ohms and 100W into 4 ohms. The distortion into 4 and 8 ohms is very low and, commendably, doesn't rise in the top octave, which suggests that the amplifier has a wide open-loop bandwidth. However, the distortion in the left channel rises at very low frequencies, especially into 4 ohms (cyan trace).

The THD+N waveform at 50W into 8 ohms was primarily the subjectively innocuous second harmonic (figs.7 and 8), though the third harmonic was dominant at the same power into 4 ohms (fig.9). But with this harmonic lying at –99dB (0.001%) it should not have audible consequences, especially as intermodulation distortion is also extremely low, even into 4 ohms (fig.10).

Musical Fidelity's Nu-Vista 800.2 offers excellent measured performance, equaling or bettering that of its predecessor.—John Atkinson