Musical Fidelity Nu-Vista 800.2 Integrated Amplifier Measurements

Link: reviewed by Phil Gold on SoundStage! Hi-Fi on October 15, 2023

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Musical Fidelity Nu-Vista 800.2 was conditioned for one hour at 1/8th full rated power (~37W into 8 ohms) before any measurements were taken. All measurements were taken with both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The 800.2 offers four unbalanced (RCA) and one balanced (XLR) set of line-level analog inputs, two pairs of line-level outputs (fixed and variable over RCA), and two pairs of speaker-level outputs. For the purposes of these measurements, the following inputs were evaluated: analog line-level balanced inputs (a 1kHz FFT using the unbalanced inputs is also provided).

Most measurements were made with a 2Vrms line-level analog input. The signal-to-noise ratio (SNR) measurements were amade with the same input signal values but with the volume set to achieve the measured output power at 1% THD, which was 276W into 8 ohms (the 800.2 did not make its rated output of 330W into 8 ohms). For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum, but with a lower input voltage to achieve the same 276W output.

Based on the accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the 800.2 volume control is likely digitally controlled but operating in the analog domain. The volume control offers a total range from -69.5dB to +30.9dB between the line-level balanced analog inputs and the speaker outputs. Volume step sizes were 0.5dB steps throughout the range.

The analyzer’s input bandwidth filter was set to 10Hz–22.4kHz for all measurements, except for frequency response (DC to 1 MHz), FFTs (10Hz to 90kHz) and THD vs Frequency (10Hz to 90kHz). The latter to capture the second and third harmonic of the 20kHz output signal. Since the 800.2 is a conventional class-AB amp, there was no issue with excessive noise above 20kHz.

Volume-control accuracy (measured at speaker outputs): left-right channel tracking

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Musical Fidelity for the 800.2 compared directly against our own. The published specifications are sourced from Musical Fidelity’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth was extended from DC to 1MHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level analog input (unless specified, assume a 1kHz sinewave at 2Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the 800.2 was able to sustain 508W into 4 ohms (~4% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (50.8W) for five seconds for five continuous minutes without inducing a fault or the initiation of a protective circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the 800.2 was warm but not hot to the touch.

Frequency response (8-ohm loading, line-level input, relative level)

In our measured frequency-response chart above, the blue and red plots show the speaker-level outputs (relative to 1kHz, at 10W into 8 ohms). The 800.2’s speaker outputs are near flat within the audioband   (about -0.1dB at 20Hz and 20kHz), and exhibit an average extended bandwidth (-2dB at 75kHz). The 800.2 appears to be AC-coupled, due the attenuation in response below 20Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response (line-level input)

Above is the phase response plot from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The 800.2 does not invert polarity. Here we find phase shifts of +20 degrees at 20Hz and -20 degrees of at 20kHz.

RMS level vs. frequency vs. load impedance (1W, left channel only)

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8ohms or 2.83Vrms) as a function of frequency, for the analog line-level input swept from 5Hz to 100kHz. The blue plot is into an 8-ohm load, the purple is into a 4-ohm load, the pink plot is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan plot is no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we can see only minor deviations of about 0.04dB from 4 ohms to no load through most of the audioband, reaching as high as about 0.1dB at 20kHz. This is an indication of a high damping factor, or low output impedance. The variation in RMS level when a real speaker was used is about the same at 0.04dB through most of the audioband.

THD ratio (unweighted) vs. frequency vs. output power

The chart above shows THD ratios at the output into 8 ohms as a function of frequency for a sinewave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 245W. The power was varied using the volume control. At 1W and 10W, THD ratios were very similar and ranged from 0.001–0.002% from 20Hz to 2kHz, then up to 0.01% at 20kHz. The 245W THD data were consistent for the right channel at 0.005% from 20Hz to 8kHz, then up to 0.01% at 20kHz. The left channel THD ratios at 245W varied from 0.02% at 20Hz, down to 0.002% from 100Hz to 2kHz, then up to 0.01% at 20kHz.

THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD ratios measured at the output of the 800.2 as a function of output power for the analog line-level input for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). The 8-ohm data outperformed the 4-ohm data by 3 to 5dB. The 8-ohm data ranged from 0.01% at 50mW, down to 0.001% at 2-3W, the up to 0.005% at the “knee” at roughly 250W. The “knee” for the 4-ohm data can be seen at roughly 400W. The 1% THD marks are at 276W (shy of the rated 330W) into 8 ohms, and 472W into 4 ohms.

THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms

The chart above shows THD+N ratios measured at the output of the 800.2 as a function of output power for the analog line level-input for an 8-ohm load (blue/red for left/right) and a 4-ohm load (purple/green for left/right). Overall, THD+N values for both loads were similar, ranging from about 0.05% at 50mW, down to 0.003% (8-ohm). The 4-ohm THD+N ratios were 2–3dB worse than the 8-ohm ratios through most of the sweep.

THD ratio (unweighted) vs. frequency at 8, 4, and 2 ohms (left channel only)

The chart above shows THD ratios measured at the output of the 800.2 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 40W at the output into 8 ohms (and roughly 80W into 4 ohms, and 160W into 2 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the 4-ohm load the purple trace, and the 2-ohm load the pink trace. We find a 5 to 10dB increase in THD every time the load is halved. Nonetheless, even into 2 ohms, these data show that the 800.2 is not only stable into 2-ohm loads, but will yield acceptably low THD values, ranging from 0.005 to 0.05%.

THD ratio (unweighted) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows THD ratios measured at the output of the 800.2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). At lower frequencies, the two-way speaker yielded the highest THD ratios, as high as 0.03% at 20Hz. From 300Hz to 20kHz however, all THD values were very similar, ranging  from around 0.001% up to 0.01%. This shows that the 800.2 will yield consistent and stable THD results into different loads at low power levels.

IMD ratio (CCIF) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the 800.2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here the CCIF IMD method was used, where the primary frequency is swept from 20kHz (F1) down to 2.5kHz, and the secondary frequency (F2) is always 1kHz lower than the primary, with a 1:1 ratio. The CCIF IMD analysis results are the sum of the second (F1-F2 or 1kHz) and third modulation products (F1+1kHz, F2-1kHz). The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). The results for the dummy load were fairly consistent, hovering around 0.001% throughout the sweep. We find that both speakers yielded IMD ratios that were close to the dummy load, but at times 5dB higher than the dummy load.

IMD ratio (SMPTE) vs. frequency into 8 ohms and real speakers (left channel only)

The chart above shows IMD ratios measured at the output of the 800.2 as a function of frequency into an 8-ohm load and two different speakers for a constant output voltage of 2.83Vrms (1W into 8 ohms) for the analog line-level input. Here, the SMPTE IMD method was used, where the primary frequency (F1) is swept from 250Hz down to 40Hz, and the secondary frequency (F2) is held at 7kHz with a 4:1 ratio. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 8-ohm load is the blue trace, the purple plot is a two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is a three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are similar enough to be judged identocal, hovering around the 0.006% level.

FFT spectrum – 1kHz (balanced line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the balanced analog line-level input. We see that the signal’s second harmonic (2kHz) dominates at around -100dBrA, or 0.001%. The other signal harmonics are below -110dBrA, or 0.0003%. There are clearly visible power-supply-related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz (right) and 120Hz (left) peaks dominating at -105dBrA, or 0.0006%.

FFT spectrum – 1kHz (unbalanced line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the unbalanced analog line-level input. The FFT is virtually identical to the FFT shown above using the balanced input.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output across an 8-ohm load at 10W for the analog line-level input. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. We see that the signal’s second harmonic (100Hz) dominates at around -100dBrA, or 0.001%. The subsequent signal harmonic peaks are below -110dBrA, or 0.0003%. There are clearly visible power-supply related noise peaks at even and odd harmonics (60Hz, 120Hz, 180Hz, 240Hz, etc.) on the left side of the main 1kHz peak, with the 60Hz (right) and 120Hz (left) peaks dominating at -105dBrA, or 0.0006%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the analog line-level input. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 10W (0dBrA) into 8 ohms at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -110dBrA, or 0.0003%. The third-order modulation products, at 17kHz and 20kHz, are higher at -105dBrA, or 0.0006%. This is a very clean IMD result.

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the speaker-level output of the 800.2 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and below the -120dBrA, or 0.0001%, level. This is another clean IMD result. The peaks that reach the -105dBrA level at lower frequencies are not IMD products but power-supply-related noise peaks.

Square-wave response (10kHz)

Above is the 10kHz squarewave response using the analog line-level input, at roughly 10W into 8 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the 800.2’s slew-rate performance. Rather, it should be seen as a qualitative representation of the 800.2’s above-average bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. Here we can see a relatively clean squarewave reproduction, with some mild softening of the corners, and no overshoot.

Damping factor vs. frequency (20Hz to 20kHz)

The final graph above is the damping factor as a function of frequency. Both channels track closely, with a higher damping factor (around 350 to 370) between 20Hz and 2kHz. Above 2kHz, we see a decline in the damping factor, as low as 150 at 20kHz.

Diego Estan
Electronics Measurement Specialist