Rotel A11 Tribute Integrated Amplifier Measurements

Link: reviewed by Dennis Burger on SoundStage! Access on April 1, 2021

General information

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

The Rotel A11 Tribute was conditioned for 1 hour at 1/8th full rated power (~6W 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 A11 Tribute offers four sets of line-level unbalanced inputs (RCA), one moving magnet (MM) phono input (RCA), a set of variable line-level preamp outputs (RCA), and two pairs of speaker outputs. Also available is Bluetooth input (untested) and a headphone output via a 3.5mm TRS jack on the front panel. Based on the accuracy of the left/right channel matching (see table below), the A11’s volume knob is not a potentiometer in the signal path, but rather provides digital control over a proprietary or integrated, analog-domain volume circuit. The volume control offers 1dB increments from -79dB to +14.6dB as measured at the preamp outputs using a line-level input.

All measurements, with the exception of signal-to-noise ratio (SNR) or otherwise stated, were made with the volume set to unity gain for the preamplifier (position 82) as measured at the preamp outputs. SNR measurements were made with the volume control set to maximum. At the unity gain volume position, to achieve 10W into 8 ohms, 383mVrms was required at the line level input and 5mVrms at the phono input.

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 Rotel for the A11 Tribute compared directly against our own. The published specifications are sourced from Rotel’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 is set at its maximum (DC to 1MHz), assume, unless otherwise stated, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the line-level inputs (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Our clipping headroom result was 1.7dB for the A11 Tribute, defined as the ratio of max power over rated power into 8 ohms. The A11 Tribute was also able to sustain 75W (1.7dB over rated output) into 8 ohms using an 80Hz tone for 500 ms, alternating with a signal at -10dB of the peak (7.5 W) for 5 seconds, for 5 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 A11 was warm to the touch, but not quite hot enough to induce pain.

Our primary measurements revealed the following using the phono-level inputs (unless specified, assume a 1kHz sinewave, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced line-level inputs at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms output, 300-ohm loading, 10Hz to 90kHz bandwidth):

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

In our measured frequency-response plot above, the A11 Tribute is essentially flat within the audioband (20Hz to 20kHz) for the line-level input. These data corroborate Rotel’s claim of 10Hz to 100kHz (+/-0.5dB), since the worst-case deviation is at 10kHz where the response is about -0.2dB. The A11 can be considered a high-bandwidth audio device as the response at 100kHz is approximately 0dB. In the chart above and most of the charts 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.

Frequency response (treble/bass at minimum and maximum settings, 8-ohm loading, line-level input)

Above are two frequency response plots for the line-level input, measured at 10W (8-ohm) at the speaker outputs, with the treble/bass controls set at both minimum and maximum. They show that the A11 Tribute will provide a maximum gain/cut of approximately 13dB at 20Hz, and a maximum gain/cut of approximately 9dB at 20kHz.

Frequency response (8-ohm loading, phono input)

The chart above shows frequency response for the phono input, and shows a maximum deviation of about +4dB at 50Hz from flat within the audio band. What is shown is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision (i.e., no deviation from the RIAA reference would yield a flat line at 0dB).

Phase response (phono input)

Above is the phase response plot from 20Hz to 20kHz for the phono input. For the phono input, since the RIAA equalization curve must be implemented, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst case -60/-50 degrees at 200/5000Hz and +20 degrees at 20Hz.

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

The chart above shows RMS level (relative to 0dBrA, which is 1W into 8 ohms or 2.83Vrms) as a function of frequency, for the 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 is an actual speaker (Focal Chora 806, measurements can be found here), and the cyan is no load connected. The chart below . . .

. . . is zoomed in to highlight differences. Here we find that there’s a total deviation of about 0.1dB throughout the audioband, which is an indication of a high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used as a load is very small, deviating by just under 0.1dB within the audioband (20Hz to 20kHz), with the lowest RMS level, which would correspond to the lowest impedance point for the load, exhibited around 200Hz, and the highest RMS level, which would correspond to the highest impedance point for the load, at around 1-2kHz. 

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 (20Hz to 20kHz) for a sine-wave stimulus at the 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 the full rated power of 50W. The power was varied using the volume control. All three THD plots exhibit a rise in THD above a “knee” at roughly 1-3kHz. The graphs are a little difficult to follow because above 1kHz, there is an up-to-10dB channel deviation in THD at all power levels, where the right channel outperformed the left one. At all power levels, THD values were remarkably close to one another. The biggest deviations were between 200 and 500Hz, where the 1W and 10W data show THD values around 0.004%, while at 50W we see 0.005%. At 20Hz, all THD values are around 0.02%. At 20 kHz, between 0.03% (left) and 0.02% (right).

THD ratio (unweighted) vs. frequency at 10W (phono input)

This chart shows THD ratios as a function of frequency for the phono input measured across an 8-ohm load at 10W output. The input sweep is EQ’d with an inverted RIAA curve. The THD values vary from near 0.1% at 20 and 30Hz, down to about 0.002% (right channel) at 2.5kHz and 0.005% (left channel) at 500Hz, then up to about 0.02/0.04% (right/left channels) at 20 kHz. As with the chart above, we again see significant channel-to-channel THD deviations, with as much as a 15dB difference in favor of the right channel over the left at 3kHz.

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 A11 Tribute as a function of output power for the line level-input, for an 8-ohm load (blue/red for left/right channel) and a 4-ohm load (purple/green for left/right channel). The 4-ohm data shows consistently slightly higher THD values compared to the 8-ohm data (about a 6-7dB difference), and the right channel outperforms the left by about 2dB in both plots. At the 50mW level, THD values measured around 0.01% and stay roughly around this level for the 4-ohm data until the “knee” around 70W, then hitting the 1% THD mark at 101W. The 8-ohm data improves on this with THD levels around 0.003/0.004% from 0.5 to 10W. The “knee” in the 8-ohm data occurs at 50W, then hitting the 1% THD mark at 74W.

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 A11 Tribute as a function of output power for the line level-input, for an 8-ohm load (blue/red for left/right channel) and a 4-ohm load (purple/green for left/right channel). The 4-ohm data shows consistently slightly higher THD+N values compared to the 8-ohm data (about a 5dB difference). At the 50mW level, THD+N values measured around 0.1% (8 ohms) and 0.2% (4 ohms), dipping down to around 0.01% at 20W to 50W for the 8-ohm data, and 0.02% for the 4-ohm data from 20 to 70W.

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 A11 as a function of load (8, 4, and 2 ohms) for a constant input voltage that yields 5W at the output into 8 ohms (and roughly 10W into 4 ohms, and 20W into 2 ohms) for the 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. However, in the case of the A11, the 2-ohm trace is not available because the protection circuit was immediately engaged when trying to drive this load, which likely means that this amplifier was not design to drive a load that demanding. Thankfully, it protects itself. We find increasing levels of THD from 8 to 4 ohms, with about a 5dB difference from 20Hz to 20kHz. Overall, even with a 4-ohm load at roughly 10W, THD values ranged from 0.02% at 20Hz to just below 0.03% at 20kHz.

FFT spectrum – 1kHz (line-level input)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the line-level input. As shown in the results above, we see that the right channel outperforms the left with slightly lower peaks. We see that the signal’s second and third harmonics, at 2 and 3kHz, are at around -90/100dBrA, while the remaining harmonics are below -100dBrA (with the exception of the left channel at 4kHz sitting at -100dBrA). Below 1kHz, we see noise artifacts, with the 60Hz peak due to power-supply noise just below -110dBrA, and the 120Hz peak at -80dBrA, and the fourth harmonic at 240Hz at just below -90dBrA.

FFT spectrum – 1kHz (phono input)

Shown above is the FFT for a 1kHz input sine-wave stimulus, measured at the output across an 8-ohm load at 10W for the phono input. The second signal harmonic at 2kHz is at -85/-95dBrA (right/left channel), with subsequent harmonics below this level. Here again, the right channel clearly outperformed the left in terms of signal distortion, for example, at the fourth signal harmonic, the peak for the left channel is at -95dBrA, while the right channel is below -110dBrA. The highest peak from power supply noise is at the fundamental (60Hz), reaching just below -60dBrA, and the second noise harmonic (120Hz) is at -80dBrA.

FFT spectrum – 50Hz (line-level input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the line-level input. The X axis is zoomed in from 40 Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The most predominant peak aside from the fundamental is that of the noise signal’s second harmonic (120Hz) at about -80dBrA. The second most significant peak is from the signal second harmonic (100Hz) at around -85dBrA.

FFT spectrum – 50Hz (phono input)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output across an 8-ohm load at 10W for the phono input. The most predominant peak aside from the signal fundamental is the noise signal’s fundamental (60Hz) at just below -60dBrA. The most predominant signal harmonic peak is the second harmonic (100Hz) at -85dBrA.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output across an 8-ohm load at 10W for the 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 very low at -115/-105dBrA (right/left channel), or 0.0002%/0.0005%, while the third-order modulation products, at 17kHz and 20kHz, are at around -105dBrA, or 0.0005%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, phono input)

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the phono input. Here we find that the second-order modulation product (i.e., the difference signal of 1kHz) is just above -100dBrA, or 0.001%, for both channels. The third-order modulation products, at 17kHz and 20kHz, are at around -105dBrA.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the 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 Rotel’s slew-rate performance. Rather, it should be seen as a qualitative representation of its extended bandwidth. An ideal square-wave 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. The A11’s reproduction of the 10kHz square-wave can be considered very clean, with sharp edges and no overshoot and/or undershoot.

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

This final chart is the damping factor as a function of frequency. Both channels show a trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies (about 15% lower at 20kHz compared to 20Hz). The right channel outperformed the left with a peak value around 150, while the left channel achieved a peak damping factor of around 140.

Diego Estan
Electronics Measurement Specialist