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Link: reviewed by Dennis Burger on SoundStage! Access on May 1, 2022

General Information

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

The A12MKII was conditioned for 1 hour at 1/8th full rated power (~8W 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 A12MKII offers three unbalanced line-level analog inputs (RCA), one unbalanced moving-magnet (MM) phono input (RCA), one S/PDIF coaxial input (RCA), one S/PDIF optical input (TosLink), one USB digital input, a Bluetooth receiver, one pair of line-level pre-outs (RCA), and two stets (A and B) of speaker level outputs. On the front panel is a 1/8” TRS headphone output. For the purposes of these measurements, the following inputs were evaluated: coaxial digital, plus the analog line-level and MM unbalanced inputs.

Most measurements were made with a standard 2Vrms line-level analog input, 5mVrms MM input, and 0dBFS digital input. For the analog inputs, the tone-control bypass function was enabled, except for the chart showing the effects of the tone controls. The signal-to-noise ratio (SNR) measurements were made with the same input signal values but with the volume set to achieve the rated output power of 60W (8 ohms). For comparison, on the line-level input, an SNR measurement was also made with the volume at maximum, where only 0.94Vrms was required to achieve 60W into 8 ohms.

Based on the high accuracy and non-repeatable results at various volume levels of the left/right channel matching (see table below), the A12MKII volume control is likely operating in the analog domain, but is digital controlled. The volume control offers a total range of 0 to 96 on the display, which measured from -53.3dB (position 1) to +27.4dB between the line-level analog input and the speaker outputs, in increments of 6 to 2dB below 6, 1dB from 6 to 80, then 0.5dB steps from 80 to 96. One oddity that was observed was that between volume steps 7 to 40, every second volume increment did not change the output voltage.

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

Volume position Channel deviation
1 0.23dB
10 0.071dB
20 0.038dB
40 0.078dB
60 0.003dB
80 0.029dB
96 0.044dB

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Rotel for the A12MKII 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 was extended from DC to 500kHz, assume, unless otherwise stated, 10W into 8 ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

Parameter Manufacturer SoundStage! Lab
Rated output power into 8 ohms (1% THD, 1kHz) 60W 91W
Rated output power into 4 ohms (1% THD, 1kHz) 120W 142W
THD (1kHz, 10W, 8 ohms) <0.018% <0.008%
SNR (A-weighted, IHF 8 ohms, line-level input) 100dB 101.4dB
SNR (A-weighted, IHF 8 ohms, digital input 24/96) 103dB 100.7dB
SNR (A-weighted, IHF 8 ohms, phono input) 90dB 79.7dB
Damping factor (ref. 8 ohms 1kHz) 220 212
Frequency response (line-level input) 10Hz-100kHz, 0±0.5dB 10Hz-100kHz, -0.8,+0.1dB
Frequency response (digital input, 24/192) 10Hz-90kHz, 0±2dB 10Hz-90kHz, -2,-1.5dB
Frequency response (phono input) 20Hz-20kHz, 0±0.5dB 20Hz-20kHz, ±0.5dB
Intermodulation distortion (60Hz:7kHz, 4:1, 10W into 8ohms) <0.03% <0.03%
Input sensitivity (line-level) 230mVrms 940mVrms
Input sensitivity (digital) 0dBFS -8.7dBFS
Input sensitivity (phono) 3.4mVrms 3.03mVrms
Input impedance (line-level) 24k ohms 25k ohms
Input impedance (phono) 47k ohms 53.5k ohms
Input overload (line-level) 4Vrms 4.08Vrms
Input overload (phono, 1kHz) 50mVrms 63.7mVrms
Tone controls ±10dB at 100Hz/10kHz ±8dB at 100Hz/10kHz

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sine wave at 2Vrms or 0dBFS, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Parameter Left channel Right channel
Maximum output power into 8 ohms (1% THD+N, unweighted) 91W 91W
Maximum output power into 4 ohms (1% THD+N, unweighted) 142W 142W
Maximum burst output power (IHF, 8 ohms) 98.6W 98.6W
Maximum burst output power (IHF, 4 ohms) 169.1W 169.1W
Continuous dynamic power test (5 minutes, both channels driven) failed failed
Crosstalk, one channel driven (10kHz) -62.5dB -64.6dB
Damping factor 216 212
Clipping no-load output voltage 31.7Vrms 31.7Vrms
DC offset <4.3mV <3.8mV
Gain (pre-out) 0.81dB 0.76dB
Gain (maximum volume) 27.4dB 27.4dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-81dB <-85dB
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) <-70dB <-78dB
Input impedance (line input, RCA) 24.8k ohms 25.0k ohms
Input sensitivity (for rated power, maximum volume) 940mVrms 940mVrms
Noise level (A-weighted) <230uVmrs <250uVmrs
Noise level (unweighted) <640uVmrs <660uVmrs
Output impedance (pre-out) 452.8 ohms 453.6 ohms
Signal-to-noise ratio (full rated power, A-weighted, 2Vrms in) 99.2dB 98.9dB
Signal-to-noise ratio (full rated power, unweighted, 2Vrms in) 90.7dB 90.5dB
Signal-to-noise ratio (full rated power, A-weighted, max volume) 99.4dB 99.2dB
Dynamic Range (full rated power, A-weighted, digital 24/96) 99.0dB 99.0dB
Dynamic Range (full rated power, A-weighted, digital 16/44.1) 93.9dB 93.9dB
THD ratio (unweighted) <0.0080% <0.0035%
THD ratio (unweighted, digital 24/96) <0.0051% <0.012%
THD ratio (unweighted, digital 16/44.1) <0.0049% <0.011%
THD+N ratio (A-weighted) <0.0095% <0.0047%
THD+N ratio (A-weighted, digital 24/96) <0.0065% <0.014%
THD+N ratio (A-weighted, digital 16/44.1) <0.0081% <0.014%
THD+N ratio (unweighted) <0.011% <0.0082%
Minimum observed line AC voltage 125VAC  125VAC

For the continuous dynamic power test, the A12MKII was able to sustain 133W into 4 ohms (~1% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (13.3W) for 5 seconds, for about 4 minutes (out of a 5- minute test) before the protection circuit shut down the unit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the A12MKII was quite warm to the touch. It should be noted that this test was conducted after a few hours of testing with an average of 10W at the output.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sine wave input, 10W output, 8-ohm loading, 10Hz to 90kHz bandwidth):

Parameter Left channel Right channel
Crosstalk, one channel driven (10kHz) -61.3dB -66.5dB
DC offset <1.1mV <-0.3mV
Gain (default phono preamplifier) 49.71dB 49.78dB
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) <-85dB <-76dB
IMD ratio (CCIF, 3kHz + 4kHz stimulus tones, 1:1) <-87dB <-76dB
Input impedance 53.2k ohms 53.5k ohms
Input sensitivity (to max power with max volume) 3.04mVrms 3.03mVrms
Noise level (A-weighted) <0.9mVrms <0.85mVrms
Noise level (unweighted) <8mVrms <7.8mVrms
Overload margin (relative 5mVrms input, 1kHz) 22.1dB 22.1dB
Signal-to-noise ratio (full rated power, A-weighted) 79.7dB 79.6dB
Signal-to-noise ratio (full rated power, unweighted) 61.1dB 61.3dB
THD (unweighted) <0.0015% <0.01%
THD+N (A-weighted) <0.01% <0.015%
THD+N (unweighted) <0.09% <0.09%

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sine wave, 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Parameter Left channel Right channel
Maximum output power into 600 ohms (1% THD+N, unweighted) 17.8mW 17.8mW
Maximum output power into 300 ohms (1% THD+N, unweighted) 48.8mW 48.6mW
Maximum output power into 32 ohms (1% THD+N, unweighted) 203mW 203mW
Gain 27.3dB 27.2dB
Output impedance 320 ohms 320 ohms
Noise level (A-weighted) <94uVmrs <96uVmrs
Noise level (unweighted) <143uVmrs <146uVmrs
Signal-to-noise (A-weighted, ref. max output voltage) 92.6dB 92.4dB
Signal-to-noise (unweighted, ref. max output voltage) 89.6dB 89.4dB
THD ratio (unweighted) <0.29% <0.29%
THD+N ratio (A-weighted) <0.33% <0.33%
THD+N ratio (unweighted) <0.29% <0.29%

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

frequency response

In our measured frequency response chart above, the A12MKII is nearly flat within the audio band (20Hz to 20kHz). At the extremes the A12MKII is about 0.25dB down at 20Hz, and 0dB at 20kHz. The A12MKII appears to be AC coupled (i.e., not flat down to DC), contradicting Rotel’s frequency response claim of 10Hz-100kHz, 0±0.5dB. At the high-frequency extremes, however, Rotel’s claim is verified as we are within 0.5dB of flat, even at 200kHz. 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.

Frequency response (8-ohm loading, line-level input, bass and treble controls)

frequency response bass treble

Above are frequency response plots measured at the speaker-level outputs into 8 ohms, with the bass and treble controls set to maximum (blue/red plots) and minimum (purple/green plots). We see that for the bass and treble controls, roughly +/- 12dB and +/- 9dB, respectively, of gain/cut are available at 20Hz and 20kHz.

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

phase response

Above are the phase response plots for the left and right channels from 20Hz to 20kHz for the line-level input, measured across the speaker outputs at 10W into 8 ohms. The A12MKII does not invert polarity and exhibits, at worst, less than 20 degrees (at 20Hz) of phase shift within the audio band.

Frequency response vs. input type (8-ohm loading, left channel only)

frequency response vs input type

The chart above shows the A12MkII’s frequency response as a function of input type measured across the speaker outputs at 10W into 8 ohms. The green trace is the same analog input data from the previous graph. The blue trace is for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial digital input, and the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, while the pink trace is for a 24/192 dithered digital input signal from 5Hz to 96kHz. The analog input shows slightly flatter response at low frequencies, with the digital input -2dB at 10Hz and -0.5dB at 20Hz. The 16/44.1 data exhibits brick-wall-type filtering, with a -3dB point at 21.0kHz. The 24/96 and 24/192 kHz data yielded -3dB points at 46.2kHz and 91.9kHz, respectively, reflecting the higher sampling frequencies and, therefore, increased bandwidth.

Frequency response (8-ohm loading, MM phono input)

frequency response phono mm

The chart above shows the frequency response for the MM phono input. What is being displayed is the deviation from the RIAA curve, where the input signal sweep is EQ’d with an inverted RIAA curve supplied by Audio Precision analyzer (i.e., zero deviation would yield a flat line at 0dB). We see a maximum deviation of about +0.5 at 40Hz and 20kHz, from 20Hz to 20kHz. The worst-case channel deviation is between about 10kHz to 20kHz, at about 0.2dB. 

Phase response (MM input)

phase response phono mm

Above is the phase-response plot from 20Hz to 20kHz for the MM phono input, measured across the speaker outputs at 10W into 8 ohms. The A12MKII does not invert polarity. 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 of about -60 degrees at 200Hz and 5kHz, and +20 degrees at 20Hz.

Digital linearity (16/44.1 and 24/96 data)

digital linearity

The chart above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the line-level pre-outs of the A12MKII for a 1Vrms (at 0dBFS) output signal. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response would be a straight flat line at 0dB. Both data were essentially perfect as of -100dBFS down to 0dBFS. At -120dBFS, the 16/44.1 data were about +3.5dB above reference, while the 24/96 data were within +1.5/2.5dB ((left/right) of reference. This is an acceptable linearity test result.

Impulse response (24/44.1 data)

impulse response 2444 1

The chart above shows the impulse response for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, measured at the line-level pre-outs of the A12MKII. We can see that the A12MKII utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

jtest coax 2448

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outs of the A12MKII. J-Test was developed by Julian Dunn in the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz square wave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g,. 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA and below.  The test file can also be used in conjunction with artificially injected sine-wave jitter by the Audio Precision, to show how well the DAC rejects jitter.

The coaxial input exhibits significant peaks in the audio band, peaking near the 12kHz primary signal, at -95dBrA and below. This is a poor J-Test result, indicating that the A12MKII DAC likely has poor jitter immunity.

J-Test (optical)

jtest optical 2448

The chart above shows the results of the J-Test test for the optical digital input measured at the line- level pre-outs of the A12MKII. The optical input exhibits significant peaks in the audio band, peaking near the 12kHz primary signal, at -85dBrA and below. This, as with the coaxial-input test above, is a poor J-Test result, indicating that the A12MKII DAC likely has poor jitter immunity.

J-Test with 100ns of injected jitter (coaxial)

jtest coax 2448 100ns

Both the coaxial and optical inputs were also tested for jitter immunity by injecting artificial sine-wave jitter at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection. Jitter immunity proved poor, with visible sidebands at only 10ns of jitter level. Clear sidebands can be seen at nearly -70dBrA. The optical input jitter result was very similar to the coaxial input result shown above. This, again, is a poor J-Test result.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

wideband fft noise plus 19 1khz 1644 1kHz

The plot above shows a fast Fourier transform (FFT) of the A12MKII’s line-level pre-outs with white noise at -4dBFS (blue/red), and a 19.1 kHz sine wave at 0dBFS (1Vrms) fed to the coaxial digital input, sampled at 16/44.1. The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of a brick-wall-type reconstruction filter. There are no aliased image peaks in the audio band above the -120dBrA noise floor. The main 25kHz alias peak is at -60dBrA. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -75dBrA and -105dBrA.

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

rms level vs frequency vs load impedance

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 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 . . .

rms level vs frequency vs load impedance

. . . is the same, but zoomed in to highlight differences. Here we can see a maximum deviation within the audio band of about 0.08dB from 4 ohms to no load, which is an indication of a relatively high damping factor, or low output impedance. The maximum variation in RMS level when a real speaker was used is even less, deviating by about 0.06dB within the flat portion of the curve (100Hz to 20kHz). Note that the dip in RMS level at lower frequencies is a result of the frequency response of the A12MKII, and not a damping-factor issue, as all four plots show the same dip, at roughly the same rate.

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

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 sine-wave stimulus at the analog line-level input. The blue and red plots are for the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at the rated 60W. The power was varied using the volume control. The A12MKII manages to maintain consistent THD ratios across a wide range of power output levels, and easily makes the Rotel spec of <0.018% THD from 20Hz to about 10kHz, from 1W to 60W. The disparity in the plots is actually a channel disparity, with the right channel outperforming the left channel by as much as almost 10dB, with the right channel dipping as low as 0.003% from 50Hz to 2kHz across all power levels. We wanted to investigate whether the disparity was in the amp or preamp section, so . . . 

thd ratio unweighted vs frequency 1Vin 10W

. . . we plotted THD ratios with a 1Vrms input (instead of 2Vrms), increasing the volume by 6dB to achieve the same 10W output into 8 ohms. Above is a chart that shows these THD ratios as a function of frequency for a 1Vrms sine-wave stimulus at the analog line-level input. Here we find better tracking between the left and right channels, and interestingly, below 2kHz, the left channel outperforming the right channel by about 5dB—the opposite result compared to a 2Vrms input. We can conclude from this that at least part of the reason for the THD channel disparity with a 2Vrms input is due to distortion in the preamp section.

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

thd ratio unweighted vs frequency phono mm

The chart above shows THD ratios as a function of frequency plots for the MM phono input measured across an 8-ohm load at 10W. The input sweep is EQ’d with an inverted RIAA curve. The THD values for the MM configuration vary from around 0.1% (20/30Hz) down to 0.001% (1kHz) for the left channel, then up to 0.03% at 20kHz. The right channel showed more constant THD ratios of roughly 0.01% from 50Hz to 10kHz.

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

thd ratio unweighted vs output power at 4 8 ohms

The chart above shows THD ratios measured at the output of the A12MKII as a function of output power for the analog line-level input, for an 8-ohm load (purple/green for left/right channels) and a 4-ohm load (blue/red for left/right channels). Both 8- and 4-ohm data sets, for the left channel, track fairly closely, with THD ratios from about 0.01% down to 0.002% at 2-3W, then back up to 0.01% at the “knees”—roughly 70W for the 8-ohm load, and about 120W for the 4-ohm load. The right channel performed worse than the left channel by as much as 10-12dB, with the exception of a crossover point just over 20W into 8 ohms, where the right channel begins to outperform the left channel with a nearly 10dB advantage at the knee. As discussed above, this may be due to distortions in the preamp section, as at the knee, we are approaching 2Vrms at the input. The 1% THD values are reached at about 91W (8 ohms) and 142W (4 ohms).

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

thd n ratio unweighted vs output power at 4 8 ohms

The chart above shows THD+N ratios measured at the output of the A12MKII as a function of output power for the line-level input, for an 8-ohm load (purple/green for left/right channels) and a 4-ohm load (blue/red for left/right channels). Overall, THD+N values for both loads were similar but slightly lower for the 8-ohm load, ranging from about 0.1%, down to 0.005%. The exception was the right channel into 4 ohms, which performed worse by almost 10dB from 10 to 50W.

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

thd vs frequency load

The chart above shows THD ratios measured at the output of the A12MKII as a function of frequency into three different loads (8/4/2 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 analog line-level input. The right channel was chosen because with these conditions (2Vrms in, 5W into 8 ohms), the right channel clearly outperformed the left channel. 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 increasing THD values from 8 to 4 to 2 ohms. Into 8 ohms, THD ratios are as low as 0.003% from 50Hz to 3kHz. The 4-ohm THD ratios are more than 10dB higher through most of the frequency range, while the 2-ohm data is about 6dB higher than the 4-ohm data. Basically, THD increases as impedance descreases.

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

thd vs frequency vs speakers

The chart above shows THD ratios measured at the output of the A12MKII 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 high frequencies (5-20kHz), all three plots show similar THD ratios from 0.005% to about 0.015%. Through the upper bass and midband, however, THD ratios were higher with real speakers, hovering around 0.01%, more than 10dB higher than the 0.003% measured with a dummy load. At very low frequencies, THD ratios were the highest with the two-way speaker, measuring 0.05%, a full 20dB higher than the 0.005% measured in the dummy load.

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

IMD CCIF vs frequency vs speakers

The chart above shows intermodulation distortion (IMD) ratios measured at the output of the A12MKII 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 the same two-way speaker as above (Focal Chora 806, measurements can be found here), and the pink plot is the same three-way speaker as above (Paradigm Founder Series 100F, measurements can be found here). In the lower frequencies (4kHz and below), all three results are similar, with relatively constant IMD ratios from as low as 0.002% to 0.008% at 2.5kHz. At higher frequencies, IMD ratios were highest with a real speaker, as high as 0.01% at 20kHz for the three-way speaker, which is 5dB higher than with the dummy load, and 15dB higher than with two-way speaker, which yielded, in general, the lowest IMD values.

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

IMD SMPTE vs frequency vs speakers

The chart above shows IMD ratios measured at the output of the A12MKII 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 the same two-way speaker (Focal Chora 806, measurements can be found here), and the pink plot is the same three-way speaker (Paradigm Founder Series 100F, measurements can be found here). All three results are nearly identical, with flat IMD results just around 0.02%.

FFT spectrum – 1kHz (line-level input)

FFT spectrum 1khz

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 analog line-level input. We find the left channel dominating at the second harmonic (2kHz) at -80dBrA, or 0.01%, compared to the right channel at below -90dBrA, or 0.003%. At the fourth harmonic (4kHz), the right channel dominates at about -95dBrA, or 0.002%, compared to the left channel at -110dBrA, or 0.0003%. At the third (3kHz) and fifth harmonics (5kHz), both channels yielded peaks at roughly -105dBrA, or 0.0006%. On the left side of the main signal peak, we find a small peak at 60Hz due to power-supply noise at about -115dBrA, or 0.0002%, for the left channel, and more dominant higher-order peaks at 120Hz, and especially 240Hz (fourth harmonic) at -105dBrA, or 0.0006%. Power-supply and signal-related harmonic peaks can be seen right out to 100kHz.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

fft spectrum 1khz 1644 1 0dbfs

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 coaxial digital input, sampled at 16/44.1. Here we find the right channel dominating at the second harmonic (2kHz) at -80dBrA, or 0.01%, compared to the left channel at below -100dBrA, or 0.001%. At the third (3kHz) harmonic, both channels yielded peaks at roughly -85dBrA, or 0.006%. The noise-related peaks on the left side of the signal peak are very similar to the line-level FFT above.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

fft spectrum 1khz 2496 0dbfs

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 coaxial digital input, sampled at 24/96. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

fft spectrum 1khz 2444 1 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 16/44.1 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. We see the 1kHz primary signal peak, at the correct amplitude, and signal harmonics are non-existent above the noise floor. The fourth (240Hz) and sixth (360Hz) power-supply related harmonics are visible at roughly -105dBrA, or 0.0006%.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

fft spectrum 1khz 2496 90dbfs

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sine-wave stimulus at the coaxial digital input, measured at the output across an 8-ohm load. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

FFT spectrum – 1kHz (MM phono input)

fft spectrum 1khz phono mm

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 MM phono input. We see the right channel dominating the even-order harmonics (2, 4, 6kHz), as high as -80dBrA, or 0.01%, at 2kHz. We see the primary (60Hz) power-supply-related peak at just under -60dBrA, or 0.1%, and subsequent power-supply-related peaks (120, 180, 240Hz, etc.) extending beyond the 1kHz signal peak, at -80dBrA (at 180Hz), or 0.01%, and below.

FFT spectrum – 50Hz (line-level input)

fft spectrum 50hz

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 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. The second harmonic (100Hz) dominates at -85/-95dBrA (left/right), or 0.006/0.002%, but even and odd signal harmonics at lower levels can be seen throughout. We also see the 60Hz power-supply-related peak at -120dBrA, or 0.0001%, with higher-order peaks at 180Hz and 240Hz, between -100 and -110dBrA, or 0.001 and 0.0003%.

FFT spectrum – 50Hz (MM phono input)

fft spectrum 50hz phono mm

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 MM phono 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. The most predominant (non-signal) peak is from the 60Hz power-supply fundamental at -60dBrA, or 0.1%. The second-order signal peak at 100Hz is dominated by the right channel at -80dBrA, or 0.01%, while the third-order noise peak (180Hz) is at the same level, but for both channels.

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

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave 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 at -90/-95dBrA (left/right), or 0.003/0.002%. The third-order modulation products, at 17kHz and 20kHz, are around -100dBrA, or 0.001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 16/44.1)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 16/44.1. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at 100/-85dBrA (left/right), or 0.001/0.006%.  The third-order modulation products, at 17kHz and 20kHz, are around -85dBrA, or 0.006%. We also see the main aliased peaks at 25.1kHz and 26.1kHz around -70dBrA, due to the 44.1kHz sample rate (e.g., 44.1kHz-19kHz = 25.1kHz).

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, coaxial digital input, 24/96)

intermodulation distortion fft 18khz 19khz summed stimulus

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the digital coaxial input at 24/96. The FFT spectrum is nearly identical within the audio band to the 16/44.1 FFT above.

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

intermodulation distortion FFT 18kHz 19kHz summed stimulus phono mm

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at around -100/-80dBRa (left/right), or 0.001/0.01%, while the third-order modulation products, at 17kHz and 20kHz, are around -105dBrA, or 0.0006%.

Square-wave response (10kHz)

square wave response 10kHz

Above is the 10kHz square-wave 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 A12MKII’s slew-rate performance. Rather, it should be seen as a qualitative representation of the A12MKII’s extended bandwidth. An ideal square wave can be represented as the sum of a sine wave 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 an exceptionally clean square-wave reproduction, with sharp corners and little-to-no ringing, indicating a high bandwidth.

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

damping factor vs frequency

The final graph above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 20kHz, right around 215. This is very close to Rotel’s claim of 220.

Diego Estan
Electronics Measurement Specialist

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