Music Hall A15.3 Integrated Amplifier Measurements

Link: reviewed by James Hale on SoundStage! Xperience on February 1, 2021

General information

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

The Music hall a15.3 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 a15.3 offers five sets of line-level unbalanced (RCA) inputs, one moving-magnet (MM) phono input, a set of variable line-level RCA pre-outputs, and one pair of speaker outputs. Also available is a headphone output via a 3.5mm TRS jack on the front panel. Based on the high accuracy of the left/right channel matching (see table below), the a15.3’s volume knob is not a potentiometer in the signal path, which is typically less accurate, but rather provides digital control over a proprietary or integrated analog-domain volume circuit. The a15.3’s volume control offers between 1 and 2dB step increments throughout most of the volume range. At the lowest end of the range, the first 3 steps offer 4dB increments, and the next 3 steps 3dB.

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 (about 3 o’clock) as measured at the pre-outputs. SNR measurements were made with the volume control set to maximum. At the unity-gain volume position, to achieve 10W into 8 ohms, 275mVrms was required at the line-level input and 2.65mVrms at the phono input.

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

Published specifications vs. our primary measurements

The tables below summarize our primary measurements performed on the a15.3. Here we can compare our results against Music Hall’s own published specifications for the a15.3, which are stated as follows:

• Rated output power: 50W into 8 ohms, 75W into 4 ohms
• Input sensitivity: 330mV (RCA), 1mV (phono)
• SNR: >90dB
• THD+N: <0.02% (20Hz-20kHz)
• Frequency response: RCA: -0.5dB (20Hz-20kHz), PHONO: RIAA compliant

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 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):

Music Hall’s power output claims of 50 and 75WPc into 8- and 4-ohm loads were corroborated with our maximum (1% THD+N) measurements of 63Wpc and 86Wpc into 8 and 4 ohms, during which time our line AC voltage never dipped below 121VAC.

Our clipping headroom result was 1dB for the a15.3, defined as the ratio of max power over rated power into 8 ohms. The Music Hall a15.3 was also able to sustain 86Wpc (1dB over rated output) into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (8.6Wpc) 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 a15.3 was very warm to the touch, causing some discomfort on the skin after 10 seconds of direct contact.

Music Hall’s input sensitivity claims (volume at maximum) were verified, where we measured 308mVrms and 3mVrms respectively for the single-end RCA and phono inputs, which are close to the 330mVrms and 1mVrms specs.

Music Hall’s 90dB SNR claim was corroborated by our own measurements, where we measured 98.5dB (A-weighted) and 96.2dB (20Hz-20kHz) for the left and right channels.

Music Hall’s THD+N claim of <0.02% (20Hz to 20kHz) was corroborated at 1KHz in our primary table; however, as our THD versus frequency graph below shows, THD ratios were above 0.02% from 6kHz to 20kHz at 10W into 8ohms.

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

Music Hall does not supply any headphone-output specifications, so there there was nothing for us to compare to.

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

In our measured frequency response plot above, the a15.3 is nearly flat within the audio band (20Hz to 20kHz) for the line-level input. These data corroborate Music Hall’s claim of 20Hz to 20kHz (-0.5dB) as the worst-case deviation is at 20kHz, where the response is about -0.2dB. At 20Hz, the response is at 0dB, and -0.5dB at 5Hz. The a15.3 can be considered a high-bandwidth audio device since the response at 100kHz is approximately +0.1dB. 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, phono input)

The plot above shows frequency response for the phono input, and shows a maximum deviation of -1dB (20Hz, right channel) from flat within the audioband. 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., zero deviation would yield a flat, horizontal line at 0dB). Between 100Hz and 500Hz, there is a 0.2dB deviation between channels, as well as at 30Hz and below, indicating small channel-to-channel differences in the RIAA curve implementaion.

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

The plots above shows RMS level (relative to 0dBrA, which is 1W into 8ohms 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 ohms load, the purple is into a 4 ohms load, the pink is an actual speaker (Focal Chora 806, measurements can found here), and the cyan line is no load connected. 

This chart is the same test as above, but the chart has been zoomed in to highlight differences. Here we find that there’s a total deviation of about 0.1 dB 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 (in this case, a Focal Chora 806, with measurements available through this link) as a load is very small, deviating by a little over 0.05dB within the flat portion of the curve (20Hz to 1kHz), 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 1kHz.

Phase response (line-level input)

Above is the phase response plot from 20Hz to 20kHz for the line-level input. The a15.3 does not invert polarity on the line-level input, and the plot shows very little phase shift, with a worst case of just under +30 degrees at 20kHz.

Phase response (phono input)

Above is the phase response plot from 20Hz to 20kHz for the phono input from 20Hz to 20kHz. 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 degrees at 200Hz and 5kHz.

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

The plot above shows THD ratios at the output into 8 ohms as a function of frequency (20Hz to 20kHz) for a sinewave stimulus at the line-level input. The blue and red plots are for left and right 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 1.5kHz. At 1W, the THD values in the flat portion (20Hz to 1kHz) are hovering between 0.005-0.006%, then rise up to about 0.07% at 20kHz. At 10W, the flat portion shows THD ratios around 0.003%, then up to 0.06% at 20Hz. The 50W data shows the flat portion around 0.004%, rising up to just above 0.1% at 20kHz. At 50W, the right channel outperforms the left throughout the sweep by about 2dB.

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

The chart above shows the THD ratio as a function of frequency plot 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 about 0.2% at 20Hz and 30Hz, down to about 0.003/0.005% (left/right channels) at 1kHz, then up to about 0.07/0.06% (left/right channels) at 20kHz.

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

The above chart shows THD ratios measured at the output of the a15.3 as a function of output power for the line level-input, for an 8-ohm load (blue/red lines for left/right chanels) and a 4-ohm load (purple/green for left/right). The 4-ohm data shows consistently slightly higher THD values compared to the 8-ohm data (about a 5dB difference), and the right channel outperforms the left in both plots by about 2dB. At the 50mW level, THD values measured around 0.01% (8 ohms) and 0.02% (4 ohms), dipping down to around 0.004/0.003% at 10W to 30W for the 8-ohm data, and 0.005% for the 4-ohm data from 10W to 50W. The “knee” in the 8-ohm data occurs near 50W, hitting the 1% THD mark at 63W. For the 4-ohm data, the “knee” occurs near 70W, hitting the 1% THD mark at 86W.

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

This chart shows THD+N ratios measured at the output of the a15.3 as a function of output power for the line level-input, for an 8-ohm load (blue/red lines for left/right channels) and a 4-ohm load (purple/green for left/right). 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 0.15% (4 ohms), dipping down to around 0.005% at 20W to 40W for the 8-ohm data, and 0.006% for the 4-ohm data from 30 to 50W.

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 a15.3 as a function of load (8, 4, and 2 ohms) for a constant input voltage that yields 10W at the output into 8 ohms (and roughly 20W into 4 ohms and 40W 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. We find increasing levels of THD from 8 to 4 to 2 ohms, with about a 5dB difference from 20Hz to 1kHz, with a smaller degradation in THD performance (1-2dB) between 4 and 2 ohms from 1kHz to 20kHz. Overall, even with a 2-ohm load at roughly 40W, THD values ranged from 0.005% at 20Hz to just below 0.2% at 20 kHz.

FFT spectrum – 1kHz (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 line-level input. As seen in the plots above, we see that the right channel (red) outperforms the left channel (blue) with slightly lower peaks. We see that the signal’s second and third harmonic, at 2kHz and 3kHz, are both at around -95dBrA, while the remaining harmonics are below -100dBRA. Below 1kHz, we see noise artifacts, with the 60Hz peak due to power-supply noise just above -110dBrA, and the 120Hz peak at -105/110dBrA (left/right channels).

FFT spectrum – 1kHz (phono input)

Shown above is the FFT for a 1kHz input sinewave stimulus, measured at the output across an 8-ohm load at 10W for the phono input. The second signal harmonic at 2kHz is at -90/-100dBRA (left/right channels), with subsequent harmonics below this level. The highest peak from power supply noise is at the fundamental (60Hz), reaching almost -55dBrA, and the third noise harmonic (180Hz) is near -75dBrA.

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 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 most predominant peak is that of the signal’s third harmonic (150Hz) at about -95dBrA. The signal second harmonic (100Hz) is at around -100dBRA, while the noise second harmonic (120Hz) is at around -105/-110dBrA (left/right channels).

FFT spectrum – 50Hz (phono 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 phono input . The most predominant peak is the noise signal’s fundamental (60Hz) at near -55dBRA. The most predominant signal harmonic peak is the third harmonic (150Hz) at -90dBrA.

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 near -105/-110dBRA (left/right channels), while the third-order modulation products, at 17kHz and 20kHz, are higher, at around -100dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, phono 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 phono input. Here we find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95/-100dBrA (left/right channels), and smaller in magnitude than some of the harmonic peaks from the noise signal surrounding it. The third-order modulation products, at 17kHz and 20kHz, are at around -100dBrA.

Square-wave response (10kHz)

Above is the 10kHz squarewave 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 Music Hall’s slew-rate performance. Rather, it should be seen as a qualitative representation of it’s extended 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. The a15.3’s reproduction of the 10kHz squarewave can be considered clean, with sharp edges and very mild overshoot.

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

The final plot above is the damping factor as a function of frequency. Both channels show a general trend of a higher damping factor at lower frequencies, and lower damping factor at higher frequencies (about 20% lower at 20kHz compared to 20Hz). The right channel outperformed the left with a peak value around 190 from 20Hz to 400Hz, while the left channel achieved a damping factor of around 160 within the same frequency range.

Diego Estan
Electronics Measurement Specialist