AudioControl The Director Model M4800 Eight-Channel Network DSP Matrix Amplifier

Link: reviewed by Dennis Burger on SoundStage! Hi-Fi on September 1, 2021

General Information

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

The M4800 was conditioned for 1 hour at 1/8th full rated power (~12W into 8 ohms) before any measurements were taken. All measurements were taken with two channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The M4800 is an eight-channel multi-zone network matrix amplifier, and not the usual type of amp that we would measure at SoundStage! Network. The M4800 offers analog (RCA) or digital (coaxial or optical) inputs for each channel and is fully configurable using an ethernet connection. Speaker output connections are four-pin screw-down-type connectors that only accept bare wire. Each stereo output can be configured for mono (bridged) operation with double the power into 8 ohms. For the purposes of these measurements, the following inputs were evaluated: inputs 7 and 8 over analog (RCA, assigned as left and right respectively in our measurements below) and digital coaxial (RCA).

Based on the accuracy and repeatability at various volume levels of the left/right channel matching (see table below), the M4800 volume control is likely operating in the digital domain. When we consider this and all the EQ and crossover functions available in the M4800 web interface menu, it’s obvious that the M4800 digitizes all incoming analog signals. This was confirmed in our measurements below. The volume control offers a total range from -70dB to +29.8dB, in increments of 2dB to 0.8dB, depending on level.

All measurements were made with the volume set to 100%. At this volume position, to achieve 10W into 8 ohms, 286mVrms was required at the analog line-level input and -17.3dBFS for the digital input.

Input sensitivities can be configured for every input pair on the M4800. If 1Vrms is chosen, then the amplifier will apply about 29dB of gain to achieve 100W into 8ohms. If 2Vrms is chosen, then the amplifier will apply about 23dB of gain to achieve the same 100W into 8ohms. Since 29dB is a very common gain found in home audio amplifiers, this is how the M4800 was evaluated.

Because the M4800 is a digital amplifier technology that exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFTs. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

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 AudioControl for the M4800 compared directly against our own. The published specifications are sourced from AudioControl’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 22.4kHz, and the worst-case measured result between the left and right channel.

Our primary measurements revealed the following using the analog or digital input (unless specified, assume a 1kHz sinewave at 286mVrms or -17.3dBFS, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the M4800 was able to sustain 207W into 4 ohms using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (7.7W) 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 M4800 stayed slightly warm to the touch. During the high-power peaks, a small low-level buzz could be heard and felt from the chassis.

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

In our measured frequency-response plot above, the M4800 is nearly flat within the audioband (20Hz to 20kHz). At the extremes the M4800 is -0.75dB down at 20Hz and -1.3dB at 20kHz. The M4800 cannot be considered a high-bandwidth audio device as the -3dB point is just past 20kHz. In fact, the M4800 exhibits brick-wall-type filtering just past 20kHz. 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 vs. input type (8-ohm loading, left channel only)

The plot above shows the M4800’s frequency response (left channel only) as a function of input type. The green trace is the same analog input data from the previous graph. The blue trace is for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple trace is for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally pink is 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same across digital input types: -0.5dB at 20Hz, compared to the -0.75dB at 20Hz for the analog input. The behavior at 20kHz for all digital sample rates is identical (about -0.6dB at 20kHz). The 24/96 and 24/192 sample rates do have slightly higher extension than the 16/44.1 sample rate, with -3dB points at 22.8kHz vs 20.8kHz at 16/44.1. Overall, the digital input offers a slightly more extended frequency response at the extremes of the audioband compared to the analog input.

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

The plot above shows the results of a linearity test for the digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the speaker outputs of the M4800. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555 and normalized to the -20dBFS result. The ideal response would be a straight flat line at 0dB. Both digital input types performed similarly, approaching the ideal 0dB relative level at -110dBFS, then yielding perfect results from -100dBFs to -10dBFS. At -120dBFS, both channels at 16/44.1 overshot the ideal output signal amplitude by 1-1.5dB, while the left/right channels at 24/96 undershot by less than 1dB. Above -10dBFS, the M4800 appears to attenuate the incoming digital signal, perhaps in an effort to eliminate digital clipping. At 0dBFS, the output was measured at -6dBFS for both sample rates. For this test, we reduced the M4800 volume control slightly to ensure analog clipping was not introduced at the speaker outputs. We also repeated the measurement at different volume levels, and the same results emerged.

Impulse response (16/44.1 and 24/96 data)

The graph 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 speaker outputs (~10W 8-ohm) of the M4800. We can see that the M4800 utilizes a typical sinc function reconstruction filter.

J-Test (coaxial)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the speaker outputs (10W into 8 ohms) of the M4800. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS, undithered 12kHz square wave sampled (in this case) at 48kHz (24 bit). 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 sinewave. 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 -144dBFS 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.

We see a few spurious peaks in the audioband below -110dBFS. This is a reasonably clean J-Test result, and an indication that the M4800 DAC has good jitter immunity. When sine-wave jitter was injected at 2kHz, which would manifest as sidebands at 10kHz and 14kHz without any jitter rejection, no further peaks, or worsening of existing peaks, were observed up to 1000ns of jitter level.

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

The chart above shows a fast Fourier transform (FFT) of the M4800’s speaker outputs (1W into 8-ohms) with white noise at -20.3dBFS (blue/red), and a 19.1 kHz sinewave at -17.3dBFS 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 audioband visible above the noise floor. There is a strong 28.9kHz alias peak at -80dBrA (as opposed to a peak at 25kHz), indicating that the M4800 likely resamples incoming 16/44.1 data to 48kHz. There is also a small 48kHz peak at -120dBrA to support this hypothesis. The second, third, and fourth distortion harmonics (38.2, 57.3, 76.4kHz) of the 19.1kHz tone are between -65dBrA and -110dBrA.

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 input swept from 10Hz 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. Zoomed in, we can see a maximum deviation within the audioband of only about 0.1dB from 4 ohms to no load, 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 was a little less, deviating by about 0.09dB within the flat portion of the curve (100Hz to 5kHz), 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 2kHz.

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 6kHz) for a sine-wave stimulus at the analog 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 100W (full rated power). The 1W data exhibited the lowest THD values, with a distinct rise at low frequencies (0.003% at 60Hz and up to 0.03% at 20Hz), then relatively flat around 0.002-0.003% to 1kHz, then up to 0.008% at 6kHz. The 10W data followed a near identical trend but with almost 10dB higher THD ratios. The 100W data was at worst at 0.3% at 25Hz, and fairly constant from 200Hz to 6kHz at 0.05-0.06%.

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

The plot above shows THD ratios measured at the output of the M4800 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). At very low output power, the 8-ohm data does offer lower THD values (0.002%) compared to the 4-ohm data (0.005%). At higher output levels (above 10W), before the “knee,” the 4-ohm data slightly outperformed the 8-ohm data by about 2-5dB. For example, at 50W, the 4-ohm THD ratios are just below 0.02%, while the 8-ohm ratios are at 0.03%. The “knee” in the 8-ohm data occurs just past 100W, hitting the 1% THD mark at 108W. For the 4-ohm data, the “knee” occurs just past 200W, hitting the 1% THD at 214W.

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

The plot above shows THD+N ratios measured at the output of the M4800 as a function of output power for the analog line-level input, for an 8-ohm load (blue/red for left/right channels) and a 4-ohm load (purple/green for left/right channels). There’s a distinct 5dB jump in THD+N when the output voltage is around 1Vrms (i.e., 0.15W into 8 ohms, 0.25W into 4 ohms). This behavior was repeatable over multiple measurement trials. Overall, THD+N values before the “knee” ranged from around 0.01% to 0.05%.

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 M4800 as a function of load (8/4/2 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 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 increasing levels of THD from 8 to 4 to 2 ohms between 80Hz and 3kHz, with about a 2-3dB increase when halving of the load. At the frequency extremes, THD ratios are near identical for all loads, and at their worst at 20Hz (0.1%). Overall, even with a 2-ohm load at roughly 40W, THD values ranged from 0.1% at around 20Hz to at or near 0.01% from 60Hz to 6kHz. One thing to take note of is that distortion does rise in the low-bass region for 8-, 4-, and 2-ohm loading. 

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 analog line-level input. We see that the signal’s second harmonic, at 2kHz, is at -95/-90dBrA (left/right channels), or 0.002/0.003%, while the third harmonic is predominant at -85dBrA, or 0.006%. The remaining signal harmonics are at or below -110dBrA. Below 1kHz, we see no power-supply noise-related peaks. There is a peak at 48kHz, and stronger peaks (-75dBrA) at 47kHz and 49kHz. These peaks are a clear indication that the 1kHz analog input signal was sampled by the M4800 at 48kHz, resulting in obvious intermodulation distortion (IMD) products (i.e., 48kHz - 1kHz and 48kHz + 1kHz).

FFT spectrum – 1kHz (digital input, 16/44.1 data at -17.3dBFS)

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. We see close to the same signal harmonic profile as with the analog input, but with any lower-level peaks obscured by the higher noise floor. The same peaks at 47/48/49kHz are visible, indicating that the incoming digital input has been resampled to 48kHz.

FFT spectrum – 1kHz (digital input, 24/96 data at -17.3dBFS)

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. We see essentially the same signal harmonic profile as with the 16/44.1 FFT above, but with lower-level peaks visible due to the lower noise floor of the 24-bit data. The same peaks at 47/48/49kHz are visible, indicating that the incoming digital input has been re-sampled to 48kHz.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -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. Since a -17.3dBFS input signal yielded 10W at the output, which is defined here as 0dBrA, then a -90dBFS digital input is 72.7dBrA below reference. The second harmonic is predominant at -125/-130dBrA (left/right channels), or 0.001/0.0003%. One thing to note is that the right-channel noise floor (red) is about 10dB lower than the left-channel noise floor (blue).

FFT spectrum – 1kHz (digital input, 24/96 data at -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. We see the 1kHz primary signal peak at the correct amplitude. Since a -17.3dBFS input signal yielded 10W at the output, which is defined here as 0dBrA, then a -90dBFS digital input is 72.7dBrA below reference. The second harmonic is predominant at around -130dBrA, or 0.0003%. Below 1kHz, the left-channel noise floor (blue) is about 10-20dB lower than the right-channel noise floor (red).

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 analog 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 the signal third harmonic (150Hz) at -80dBrA, or 0.01%, while the second signal harmonic (100Hz) is seen at -90/-95dBrA (left/right channels), or 0.003/0.002%.

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 sine-wave stimulus tone measured at the output across an 8-ohm load at 10W for the analog 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 -100/-95dBrA (left/right channels), or 0.001/0.002%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at around -75dBrA, or 0.02%. We also see the main aliased peaks at 29kHz and 30kHz around -90dBrA, again indicating that the M4800 ADC is digitizing the incoming analog signal at 48kHz (i.e., 48kHz-19kHz = 29kHz).

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

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 just below -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are just below -70dBrA, or 0.03%. We also see the main aliased peaks at 29kHz and 30kHz around -90dBrA, again indicating that the M4800 is resampling to 48kHz.

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

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. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just below -90dBRA, or about 0.003%, while the third-order modulation products, at 17kHz and 20kHz, are just below -70dBrA, or 0.03%. We also see the main aliased peaks at 29kHz and 30kHz around -90dBrA, once again indicating that the M4800 is resampling all inputs (digital and analog) to 48kHz.

Square-wave response (10kHz)

Above is the 10kHz square-wave response using the analog 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 M4800’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very limited 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, as well as softening of the edges. Due to the M4800’s very limited bandwidth, only the square wave’s fundamental (10kHz) sine wave is reproduced here. In addition, we can see the 400kHz switching oscillator frequency used in the digital amplifier section clearly visible modulating the waveform.

Square-wave response (1kHz, bandwidth limited to 250kHz)

Above is the 1kHz square-wave response using the analog input, at roughly 10W into 8 ohms. The Audio Precision’s input bandwidth filter has been lowered to 250kHz to filter out the switching oscillator frequency. Due to the M4800’s very limited bandwidth, even with a 1kHz fundamental frequency, we see clear over and undershoot at the corners of the square wave.

FFT spectrum of 400kHz switching frequency relative to a 1kHz tone

The M4800’s class-D amplifier relies on a switching oscillator to convert the input signal to a pulse-width modulated (PWM) square-wave (on/off) signal before sending the signal through a low-pass filter to generate an output signal. The M4800 oscillator switches at a rate of about 400kHz, and this graph plots a wide-bandwidth FFT spectrum of the amplifier’s output at 10W into 8 ohms as it’s fed a 1kHz sine wave.

We can see that the 400kHz peak is quite evident, and at -45dBrA. There is also a peak at 800kHz and 1200kHz (the second and third harmonic of the 400kHz peak) at -70 and -115dBrA. Those three peaks—the fundamental and its second/third harmonics—are direct results of the switching oscillators in the M4800 amp modules. Also obvious are the 47/48/49kHz peaks due to the ADC sampling the incoming signal to 48kHz. The noise around those very-high-frequency signals are in the signal, but all that noise is far above the audioband—and therefore inaudible—as well as so high in frequency that any loudspeaker the amplifier is driving should filter it all out anyway.

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

The final chart above is the damping factor as a function of frequency. Both channels show a relatively constant damping factor from 20Hz to 20kHz. The damping factor is at around 200/230 (left/right channels) from 20Hz to 6kHz, then down to 180/190 (left/right channels) at 20kHz. These values represent high damping factors.

Diego Estan
Electronics Measurement Specialist