Link: reviewed by Thom Moon on SoundStage! Access on January 15, 2025
General information
All measurements taken using an Audio Precision APx555 B Series analyzer.
The Dayton Audio A400 was conditioned for 1 hour at 1/8th full rated power (~20W 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.
Although marketed as a power amplifier, the A400 was evaluated on the test bench as an integrated amplifier because it offers a prominent, non-defeatable volume control. It offers two line-level analog inputs (balanced XLR and single-ended RCA, switchable on the rear panel), left/right fixed line-level outputs (single-ended RCA and balanced XLR, both with no gain), and one set of speaker-level outputs (configurable for bridged operation).
For the purposes of these measurements, the following input was evaluated: balanced analog line-level. There were no appreciable differences in terms of THD, noise, and gain between the unbalanced and balanced analog inputs (FFTs provided in this report for comparison).
Most measurements were made with a 2Vrms line-level analog input. The signal-to-noise (SNR) measurements were made with the default input signal values but with the volume set to achieve the achievable output power of 150W into 8 ohms. For comparison, on the line-level input, an SNR measurement was also made with the volume at maximum.
Based on the accuracy and randomness of the left/right volume channel matching (see table below), the A400 volume control is a potentiometer operating in the analog domain. The A400 overall volume range is from -62dB to +38dB (balanced line-level input, speaker output).
Our typical input bandwidth filter setting of 10Hz-22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz-90kHz was used. Frequency response measurements utilize a DC to 1 MHz input bandwidth.
Volume-control accuracy (measured at speaker outputs): left-right channel tracking
Volume position | Channel deviation |
min | 0.319dB |
8 o'clock | 2.697dB |
10 o'clock | 0.136dB |
12 o'clock | 0.092dB |
2 o'clock | 0.008dB |
4 o'clock | 0.322dB |
max | 0.092dB |
Published specifications vs. our primary measurements
The table below summarizes the measurements published by Dayton Audio for the A400 compared directly against our own. The published specifications are sourced from Dayton Audio’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 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.
Parameter | Manufacturer | SoundStage! Lab |
Amplifier rated output power into 8 ohms (1% THD) | 150W | 190W |
Amplifier rated output power into 4 ohms (1% THD) | 300W | 296W |
THD+N | <0.05% | <0.018% |
Frequency response | 15Hz-20kHz (±0.5dB) | 15Hz-20kHz (-0.3/-0.1dB) |
Signal-to-noise ratio (150W, 8-ohm, A-wgt, max volume) | 103dB | 102dB |
Input sensitivity (150W, 8-ohm) | 0.43Vrms | 0.43Vrms |
Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):
Parameter | Left channel | Right channel |
Maximum output power into 8 ohms (1% THD+N, unweighted) | 190W | 190W |
Maximum output power into 4 ohms (1% THD+N, unweighted) | 296W | 296W |
Maximum output power into 8 ohms bridged (1% THD+N, unweighted) | 628W | n/a |
Maximum burst output power (IHF, 8 ohms) | 227W | 227W |
Maximum burst output power (IHF, 4 ohms) | 380W | 380W |
Continuous dynamic power test (5 minutes, both channels driven) | passed | passed |
Crosstalk, one channel driven (10kHz) | -26dB | -27dB |
Damping factor | 187 | 198 |
DC offset | <25mV | <12mV |
Gain (maximum volume, XLR in) | 38.2dB | 38.3dB |
Gain (maximum volume, RCA in) | 38.1dB | 38.2dB |
IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-71dB | <-73dB |
IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-65dB | <-64dB |
Input impedance (line input, XLR) | 23.5k ohms | 23.7k ohms |
Input impedance (line input, RCA) | 18.1k ohms | 18.4k ohms |
Input sensitivity (150W 8 ohms, maximum volume) | 434mVrms | 430mVrms |
Noise level (with signal, A-weighted) | <146uVrms | <135uVrms |
Noise level (with signal, 20Hz to 20kHz) | <346uVrms | <403uVrms |
Noise level (no signal, A-weighted, volume min) | <116uVrms | <158uVrms |
Noise level (no signal, 20Hz to 20kHz, volume min) | <278uVrms | <515uVrms |
Output impedance (pre-out, XLR) | 324 ohms | 325 ohms |
Output impedance (pre-out, RCA) | 492 ohms | 492 ohms |
Signal-to-noise ratio (150W 8 ohms, A-weighted, 2Vrms in) | 110dB | 110dB |
Signal-to-noise ratio (150W 8 ohms, 20Hz to 20kHz, 2Vrms in) | 103dB | 103dB |
Signal-to-noise ratio (150W 8 ohms, A-weighted, max volume) | 102dB | 102dB |
THD ratio (unweighted) | <0.016% | <0.017% |
THD+N ratio (A-weighted) | <0.018% | <0.019% |
THD+N ratio (unweighted) | <0.017% | <0.018% |
Minimum observed line AC voltage | 125VAC | 125VAC |
For the continuous dynamic power test, the A400 was able to sustain 310W into 4 ohms (~3% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (31W) for 5 seconds, for 5 continuous minutes without inducing a fault-protection circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top and sides of the A400 were warm to the touch.
Frequency response (8-ohm loading)
In our frequency response plots above (relative to 1kHz), measured across the speaker outputs at 10W into 8 ohms, the A400 is nearly flat within the audioband (-0.2/-0.1dB at 20Hz/20kHz). The -3dB point is just past the 100kHz mark. The A400 appears to be AC coupled, yielding roughly -2dB at 5Hz. 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 (8-ohm loading)
Above are the phase-response plots from 20Hz to 20kHz for the line level input, measured across the speaker outputs at 10W into 8 ohms. The A400 yielded just under +20 degrees of phase shift at 20Hz, and -20 degrees 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 see that the deviations between no load and 4 ohms are relatively small at roughly 0.1dB. This is a mid-tier result and an indication of a low to average output impedance, or mid-tier damping factor. With a real speaker load, deviations were smaller, at roughly 0.05dB.
THD ratio (unweighted) vs. frequency vs. output power
The chart above shows THD ratios at the speaker-level outputs 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 the left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 160W (just above the rated output of 150W). The power was varied using the A400 volume control. All data are fairly closely lumped together, an indication of a strong result for this test, except for the high THD ratios overall. THD ratios ranged from 0.015% to 0.07%. While these THD ratios are well below the threshold of audibility, they are nonetheless one to two orders of magnitude higher than many modern solid-state amplifiers.
THD ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms
The chart above shows THD ratios measured at the speaker-level outputs of the A400 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). THD ratios into 4 and 8 ohms are close (within roughly 5-8dB). For the 8-ohm load, THD ratios ranged from 0.05% at 50mW, down to 0.015% from 10W to the “knee” at roughly 170W, then up to the 1% THD mark at 190W. For the 4-ohm load, THD ratios ranged from 0.06% at 50mW, down to 0.03% from 1W to the “knee” at roughly 270W, then up to the 1% THD mark at 296W.
THD+N ratio (unweighted) vs. output power at 1kHz into 4 and 8 ohms
The chart above shows THD+N ratios measured at the speaker-level outputs of the A400 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). THD+N ratios into 4 and 8 ohms are close (within 3-6dB—note that since THD ratios are relatively high, these dominate the data in this graph). THD+N ratios range from just under 0.1% (50mW) to 0.015% at the “knee” for the 8-ohm load, and to just over 0.1% (50mW) to 0.03% to the “knee” for the 4-ohm load.
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 A400 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 50W at the output into 8 ohms (and roughly 100W into 4 ohms, and 200W 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. There is roughly a 5dB increase in THD every time the load is halved. Into 2 ohms, THD ratios ranged from 0.05% at 20Hz up to 0.15% at 20kHz.
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 A400 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). Generally, THD ratios into the real speakers were close to those measured across the resistive dummy load, which is a good result, except that THD ratios are all relatively high. The two-way speaker yielded THD ratios from 0.06% (20Hz) to 0.01% (80/1.5kHz), and up to 0.06% (20kHz). The three-way speaker yielded THD ratios from 0.015% (20Hz) to 0.03% (100Hz), and up to 0.08% (20kHz).
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 A400 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 two-way speaker (Paradigm Founder Series 100F, measurements can be found here). We find that all three IMD traces are close to one another, ranging from 0.02% to 0.05%.
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 A400 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). We find very similar IMD ratios into all three loads; 0.05% up to 500Hz, then down to 0.005% up to 1kHz.
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 analog line-level balanced input. We see significant signal harmonic peaks up to the limits of the FFT (90kHz), ranging from the highest at 2kHz (-75dBrA, or 0.02%) down to the -120dBrA, or 0.0001%, level. On the right side of the signal peak, we find significant power-supply-related noise peaks up to the limits of the FFT (90kHz), ranging from the highest at 60/120/180/240Hz (-90 to -100dBrA, or 0.003-0.001%) down to the -130 to -140dBrA, or 0.00003-0.00001%, level.
FFT spectrum – 1kHz (line-level input, unbalanced)
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 analog line-level unbalanced input. We see effectively the same result as with the balanced-input FFT above.
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. The most predominant (non-signal) peak is the second (100Hz) signal harmonic at -80dBrA, or 0.01%, and subsequent signal harmonics can be seen at the -90dBrA, or 0.003%, and below level. Power-supply-related noise peaks are prevalent through the FFT at the -90dBrA, or 0.003%, and below level.
Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, line-level input, two-channel mode)
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 at -80dBrA, or 0.01%, while the third-order modulation products, at 17kHz and 20kHz, are at roughly the -90dBrA, or 0.003%, level.
Intermodulation distortion FFT (line-level input, APx 32 tone)
Shown above is the FFT of the speaker-level output of the A400 with the APx 32-tone signal applied to the analog input. The combined amplitude of the 32 tones is the 0dBrA reference, which corresponds to 10W into 8 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and are at and below the -100dBrA, or 0.001%, level. The other larger peaks are from power-supply-related noise.
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 A400’s slew-rate performance. Rather, it should be seen as a qualitative representation of the A400’s high 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. In this case, we find clean corners with some mild softening and no over/undershoot.
Damping factor vs. frequency (20Hz to 20kHz, two-channel mode)
The final graph above is the damping factor as a function of frequency. Both channels track very closely. We can see damping factors ranging from about 190 from 20Hz to 2kHz, then down to just above 100 at 20kHz. This is a mid-tier result for an integrated solid-state amplifier.
Diego Estan
Electronics Measurement Specialist