Link: reviewed by Dennis Burger on *SoundStage! Access* on July 1, 2024

**General Information**

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

The Arcam Radia A25 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 both channels driven, using a 120V/20A dedicated circuit, unless otherwise stated.

The A25 offers three set of RCA line-level analog inputs, one RCA phono input, two digital RCA coaxial inputs, one digital TosLink optical input, one USB digital input, left/right RCA pre-outs, one set of speaker-level outputs, and one headphone output over a 1/8″ TRS connector. A Bluetooth input is also offered. For the purposes of these measurements, the following inputs were evaluated: digital coaxial, analog line-level, and phono, as well as the headphone output.

Most measurements were made with a 2Vrms line-level analog input and 0dBFS digital input. The signal-to-noise (SNR) measurements were made with the default input signal values but with the volume set to the rated output power of 100W into 8 ohms. For comparison, on the line-level input, a SNR measurement was also made with the volume at maximum. The A25 offers three digital filters for the digital inputs: Linear Phase Fast, Hybrid Fast, Minimum Phase Slow. Unless otherwise noted, the Linear Phase Fast filter was used.

Based on the accuracy and randomness of the left/right volume channel matching (see table below), the A25 volume control is digitally controlled but operating in the analog domain. The A25 overall volume range is from -77dB to +44.dB (line-level input, speaker output). It offers 4dB increments from position 0 to 10, 2dB increments from positions 10 to 20, 1dB from 20 to 50, and 0.5dB from 50 to 99.

Our typical input bandwidth filter setting of 10Hz to 22.4kHz was used for all measurements except FFTs and THD versus frequency, where a bandwidth of 10Hz to 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 |

1 | 0.112dB |

10 | 0.048dB |

30 | 0.035dB |

50 | 0.011dB |

70 | 0.008dB |

80 | 0.008dB |

99 | 0.036dB |

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Arcam for the A25 compared directly against our own. The published specifications are sourced from Arcam’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 (0.5% THD) | 100W | 108W |

Amplifier rated output power into 4 ohms (0.5% THD) | 165W | 159W |

THD (80W, 8-ohm, 1kHz) | 0.002% | 0.0007% |

Frequency response (20Hz-20kHz) | ±0.2dB | 0/-0.09dB (20Hz, 20kHz) |

Signal-to-noise ratio (A-wgt, ref 50W, 1V input) | 106dB | 105.2dB |

Phono input impedance | 47k ohms | 53.6k ohms |

Phono frequency response (20Hz-20kHz, Ref RIAA) | ±0.2dB | -3/-0.1dB (20Hz, 20kHz) |

Max headphone output level (32/300 ohms) | 2.5/5Vrms | 2.7/5.5Vrms |

Our primary measurements revealed the following using the line-level analog input and digital coaxial input (unless specified, assume a 1kHz sinewave at 2Vrms or 0dBFS, 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) | 109W | 109W |

Maximum output power into 4 ohms (1% THD+N, unweighted) | 160W | 160W |

Maximum burst output power (IHF, 8 ohms) | 131W | 131W |

Maximum burst output power (IHF, 4 ohms) | 204W | 204W |

Continuous dynamic power test (5 minutes, both channels driven) | fail | fail |

Crosstalk, one channel driven (10kHz) | -87dB | -95dB |

Damping factor | 176 | 189 |

DC offset | <0.3mV | <0.6mV |

Gain (pre-out) | 13.6dB | 13.6dB |

Gain (maximum volume) | 44.8dB | 44.8dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-103dB | <-101dB |

IMD ratio (SMPTE, 60Hz + 7kHz stimulus tones, 4:1 ) | <-98dB | <-100dB |

Input impedance (line input, RCA) | 11.7k ohms | 11.7k ohms |

Input sensitivity (100W 8 ohms, maximum volume) | 164mVrms | 164mVrms |

Noise level (with signal, A-weighted) | <50uVrms | <50uVrms |

Noise level (with signal, 20Hz to 20kHz) | <67uVrms | <67uVrms |

Noise level (no signal, A-weighted, volume min) | <50uVrms | <50uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min) | <63uVrms | <63uVrms |

Output impedance (pre-out) | 231 ohms | 231 ohms |

Signal-to-noise ratio (100W 8 ohms, A-weighted, 2Vrms in) | 110.6dB | 110.7dB |

Signal-to-noise ratio (100W 8 ohms, 20Hz to 20kHz, 2Vrms in) | 108.4dB | 108.5dB |

Signal-to-noise ratio (100W 8 ohms, A-weighted, max volume) | 90.2dB | 90.2dB |

Dynamic ange (100W 8 ohms, A-weighted, digital 24/96) | 110.4dB | 110.3dB |

Dynamic range (100W 8 ohms, A-weighted, digital 16/44.1) | 95.8dB | 95.9dB |

THD ratio (unweighted) | <0.00025% | <0.00022% |

THD ratio (unweighted, digital 24/96) | <0.0005% | <0.0014% |

THD ratio (unweighted, digital 16/44.1) | <0.0006% | <0.0014% |

THD+N ratio (A-weighted) | <0.0006% | <0.0006% |

THD+N ratio (A-weighted, digital 24/96) | <0.0009% | <0.0017% |

THD+N ratio (A-weighted, digital 16/44.1) | <0.0018% | <0.0014% |

THD+N ratio (unweighted) | <0.0008% | <0.0008% |

Minimum observed line AC voltage | 122VAC | 122VAC |

For the continuous dynamic power test, the A25 was able to sustain 161W into 4 ohms (~3% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (16.1W) for 5 seconds, for 200 seconds of the 500-second test before inducing the fault protection circuit. This test is meant to simulate sporadic dynamic bass peaks in music and movies. During the test, the top of the A25 was slightly warm to the touch.

Our primary measurements revealed the following using the phono-level input, MM configuration (unless specified, assume a 1kHz 5mVrms sinewave input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -72dB | -64dB |

DC offset | <0.3mV | <0.6mV |

Gain (default phono preamplifier) | 39.0dB | 38.9dB |

IMD ratio (CCIF, 18kHz + 19kHz stimulus tones, 1:1) | <-92dB | <-92dB |

IMD ratio (CCIF, 3kHz + 4kHz stimulus tones, 1:1) | <-88dB | <-88dB |

Input impedance | 52.7k ohms | 53.6k ohms |

Input sensitivity (to 100W with max volume) | 1.82mVrms | 1.84mVrms |

Noise level (with signal, A-weighted) | <750uVrms | <700uVrms |

Noise level (with signal, 20Hz to 20kHz) | <2.3mVrms | <2.1mVrms |

Noise level (no signal, A-weighted, volume min) | <50uVrms | <50uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min) | <64uVrms | <64uVrms |

Overload margin (relative 5mVrms input, 1kHz) | 23.8dB | 23.8dB |

Signal-to-noise ratio (100W, A-weighted, 5.1mVrms in) | 81.3dB | 81.6dB |

Signal-to-noise ratio (100W, 20Hz to 20kHz, 5.1mVrms in) | 74.9dB | 76.0dB |

THD (unweighted) | <0.0012% | <0.002% |

THD+N (A-weighted) | <0.008% | <0.008% |

THD+N (unweighted) | <0.025% | <0.025% |

Our primary measurements revealed the following using the analog input at the headphone output (unless specified, assume a 1kHz sinewave, 2Vrms input/output, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

Parameter | Left and right channels |

Maximum gain | 27.3dB |

Maximum output power into 600 ohms | 50mW |

Maximum output power into 300 ohms | 99mW |

Maximum output power into 32 ohms | 225mW |

Output impedance | 1.7 ohms |

Maximum output voltage (100k ohm load) | 5.5Vrms |

Noise level (with signal, A-weighted) | <5.2uVrms |

Noise level (with signal, 20Hz to 20kHz) | <8.3uVrms |

Noise level (no signal, A-weighted, volume min) | <4.2uVrms |

Noise level (no signal, 20Hz to 20kHz, volume min) | <7uVrms |

Signal-to-noise ratio (A-weighted, 1% THD, 5.5Vrms out) | 112dB |

Signal-to-noise ratio (20Hz - 20kHz, 1% THD, 5.5Vrms out) | 109dB |

THD ratio (unweighted) | <0.00015% |

THD+N ratio (A-weighted) | <0.0003% |

THD+N ratio (unweighted) | <0.00047% |

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

In our frequency response plots above (relative to 1kHz), measured across the speaker outputs at 10W into 8 ohms, the A25 is essentially perfectly flat within the audioband (20Hz to 20kHz, 0/-0.1dB). The -3dB point is at roughly 70kHz, and 0dB at 5Hz. The A25 appears to be DC coupled. 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, line-level input)**

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 A25 yielded only about -30 degrees of phase shift at 20kHz.

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

The chart above shows the frequency response for the MM phono input. 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 line at 0dB). We see a very flat response from 200Hz to 20kHz, and worst-case channel-to-channel deviations of roughly 0.1dB at 100 to 200Hz. Below 200Hz, there is steep attenuation (-3dB at 20Hz), as Arcam appears to have implemented an anti-rumble filter on their phono input.

**Phase response (MM input)**

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 A25 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 +30 degrees at 20Hz and -100 degrees at 20kHz.

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

The chart above shows the A25’s frequency response (relative to 1kHz) as a function of input type measured across the speaker outputs at 10W into 8 ohms. The two green traces are the same analog input data from the speaker-level frequency-response graph above. The blue and red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz using the coaxial input, and the purple and green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and the pink and orange traces are for a 24/192 dithered digital input signal. At low frequencies, the digital signals yielded a flat response down to 5Hz, same as the analog response. The -3dB points are: 21kHz for the 16/44.1 data, 46kHz for the 24/96, 78kHz for the 24/192 data, and 72kHz for the analog input. Also of note, the 16/44.1 and 24/96 input data showed brick-wall-type high-frequency filtering, while the 24/912 and analog data did not.

**Frequency response vs. filter type (16/44.1, left channel only)**

The chart above shows the A25’s frequency response (relative to 1kHz) as a function of filter type measured across the speaker outputs at 10W into 8 ohms for a 16/44.1 digital input (left channel only). The blue plot is the default Linear Phase Fast filter, the purple trace is the Hybrid Fast Filter, while the pink trace is the Minimum Phase Slow filter. We can see how the Linear Phase Fast filter offers true brick-wall filtering just past 20kHz (21kHz), while the Hybrid Fast and Minimum Phase Slow filters yielded -3dB points at 19kHz.

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

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-outputs of the A25, where 0dBFS was set to yield 1Vrms (2Vrms caused excessive THD). 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. The 24/96 data were at -3dB at -120dBFS, while the 16/44.1 data were +/-1dB at -120 to -110dBFS.

**Impulse response (24/44.1 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 line-level pre-outs of A25. The blue trace is for the Linear Phase Fast filter, which yielded a typical symmetrical sinc function response. The purple trace is for the Hybrid Fast filter, which yielded little pre-ringing and extended post-ringing, while the Minimum Phase Slow filter (orange) yielded essentially no pre-ringing and minimized post-ringing.

**J-Test (coaxial)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level pre-outputs of the A25 where 0dBFS is set to 1Vrms. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave 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 sinewave jitter by the Audio Precision, to show how well the DAC rejects jitter.

Here we see a poor J-Test result, with several peaks in the audioband, ranging from -105dBFS down to -150dBFS. This is an indication that the A25 DAC may have poor jitter immunity.

**J-Test (optical)**

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level pre-outputs of the A25. The optical input yielded similar but slightly worse results compared to the coaxial input.

**J-Test (coaxial, 10ns jitter)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the A25, with an additional 10ns of 2kHz sinewave jitter injected by the APx555. The telltale peaks at 10kHz and 12kHz can be seen at -120dBFS.

**J-Test (coaxial, 100ns jitter)**

The chart above shows the results of the J-Test test for the coaxial digital input measured at the line-level output of the A25, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The telltale peaks at 10kHz and 12kHz can be seen at a higher -100dBFS.

**J-Test (optical, 100ns jitter)**

The chart above shows the results of the J-Test test for the optical digital input measured at the line-level output of the A25, with an additional 100ns of 2kHz sinewave jitter injected by the APx555. The results are essentially the same as with the coaxial input.

**Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (Linear Phase Fast filter, coaxial input)**

The chart above shows a fast Fourier transform (FFT) of the A25’s line-level pre-outputs with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 using the Linear Phase Fast filter. The steep roll-off around 20kHz in the white-noise spectrum shows that the Linear Phase Fast filter is of the brick-wall type variety. There are a few low-level aliased image peaks within the audioband at the -115dBrA and below level. The primary aliasing signal at 25kHz is highly suppressed at -80dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are near the same level.

**Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (Hyrbrid Fast filter, coaxial input)**

The chart above shows a fast Fourier transform (FFT) of the A25’s line-level pre-outputs with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Hybrid Fast filter. The steep roll-off around 20kHz in the white-noise spectrum shows that the Hybrid Fast filter is of the brick-wall type variety. There are a few low-level aliased image peaks within the audioband at the -120dBrA and below level. The primary aliasing signal at 25kHz is highly suppressed at -80dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are near the same level.

**Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (Minimum Phase Slow filter, coaxial input)**

The chart above shows a fast Fourier transform (FFT) of the A25’s line-level pre-outputs with white noise at -4dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1, using the Minimum Phase Slow filter. The shallower roll-off around 20kHz in the white-noise spectrum shows that the Minimum Phase Slow filter does not use Brick-Wall type of filtering. There are a few low-level aliased image peaks within the audioband at the -120dBrA and below level. The primary aliasing signal at 25kHz is mildly suppressed at -35dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone are around the -80 to -90 dBrA level.

**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 50kHz. 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 very small at roughly 0.09dB. This is a strong result and an indication of a low output impedance, or high damping factor. With a real speaker load, deviations measured lower at roughly 0.07dB.

**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 left and right at 1W output into 8 ohms, purple/green at 10W, and pink/orange just at 90W. The power was varied using the A25 volume control. The 10W THD ratios were the lowest, ranging from 0.0005% at 20Hz down to 0.0002-0.0003% from 100Hz to 2kHz, then up to 0.001% at 20kHz. The 1W THD ratios were higher and relatively flat across the audioband, from 0.0005% to 0.001%. At 90W, THD ratios ranged from 0.0005% at lower frequencies, up to 0.01% at 20kHz.

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

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 0.03% (20Hz) down to 0.0005% (2/3kHz left channel), up to 0.003% at 20kHz.

**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 A25 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 remarkably close (with 2-3dB). They range from 0.002% at 50mW, down to 0.0003% in the 5 to 30W range. The “knee” into 8 ohms can be found right around the rated output power of 100W, while the 4-ohm knee can be seen around 150W. The 1% THD marks were hit at 109W and 160W into 8 and 4 ohms.

**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 A25 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 remarkably close (with 2-3dB). They range from 0.02% at 50mW, down to 0.001% in the 20 to 100W range.

**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 A25 as a function of frequency into three different loads (8/4/2 ohms) for a constant input voltage that yields 20W at the output into 8 ohms (and roughly 40W into 4 ohms, and 80W into 2 ohms) for the analog line-level input. Note that for the 2-ohm load, the A25’s protection circuit was almost immediately initiated and shut the unit down, so it is not visible. The 8-ohm load is the blue trace and the 4-ohm load the purple trace. We find a roughly 5-10dB increase in THD from 8 to 4 ohms from 400Hz to 20kHz. These ranged from 0.0003% at 20Hz down to 0.0002% at 1-2kHz, then up to 0.0015% at 20kHz for the 8-ohm load. The 4-ohm load ranged from 0.0005% at 20Hz down to 0.0002% at 100-200Hz, then up to 0.002% 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 A25 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 higher than those measured across the resistive dummy load. The differences ranged from 0.06% at 20Hz for the two-way speaker versus 0.0008% for the resistive load, and 0.004% at 20kHz into the three-way speaker versus 0.001% for the resistive load. Between the important frequencies of 500Hz to 6kHz, all three THD traces were very close, around the 0.0006-0.0007% mark. This is a strong result.

**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 A25 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 three-way speaker (Paradigm Founder Series 100F, measurements can be found here). We find that all three IMD traces are close to one another, with the three-way speaker yielding 5dB higher results in the 4-5kHz range. Most of the IMD results are hovering around the 0.001% level.

**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 A25 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, between 0.002 and 0.003% across the sweep. Another strong result.

**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 input. We see that the signal’s second (2kHz) and third (3kHz) harmonics dominate at a very low -115dBrA, or 0.0002%. There are subsequent signal harmonics visible at and below the extremely low -130dBrA, or 0.00003%, level. On the right side of the signal peak, we find power-supply-related noise peaks, with the second harmonic (120Hz) dominating at just above -120dBrA, or 0.0001%. Other noise peaks can be seen below the -120dBrA level. Overall, this is a very clean FFT.

**FFT spectrum – 1kHz (MM phono 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 MM phono input. We see that the signal’s second (2kHz) harmonic dominates (right channel) at a low -100dBrA, or 0.001%. On the right side of the signal peak, we find power-supply-related noise peaks, with the fundamental (60Hz) and second harmonic (120Hz) dominating at just above -80dBrA, or 0.01%. Other noise peaks can be seen at and below the -90dBrA level.

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

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 coaxial digital input, sampled at 16/44.1. Here the second (2kHz) signal harmonic dominates at a higher -105/-100dBrA (left/right), or 0.0006/0.001%. Subsequent harmonics can be seen at and below the -120dBrA, or 0.0001%, level. Noise peaks are essentially the same as with the analog FFT above.

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

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 coaxial digital input, sampled at 24/96. We see essentially the same result as with the 16/44.1 FFT above, but with a lower noise floor due to the increased bit depth.

**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 sinewave 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, with no signal harmonics above the -135dBrA noise floor, and the same -120dBrA 120Hz noise peak.

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

Shown above is the FFT for a 1kHz -90dBFS dithered 24/96 input sinewave 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 a single signal harmonic at 3kHz just barely above the -145dBrA noise floor, and the same -120dBrA 120Hz noise peak. Other power-supply-related noise peaks can be seen below the -120dBrA level.

**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 a low -110dBrA, or 0.0003%. Other peaks (both signal harmonics and power-supply noise-related harmonics) can be seen at -120dBrA and below.

**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 MM phono 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 (non-signal) peaks are the 60Hz fundamental power-supply noise peak and its second (120Hz) harmonic at -80dBrA, or 0.01% (left channel). The highest signal harmonic is at 100Hz, at -90dBrA, or 0.003%.

**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 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 -120dBRa, or 0.0001%, while the third-order modulation products, at 17kHz and 20kHz, are just below the -110dBrA, or 0.0003%, level. This is a very clean IMD result.

**Intermodulation distortion FFT (line-level input, APx 32 tone)**

Shown above is the FFT of the speaker-level output of the A25 with the APx 32-tone signal applied to the analog input. The combined amplitude of the 32 tones is the 0dBrA reference, and that 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 very low -135dBrA, or 0.00002%, level.

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

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 digital coaxial input at 16/44.1 (0dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110/100dBrA (left/right), or 0.0003/0.001%, while the third-order modulation products, at 17kHz and 20kHz, are very low at -120dBrA, or 0.0001%.

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

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 digital coaxial input at 24/96 (0dBFS). We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -110/100dBrA (left/right), or 0.0003/0.001%, while the third-order modulation products, at 17kHz and 20kHz, are very low at -120dBrA, or 0.0001%.

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

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 MM phono input. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at around -100dBrA, or 0.001%, while the third-order modulation products, at 17kHz and 20kHz, are lower at -115dBrA, or 0.0002%.

**Squarewave 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 A25’s slew-rate performance. Rather, it should be seen as a qualitative representation of the A25’s mid-to-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 relatively clean corners, with some softening and overshoot.

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

The final graph above shows the damping factor as a function of frequency. We can see here a constant damping factor just under 200 through most of the audioband. This is a strong result for a medium-powered solid-state integrated amplifier.

*Diego Estan*

Electronics Measurement Specialist