Ars-Sonum Armonía Stereo Amplifier Measurements

Link: reviewed by Matt Bonaccio on SoundStage! Hi-Fi on September 1, 2023

General Information

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

The Ars-Sonum Armonía was conditioned for one hour at 1/8th full rated power (~3.5W 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 Armonía has two sets of unbalanced inputs (RCA) and a pair of speaker-level outputs. A level of 360mVrms was required at the input to achieve the reference 10W into 8 ohms for the Line 2 input, which we measured yielding a typical gain of about 28dB. The Line 1 input offers more gain—we measured about 34dB. For the purposes of these measurements, unless otherwise specified, Line 2 was used.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Ars-Sonum for the Armonía compared directly against our own. The published specifications are sourced from Ars-Sonum’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 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.

Our primary measurements revealed the following using the Line 2 unbalanced analog input (unless specified, assume a 1kHz sinewave at 360mVrms at the input, 10W output, 8-ohm loading, 10Hz to 22.4kHz bandwidth):

For the continuous dynamic power test, the Armonía was able to sustain 20W into 4 ohms (~5% THD) using an 80Hz tone for 500ms, alternating with a signal at -10dB of the peak (2W) 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 Armonía was very hot to the touch, which is the same as its heat output when powered on with no signal. 

Frequency response (8-ohm loading)

In our frequency-response plots above, measured across the speaker outputs at 10W into 8 ohms, the Armonía is near flat within the audioband (-0.5/-0.4dB, 20Hz/20kHz). The -3dB points are at about 5.5Hz and 56kHz. There is also a rise in the response at around 8Hz—nearly 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.

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 Armonía does not invert polarity and exhibits, at worst, about +/-20 degrees (at 20Hz/20kHz) of phase shift within the audioband.

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

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 cyan plot is an actual speaker (Focal Chora 806, measurements can be found here). As per the manufacturer, we avoided applying a signal to the Armonía with no load connected. The chart below . . .

. . . is the same but zoomed in to highlight differences. Here we find the maximum deviation between an 8-ohm and 4-ohm load to be around 1.4dB. This is an indication of a very low damping factor, or high output impedance. With a real speaker, the deviations are worse—as much as 3dB (which would be clearly audible) between 200Hz and 1.5kHz.

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 sinewave stimulus at the analog line-level input. The blue and red plots are for left and right channels at 1W output into 8 ohms, purple/green at 10W, and pink/orange at 24W. The 1W data yielded the lowest THD figures, from 0.4% (left) at 20Hz, down to 0.02% from 200Hz to 3kHz, then up to 0.1% at 20kHz. The right channel had a flatter response, but consistently yielded higher THD figures (5dB) from 200Hz to 3kHz at all power levels. At 10W, THD data for the left channel ranged from 0.5%, down to 0.05%, then up to 0.3%. At 24W, THD data were flatter, around 1 to 3%.

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

The chart above shows THD ratios measured at the output of the Armonía 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). The 8-ohm data ranged from about 0.02% from 10 to 300mW, then a steady climb to 0.3/0.5% (left/right at 20W), reaching the 1% THD mark around 24W. The 4-ohm data ranged from about 0.02% from 10 to 200mW, then a steady climb to 0.5/1% (left/right at 10W), reaching the 1% THD mark jut past 11W, and the 5% THD mark just shy of 20W. Again here, the left channel yielded lower THD ratios (5-10dB) compared to the right channel through the most-likely-used power band of the amplifier (1W to 10W).

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

The chart above shows THD+N ratios measured at the output of the Armonía 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). THD+N values for the 8-ohm data ranged from 0.4% at 10mW down to just 0.04% (left channel) at 2-5W. The 4-ohm data yielded similar THD+N values up to about 300mW, then higher values (10dB) then the 8-ohm data due to the higher THD.

THD ratio (unweighted) vs. frequency at 8 and 4 ohms

The chart above shows THD ratios measured at the output of the Armonía as a function of frequency into two different loads (8/4 ohms) for a constant input voltage that yields 10W at the output into 8 ohms.  As per the manufactuerer, we avoided applying a signal to the Armonía with a 2-ohm load connected, which we would normally do. The 8-ohm load data are the blue/red traces and the 4-ohm load are the purple/green traces. The 8-ohm THD data for the left channel ranged from 0.5%, down to 0.05%, then up to 0.3%, with the right channel yielding higher THD ratios (5-10dB) from 200Hz to 3kHz. The 4-ohm THD data were much higher, between 1 and 2% from 20Hz to 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 Armonía 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). There are significant varations in THD ratios between the resisitve dummy load and real speakers. The two-way speaker yielded the largest swings, from 4% at 20Hz, down to 0.007% at 2kHz. This is likely due to the Armonía’s low damping factor, and just like with frequency repsonse, shows that THD results will vary greatly depending on the speaker connected to it.

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 Armonía 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). Here the three-way speaker yielded the highest IMD results, from 0.05% to 0.1%. The dummy load and two-way speaker were as low as 0.01-0.02%.

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 Armonía 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). The resistive dummy load was fairly constant at 0.06-0.08%, with the speaker IMD results fluctuating above and below these values by as much as 10dB.

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 fifth (5kHz) harmonic dominate, as high as -60dBrA (right channel), or 0.1%. Overall, the right channel yielded higher signal harmonic related peaks, but not always. At 3kHz, the right channel dominates at -70dBrA, or 0.03%, while at 4kHz, the left channel dominates at -80dBRa, or 0.01%. Power-supply-related noise peaks are pervasive, with the fundamental (60Hz) and second (120Hz) harmonics dominating between -80 and -90dBrA, or 0.01% and 0.003%.

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 dominant (non-signal) peaks are the second (100Hz) and third (150Hz) signal harmonics at roughly -60dBrA, or 0.1%. Power-supply-related harmonics can be seen here at the fundamental (60Hz) and second harmonic (120Hz) at -80dBrA, or 0.01%, as well as multiples and resulting IMD products at lower levels.

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 -80/-60dBrA (left/right), or 0.01/0.1%, and the third-order modulation products, at 17kHz and 20kHz, are slightly below -80/-70dBrA (left/right), or 0.01/0.03%.

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

Shown above is the FFT of the speaker-level output of the Armonía with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and 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 below the -70dBrA, or 0.03%, level.

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 Armonía’s slew-rate performance. Rather, it should be seen as a qualitative representation of the Armonía’s mid-tier 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 the case of the Armonía however, we see a relatively clean squarewave reproduction, although with softened cornered, but no over/undershoot.

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

The final graph above is the damping factor as a function of frequency. We find poor damping factor values, characteristic of tube amplifiers. The right channel was higher, hovering between 6 and 8, while the left channel yielded a fairly constant 3.4 from 30Hz to 20kHz.

Diego Estan
Electronics Measurement Specialist