Arcam FMJ A19 integrated amplifier Measurements

Sidebar 3: Measurements

I measured the Arcam FMJ A19 using Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com, and the January 2008 "As We See It"). Before performing any measurements, I ran the FMJ A19 for an hour at one-third its specified maximum power of 50Wpc into 8 ohms, thermally the worst case for an amplifier with a class-B or -AB output stage. At the end of that period, the internal heatsinks, which are shielded by the top panel, were at 140.3°F (60.2°C). The top panel had a more reasonable temperature of 102.6°F (39.3°C). The THD+noise remained at a very low 0.0054% throughout this period.

The maximum volume-control setting was "99"; it operated in accurate 0.5dB steps, meaning that an indicated "69" was 15dB below maximum. With a 1kHz tone, the A19 offered a maximum voltage gain for line-level sources of 39.1dB into 8 ohms. Measured at the Preamplifier outputs, the maximum gain was 9.7dB, sourced from an output impedance of 227 ohms at all audio frequencies. This output preserved absolute polarity for both phono and line inputs (ie, was non-inverting), as did the speaker outputs.

Looking first at the phono input, this offered a gain at 1kHz of 48.8dB measured at the Preamplifier outputs, which is appropriate for moving-magnet and high-output moving-coil cartridges. The input impedance was 44.5k ohms at low and middle frequencies, slightly lower than the specified 47k ohms, dropping further at 20kHz to 39k ohms. The error in the RIAA de-emphasis was impressively small and well matched between channels (fig.1), though this graph reveals that the A19 implements the IEC-recommended infrasonic rolloff, reaching –3dB at 21Hz. The phono input was also quiet, the wideband signal/noise ratio measuring 75dB in both channels, ref. an input signal of 1kHz at 5mV and taken with the input shorted. This improved to 83.5dB when A-weighted. Channel separation via the phono input (not shown) was excellent, at >80dB below 2kHz and still 73dB at 20kHz.

114A19fig01.jpg

Fig.1 Arcam FMJ A19, MM input, response with RIAA correction (left channel blue, right red) (0.5dB/vertical div.).

The phono-input overload margin was an even 20–21dB throughout the audioband, which is excellent. With a 1kHz signal at 5mV, the only harmonic visible in the output spectrum (fig.2) is the second, at –100dB (0.001%), just more than 20dB below the level of the highest power-supply–related spurious tone, at 60Hz. High-frequency intermodulation was also very low (fig.3).

114A19fig02.jpg

Fig.2 Arcam FMJ A19, MM input, spectrum of 1kHz sinewave, DC–10kHz, at 2V into 100k ohms (linear frequency scale).

114A19fig03.jpg

Fig.3 Arcam FMJ A19, MM input, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 2V peak into 100k ohms (linear frequency scale; left channel blue, right red).

Turning to the A19's performance via its line-level inputs and measured at the speaker terminals, these offered an input impedance of 10k ohms from 20Hz to 20kHz, as specified. The output impedance was 0.155 ohm at 20Hz and 1kHz, rising slightly to 0.19 ohm at 20kHz. The modification of the Arcam's response due to the Ohm's law interaction between this source impedance and that of our standard simulated load was ±0.2dB (fig.4, gray trace). The low frequencies in this graph roll off by a low 0.25dB at 20Hz but more rapidly above the audioband, reaching –3dB at 55kHz, which slows a little the leading edges of the A19's reproduction of a 10kHz squarewave (fig.5). A small overshoot is visible in this graph, as there is with a 1kHz squarewave (fig.6), but there is no ringing. The traces in fig.4 were taken with the volume control set to "99"; there was no change in the measured frequency response at lower settings of the volume control, which is commendable.

114A19fig04.jpg

Fig.4 Arcam FMJ A19, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (0.5dB/vertical div.).

114A19fig05.jpg

Fig.5 Arcam FMJ A19, small-signal 10kHz squarewave into 8 ohms.

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Fig.6 Arcam FMJ A19, small-signal 1kHz squarewave into 8 ohms.

Channel separation for line-level signals was good rather than great, at 70dB R–L and 87dB L–R at 2kHz, decreasing to 52 and 68dB, respectively, at 20kHz. The wideband, unweighted S/N ratio, ref. 2.83V into 8 ohms and taken with the input shorted but the volume control set to its maximum, was an okay 70.3dB in both channels, improving to 77.3dB when A-weighted. These ratios will increase at lower volume-control settings, of course.

The way the traces slope upward to the left in figs. 7 and 8, which plot the THD+noise percentage against output power into 8 and 4 ohms, respectively, means that any distortion is below the level of the noise, up to a few tens of watts. (A constant level of noise becomes an increasing percentage of the signal level as the latter drops.) We define an amplifier's clipping point as when the THD+N reaches 1%; these graphs indicate that with a 1kHz tone, the A19 clips at 61W into 8 ohms (17.7dBW) and 85Wpc into 4 ohms (16.3dBW)—slightly more and slightly less, respectively, than the specified maximum powers of 50Wpc into 8 ohms (17dBW) and 90Wpc into 4 ohms (16.5dBW).

114A19fig07.jpg

Fig.7 Arcam FMJ A19, distortion (%) vs 1kHz continuous output power into 8 ohms.

114A19fig08.jpg

Fig.8 Arcam FMJ A19, distortion (%) vs 1kHz continuous output power into 4 ohms.

I didn't test the A19's power delivery into 2 ohms, as it had difficulty with low impedances. For example, I had some problems when I tried to measure how the percentage of THD+N changes with frequency. Usually, I choose a level where I can be sure I am seeing actual distortion rather than noise when I look at the THD+N vs power graphs. I first tried plotting the THD+N at 9V, which is equivalent to 10W into 8 ohms or 20W into 4 ohms. However, as you can see in fig.9, the THD+N into 4 ohms (cyan and magenta traces) rises dramatically below 125Hz. This is because, with continuous drive into 4 ohms at this level, the A19's protection circuit cuts in at frequencies below the midrange. Investigating this, it appears that the amplifier will protect itself when asked to deliver a low-frequency signal at more than 15W into 4 ohms for more than 5 seconds or so. I therefore repeated this test at a level of 2.83V, equivalent to 1W into 8 ohms or 2W into 4 ohms. Now all appears well (fig.10), with the small rise in THD at the top of the audioband due to the circuit's reduced open-loop bandwidth in this region.

114A19fig09.jpg

Fig.9 Arcam FMJ A19, THD+N (%) vs frequency at 9V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).

114A19fig10.jpg

Fig.10 Arcam FMJ A19, THD+N (%) vs frequency at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).

Not only is the Arcam A19's distortion very low at small-signal levels, it is predominantly the subjectively innocuous second harmonic in nature (figs. 11 and 12). Fig.12 reveals that the supply-related spurious tone at 120Hz is also low in level. Intermodulation distortion is also negligible, even with an equal mix of 19 and 20kHz tones driven into 4 ohms at a level a couple of dB below visible clipping on the oscilloscope screen (fig.13).

114A19fig11.jpg

Fig.11 Arcam FMJ A19, 1kHz waveform at 10W into 8 ohms, 0.004% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

114A19fig12.jpg

Fig.12 Arcam FMJ A19, spectrum of 50Hz sinewave, DC–1kHz, at 30W into 8 ohms (linear frequency scale).

114A19fig13.jpg

Fig.13 Arcam FMJ A19, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 50W peak into 4 ohms (linear frequency scale).

Arcam's FMJ A19 shouldn't be used for driving loads much below 6 ohms at high levels, but that is a sensible compromise made to keep its price affordable. Other than that caveat, the A19's measured performance is excellent.—John Atkinson

COMPANY INFO
Arcam
US distributor: American Audio & Video
4325 Executive Drive, Suite 300
Southaven, MS 38672
(866) 916-4667
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