Acurus DIA 100 integrated amplifier Measurements

Sidebar 2: Measurements

The Acurus DIA would not complete the one-hour, 1/3-power preconditioning we perform as a matter of routine. The test was run four times: twice, before going to CG for listening, the DIA shut down after 45 minutes; twice more, after CG was finished with it, it shut down 20 minutes into the conditioning. It was running hot at shutdown (footnote 1), but not unusually so, indicating that its thermal protection may have been set to trip early.

The Acurus's input impedance into its CD input measured between 11k ohms and 12k ohms at volume-control settings up to 3:00, decreasing to just over 9k ohms at maximum gain. These are moderately low values; the DIA should not be used with sources having unusually high output impedances (thankfully, not all that common today). The maximum voltage gain itself (with the amp loaded with 8 ohms) measured a very high 43dB, compensating for the lack of preamp gain. Volume-control tracking was good though not outstanding, the worst case being an interchannel mismatch of 0.8dB at a setting of 12:00.

The DIA's output impedance was very low, ranging between 0.037 and 0.046 ohms depending on frequency and load. This suggests a very consistent performance—at least with respect to the DIA's frequency response into real-world loudspeakers as contrasted to a constant test-bench load. DC offset measured 19.2mV in the left channel, 1.26mV in the right. Noise (unweighted, referenced to 1W into 8 ohms) measured –78.3dB. The DIA was noninverting, a positive impulse at the input remaining positive at the output.

Fig.1 shows the frequency response of the DIA at 1W into 8 ohms. The response at 2W into 4 ohms (not shown) is virtually identical. Fig.2 is the output waveform with a 10kHz squarewave input; a very slight rounding of the leading edge reflects the response rolloff above 20kHz visible in fig.1. The 1kHz squarewave, not shown, is virtually perfect.

Fig.1 Acurus DIA 100, frequency response at (from top to bottom): 1W into 8 ohms (0.5dB/vertical div., right channel dashed).

Fig.2 Acurus DIA 100, small-signal 10kHz squarewave into 8 ohms.

Fig.3 shows the DIA's crosstalk, measured at 1kHz. It is reasonably consistent between channels above 3kHz, and low enough across the audible range that small interchannel differences below this frequency should not have any audible effect. The typical increase at higher frequencies is likely due—as usual—to interchannel capacitive coupling, probably between the two halves of the stereo volume control. I did note, when testing the squarewave response, that there seemed to be some high-frequency crosstalk between inputs; measurements of this, however, revealed a more than adequate separation of 48dB at 50kHz and 98dB at 100Hz (footnote 2).

Fig.3 Acurus DIA 100, channel separation, L–R (top), R–L (bottom) (5dB/vertical div.).

The THD+noise vs frequency results in fig.4, while not the lowest we have ever measured, are nevertheless very good. Consistently low regardless of load, they're slightly but not appreciably higher at lower load impedances and higher frequencies. The commonly encountered distortion rise at higher frequencies is moderate and does not rise above 0.12%. One exception to these observations is the distortion into a 2 ohm load, which drops slightly at higher frequencies.

Fig.4 Acurus DIA 100, THD+noise (%) vs frequency at (from top to bottom at 1kHz): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms (right channel dashed).

Fig.5 shows the THD+noise waveform of the Acurus in response to a 1kHz input into an 8 ohm load at 1W. It appears to be primarily second-harmonic overlaid with noise. A similar result was obtained into 4 and 2 ohm loads (curves not shown). Confirming this, fig.6 shows the response of the Acurus to a 50Hz input at an output of 100W into a 4 ohm load (2/3 power). The distortion components are low, the highest being the second harmonic (100Hz) at –72dB, or about 0.025%. Power-line noise components are also visible, the highest in level being 180Hz at –86dB.

Fig.5 Acurus DIA 100, 1kHz waveform at 1W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.6 Acurus DIA 100, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 4 ohms (linear frequency scale).

The intermodulation in the output at a level of 100W into 4 ohms caused by a combined 19+20kHz input is shown in fig.7. The largest artifact here is the 21kHz product at –56dB, or about 0.15%. The 1kHz distortion product is at –62dB, or about 0.08%. The spectrum resulting from a 67W output into an 8 ohm load (not shown) has considerably fewer, and lower, artifacts throughout. Interestingly, however, it shows a low but visible artifact at 2kHz but none (or at least none higher than the background noise) at 3kHz, whereas the spectrum in fig.7 shows a 3kHz artifact but none at 2kHz.

Fig.7 Acurus DIA 100, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 100W into 4 ohms (linear frequency scale).

Finally, fig.8 shows the Acurus's THD+noise vs output power curves, one channel driven. (The discrete clipping measurements—by our convention, 1% THD+noise at 1kHz—to the nearest watt, are shown in Table 1.) The power output decreases into loads of less than 4 ohms, suggesting that the DIA 100 is not ideal for those few loudspeakers which dip below 4 ohms for a considerable portion of their frequency range. The still-significant maximum 2 ohm output, however—127W—and the fact that the DIA blew no fuses during its bench tests, indicate that it should have no difficulty with loads which drop to between 2 and 4 ohms at one or two points on their impedance curves—a category which includes most loudspeakers. Note also that CG did not have any problems driving the demanding Thiel CS3.6, presumably due to its benign phase angle across the audio band and its generally higher impedance magnitude in the bass.

Fig.8 Acurus DIA 100, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

TABLE 1: Acurus DIA 100 Clipping (1% THD+N at 1kHz)

Both Channels DrivenOne Channel Driven
LoadW (dBW)W (dBW)
8126 (21)124 (21)152 (21.8)
4183 (19.6)180 (19.6)235 (20.7)
2127 (15)

The Acurus DIA 100 performed well on the bench. Its main limitations were a somewhat low input impedance and a drop-off of power into loads under 4 ohms. Both of these limitations can be minimized by care in system matching: choosing sources with low output impedances and loudspeakers which do not present awkward loads. The early shutdown during the 1/3-power test—a worst-case condition—should not have audible consequences, but such premature shutdowns are rare in modern amplifiers.—Thomas J. Norton

Footnote 1: For a class-B amplifier, 1/3-power results in the greatest dissipation in the output stage devices, leading to maximum thermal stress on the amplifier. Which is why we precondition them in this way.—John Atkinson

Footnote 2: Unusually, when the amplifier was measured before it was sent to CG, there was some positive feedback from output to input with the volume control full up—it disappeared with the control reduced even by a smidgen—resulting in mild ultrasonic oscillation. This was not apparent when the amplifier was returned to us.—John Atkinson

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