Sidebar 3: Measurements
Before I tested one of the Dan D'Agostino M400 MxV amplifiers, serial number 5879, with my Audio Precision SYS2722, I preconditioned it by following the CEA's recommendation: I ran it at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the side-mounted heatsinks were hot, at 106.1°F (41.2°C), and the top panel was hotter, at 110.5°F (43.7°C). After testing the amplifier at high powers, the temperature of the top panel had risen to 120.4°F/49.1°C. The M400 MxV needs to be well-ventilated.
The D'Agostino's voltage gain was 26.9dB into 8 ohms, and the amplifier preserved absolute polarity (ie, was noninverting). The specified input impedance is 1M ohm. I measured 183k ohms at 20Hz, 152k ohms at 1kHz, and 129k ohms at 20kHz. Though lower than the specified value, these impedances are still usefully high.
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When the M400 MxV drove an equal mix of 19 and 20kHz tones with a peak level of 50W into 8 ohms (fig.10), both the second-order difference product at 1kHz and the higher-order intermodulation products at 18kHz and 21kHz lay at –77dB (0.014%). At the same voltage into 4 ohms, these intermodulation products rose by 7dB, but this is still a respectably low level. The Dan D'Agostino M400 MxV did well on the test bench, offering very high power with sufficiently low levels of distortion and noise.—John Atkinson
Fig.1 Dan D'Agostino M400 MxV, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), 2 ohms (red) (0.25dB/vertical div.).
Fig.2 Dan D'Agostino M400 MxV, small-signal 10kHz squarewave into 8 ohms.
The D'Agostino's output impedance was 0.5 ohm at 20Hz and 1kHz, increasing slightly to 0.55 ohm at 20kHz. (These figures include the series impedance of 6' of spaced-pair loudspeaker cable.) The modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was small, at ±0.25dB (fig.1, gray trace). The response into an 8 ohm resistive load (fig.1, blue trace) was down by 3dB just above 100kHz, though the increasing output impedance at very high frequencies means that the ultrasonic rolloff into 4 ohms (magenta) and 2 ohms (red) was slightly greater. The M400 MxV's reproduction of a 10kHz squarewave into 8 ohms (fig.2) was superb, with no overshoot or ringing.
Fig.3 Dan D'Agostino M400 MxV, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (blue) and ay 100W into 8 ohms (red) (linear frequency scale).
Measured with the balanced input shorted to ground, the amplifier's unweighted, wideband signal/noise ratio was a very good 79.3dB ref. 1W into 8 ohms. This ratio improved to 81dB when the measurement bandwidth was restricted to 22Hz–22kHz and to 84dB when A-weighted. While spuriae at the 60Hz power-supply frequency and its harmonics were present in the M400 MxV's noisefloor (fig.3), these are negligible at –90dB and below ref. 1W into 8 ohms (blue). As expected, the levels of these spuriae ref. 100W into 8 ohms (red trace) were as much as 20dB lower.
Fig.4 Dan D'Agostino M400 MxV, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Dan D'Agostino M400 MxV, distortion (%) vs 1kHz continuous output power into 4 ohms.
The Dan D'Agostino M400 MxV is specified as offering maximum output powers of 400W into 8 ohms, 800W into 4 ohms, and 1600W into 2 ohms (all powers equivalent to 26dBW). With our definition of clipping, which is when the output's percentage of THD+noise reaches 1%, the M400 MxV clipped with a 1kHz signal at 442W into 8 ohms (26.45dBW, fig.4) and at 778W into 4 ohms (25.9dBW, fig.5). The AC wall voltage had dropped from 119.7V with the amplifier idling to 118.7V with the amplifier clipping into 4 ohms, which is why the M400 MxV didn't quite meet its specified power into this load. Similarly, although I measured a clipping power of 1050W into 2 ohms, the wall voltage had dropped to 116V at this power.
Fig.6 Dan D'Agostino M400 MxV, THD+N (%) vs frequency at 20V into: 8 ohms (blue), 4 ohms (magenta), 2 ohms (red).
I examined how the percentage of THD+N changed with frequency at 20V, which is equivalent to 50W into 8 ohms, 100W into 4 ohms, and 200W into 2 ohms. The distortion was low into 8 ohms, with a slight rise in the top of the audioband (fig.6, blue trace), but rose into 4 ohms (magenta) and 2 ohms (red).
Fig.7 Dan D'Agostino M400 MxV, 1kHz waveform at 50W into 8 ohms, 0.029% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Dan D'Agostino M400 MxV, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 8 ohms (linear frequency scale).
Fig.9 Dan D'Agostino M400 MxV, spectrum of 50Hz sinewave, DC–1kHz, at 50W into 4 ohms (linear frequency scale).
The M400 MxV's distortion driving 1kHz at 50W into 8 ohms was predominantly the second harmonic (fig.7), though higher-order harmonics are present at lower levels (fig.8). The third harmonic rose almost to the same level as the second, close to –70dB (0.03%), at the same power into 4 ohms (fig.9) and at 100W into 8 ohms (not shown).
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Fig.10 Dan D'Agostino M400 MxV, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 8 ohms (linear frequency scale).
When the M400 MxV drove an equal mix of 19 and 20kHz tones with a peak level of 50W into 8 ohms (fig.10), both the second-order difference product at 1kHz and the higher-order intermodulation products at 18kHz and 21kHz lay at –77dB (0.014%). At the same voltage into 4 ohms, these intermodulation products rose by 7dB, but this is still a respectably low level. The Dan D'Agostino M400 MxV did well on the test bench, offering very high power with sufficiently low levels of distortion and noise.—John Atkinson
