Halcro dm58 monoblock power amplifier:
Measurements part 2
Fig.6 is a spectrum of the dm58's output at 200W into 8 ohms, taken with a very linear signal generator: the National Instruments NI4451 PCI card that Miller Audio Research uses as one of the platforms for its QC Suite measurement software. The second, third, and fourth harmonics are all between -106dB and -110dB! The summed total of the harmonics is around 0.0008%, but even then, this is not appreciably higher than the level of the residual harmonics present in the NI card's output (0.00055%).

Fig.6 Halcro dm58, spectrum of 1kHz sinewave, DC-10kHz, at 200W into 8 ohms (linear frequency scale).

Given this very low THD, it seemed academic to examine the Halcro's intermodulation behavior with the Audio Precision System One. Nevertheless, I include fig.7 because it shows that, even at the high power level featured in this measurement—220W into 4 ohms, just below visible clipping on the 'scope screen—the only intermodulation products present are those in the output of the DAC used to drive the amplifier.

Fig.7 Halcro dm58, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 220W into 4 ohms (linear frequency scale).

Finally, fig.8 shows how the percentage of THD+noise present in the dm58's output varied with continuous output power into 8, 4, and 2 ohms. The constant downward slope with increasing frequency reveals that the measured figure is dominated by noise below the "knee" of the traces, not distortion. In addition, the slight sawteeth present in the traces, due to the Audio Precision System One's automatic gain-ranging, reveal that the Halcro's residual THD+N is at the lower limit of what is possible to measure with this setup. The actual clipping points (1% THD) are 260W into 8 ohms (24.1dBW) and 490W into 4 ohms (23.9dBW). Into 2 ohms, the amplifier's protection circuitry cut in at 210W, muting the output. Into 4 ohms, it cut in at 500W.

Fig.8 Halcro dm58, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, 2 ohms.

Halcro's dm58 offers astonishing measured performance for an amplifier, particularly when it comes to harmonic and intermodulation distortion. Most important, this does not appear to have been achieved by compromising other aspects of the amplifier's performance, as was the case in the "THD Wars" of the 1970s.—John Atkinson