Halcro dm58 monoblock power amplifier Measurements

Sidebar 3: Measurements

Following the usual one-hour preconditioning period at 1/3 power into 8 ohms, the Halcro dm58's chassis was cool to the touch, suggesting efficient heatsinking. The input impedance was a usefully high 102k ohms unbalanced and 194k ohms balanced, and the amplifier didn't invert signal polarity through either input. (The XLRs are wired with pin 2 hot.) The voltage gain was higher than average at 33.45dB through the balanced input, and, as expected 6dB lower through the unbalanced input.

The output impedance was a very low 0.06 ohm across most of the audioband, rising inconsequentially to 0.1 ohm at 20kHz. (Note that this figure includes 0.5m of speaker cable.) As a result, any modification of the Halcro's response into our simulated speaker load (fig.1, top trace between 1kHz and 3kHz) was minimal. Into resistive loads, the Halcro's response was 1dB down at 13Hz and 75kHz, the latter meaning that the amplifier's reproduction of a 10kHz squarewave was essentially perfect (fig.2).

Fig.1 Halcro dm58, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div.).

Fig.2 Halcro dm58, small-signal 10kHz squarewave into 8 ohms.

Any DC offset present in the dm58's output was below 2mV and therefore inconsequential. The Halcro's signal/noise ratio, ref. 2.83V into 8 ohms (1W), was very respectable at 82.8dB, wideband unweighted, this figure improving to 96.8dB when A-weighted. As implied by its specifications, measuring the amount of distortion present in the dm58's output was very difficult, as it was below the noise floor except at very high output powers. Fig.3 shows the THD+noise percentage plotted against frequency at 157W into 8 ohms. It is 0.0006% or less across most of the band, rising very slightly above 10kHz. I can't swear to the accuracy of this figure, however; even at this high power, the Halcro's intrinsic distortion level appears to be of the same order as the output of the Audio Precision System One signal generator!

Fig.3 Halcro dm58, THD+noise (%) vs frequency at 157W into 8 ohms.

The bottom trace in fig.4 shows the waveform of the distortion and noise with a 1kHz sinewave signal at the same level used for fig.3. The oscilloscope's input was set to its maximum sensitivity for this graph, but even so, most of what can be seen is noise, even when 32 separate readings were averaged to drop the noise content by 15dB. Halving the load to 4 ohms, with the output power now 310W, did give a residue waveform that is recognizable as distortion (fig.5). It appears to be almost pure third-harmonic in nature.

Fig.4 Halcro dm58, 1kHz waveform at 157W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 Halcro dm58, 1kHz waveform at 310W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

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