McCormack DNA-225 power amplifier Measurements

Sidebar 3: Measurements

After a one-hour preconditioning period at one-third power, which maximally works an amplifier with a class-B output stage, the McCormack's heatsinks were too hot to keep my hand on. After another 30 minutes of continuous high-power testing, the DNA-225's thermal trip circuit turned off the power until the amplifier had cooled down.

The DNA-225's input impedance was high at 91k ohms, dropping slightly to 73k ohms at 20kHz. The voltage gain into 8 ohms was higher than usual, at 30.5dB. Both of these factors will make the McCormack a good choice for use with a passive volume control, though pairing it with a typical tube preamplifier will result in too much system gain. The amplifier didn't invert absolute polarity.

The output impedance was very low, at 0.07 ohms at lower frequencies and 0.1 ohm at 20kHz. As a result, any modification of the amplifier's frequency response due to interaction between its source impedance and the loudspeaker impedance will be minimal (fig.1). The response is sensibly arranged to roll off above the audioband, reaching -3dB around 140kHz, which slightly increases the risetime on a 10kHz squarewave (fig.2). Channel separation (not shown) was below the noise floor in the midrange and bass, but diminished at the usual 6dB/octave above 1kHz to reach 66dB (R-L) and 58dB (L-R) at 20kHz; both figures are quite acceptable.

Fig.1 McCormack DNA-225, frequency response at (from top to bottom at 20kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, and 2W into 4 ohms (0.5dB/vertical div.).

Fig.2 McCormack DNA-225, small-signal 10kHz squarewave into 8 ohms.

The unweighted S/N ratio, measured over a wide 10Hz-500kHz bandwidth and referred to 1W into 8 ohms, was a slightly low 67dB, this improving to 83.2dB with A-weighting. Because of this slightly higher noise, I plotted the change in THD+noise with frequency at 10W into 8 ohms rather than the usual 1W, in order to reduce the contribution of the noise. The results are shown in fig.3. Reducing the load impedance to below 1kHz makes little change in the distortion content, but there is more of a change in the treble, as well as an overall increase in THD at high frequencies. While the THD+N figure only just rises above the 0.1% or -60dB level above 20kHz, this HF rise does suggest that the amplifier has a limited open-loop bandwidth, with only moderate negative feedback, despite the low output impedance.

Fig.3 McCormack DNA-225, THD+noise (%) vs frequency at (from top to bottom at 4kHz): 2.83V into simulated loudspeaker load, 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms (right channel dashed).

The residual distortion waveform (fig.4) has considerable third-harmonic content, this also apparent from the spectrum of a high-level low-frequency tone (fig.5). The third harmonic doesn't change in level as the load impedance drops, but is joined by increasing amounts of the second harmonic and by increasing 60Hz, 120Hz, 180Hz, and 240Hz components, due to the higher stress on the power supply. Nevertheless, the overall distortion and level is low: Even at this high power, the individual harmonic and power-supply components sum to 0.04%. The decreasing amount of corrective feedback at ultrasonic frequencies leads to more intermodulation products than usual with the punishing high-level mix of 19kHz and 20kHz tones (fig.6), but these products are all at very low levels.

Fig.4 McCormack DNA-225, 1kHz waveform at 20W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.5 McCormack DNA-225, spectrum of 50Hz sinewave, DC-1kHz, at 100W into 8 ohms (linear frequency scale).

Fig.6 McCormack DNA-225, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 213W into 4 ohms (linear frequency scale).

2733 Merrilee Drive
Fairfax, VA 22031
(703) 573-9665