The big McCormack more than meets its specification under continuous drive conditions with both channels driven (fig.7). Defining clipping as 1% THD+N, the maximum power raised was 262W into 8 ohms (24.2dBW), 430W into 4 ohms (23.3dBW), and 795W into 2 ohms (23dBW, only one channel driven). The shape of the traces in this graph reveals that noise dominates the measured figure until quite high levels.

Fig.7 McCormack DNA-225, distortion (%) vs continuous output power into (from bottom to top at 2kHz): 8 ohms, 4 ohms, and 2 ohms (both channels driven).

With a low-duty-cycle 1kHz toneburst, which generates results that more closely resemble what happens when the amplifier is amplifying music, the DNA-225 was revealed as a powerhouse. Fig.8 again plots the change in THD with output power (noise not now included), with the 1% clipping point shown as the horizontal magenta line. No less than 303W were raised into 8 ohms (black trace), with 575W available into 4 ohms (red), 1018W into 2 ohms (blue), and 1532W into 1 ohm (green).

Fig.8 McCormack DNA-225, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

But that's not the end of the story. I plotted this graph using a decibel vertical scale rather than the usual percentage scale, which more clearly shows what happens at high powers. The McCormack actually clips very gently, meaning that to pick the 1% THD point is more arbitrary than with an amplifier that hard-clips. Under real-world conditions into low impedances, the DNA-225 rolls up its sleeves and puts out serious current—more than 40A into 1 ohm!

The McCormack DNA-225's measured performance reveals a good balance between the designer's use of what must be a relatively low level of overall negative feedback and the gain/noise/distortion tradeoff. Especially considering its price, the amplifier behaves impressively close to a voltage source under the dynamic conditions of music.—John Atkinson