McCormack Power Drive DNA-1 power amplifier 1992 Measurements part 2
Fig.5 McCormack DNA-1, 1kHz waveform at 107W into 4 ohms (top), noise and distortion waveform with fundamental notched out (bottom, not to scale).
This analysis is confirmed by fig.6, a spectral analysis of the harmonic products created when the DNA-1 reproduced a 50Hz test signal at 72W into 4 ohms. The presence of harmonics is readily apparent as the peaks to the right of the 50Hz test tone. In fact, the entire harmonic series is visible, from the second (100Hz) all the way to the tenth (500Hz). Previous experience has indicated that amplifiers with strong upper-order harmonics tend to sound harder and brighter than amplifiers whose harmonic products are primarily second or third harmonic (footnote 1). This generalization was at odds with the auditioning; I found the DNA-1 smooth and silky in the treble. In fact, the DNA-1 had a noticeably softer treble than the Parasound HCA-2200 also reviewed in this issue, which had a distortion spectrum almost devoid of harmonic products above the third harmonic (footnote 2).
Fig.6 McCormack DNA-1, spectrum of 50Hz sinewave, DC-1kHz, at 72W into 4 ohms (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level at -56dB (0.15%).
Similarly confounding, a spectral analysis of the DNA-1's intermodulation products (fig.7) when reproducing an even mixture of 19kHz and 20kHz at two-thirds rated power into 4 ohms showed a relatively strong 1kHz component (20kHz minus 19kHz). (The 1kHz intermodulation product is 60dB below the test signal; the products above 21kHz are due to the CD player source for this test.) Again, compared with the exemplary performance of the HCA-2200 on this test, one would expect the HCA-2200 to have a more natural presentation of instrumental timbres. This was not the case, however, demonstrating the need for listening rather than relying purely on measurement to assess an amplifier's suitability for long-term musical enjoyment.
Fig.7 McCormack DNA-1, HF intermodulation spectrum, 300Hz-30kHz, 19+20kHz at 72W into 4 ohms (linear frequency scale).
One area where the DNA-1 performed superlatively was in its ability to deliver current to low-impedance loads and maintain nearly perfect voltage-source behavior. With one channel driven, its maximum power output (the onset of clipping—defined as 1% THD) was 191W into 8 ohms (22.8dBW), 352W into 4 ohms (22.5dBW), and 510W into 2 ohms (21.1dBW). With both channels driven, the DNA-1 put out 180W and 183W (22.6dBW, 8 ohms) and 331W and 333W (22.2dBW, 4 ohms) at clipping from the left and right channels, respectively. The very small drop in dBW value (from 22.6 to 22.2dBW) when switching from an 8 ohm to a 4 ohm load is extraordinary. The fact that the line voltage dropped from 117V at 8 ohm testing to 116V at 4 ohm testing and 114V at 2 ohm testing may indicate that the DNA-1 may be able to act as a virtually perfect voltage source if supplied a constant 120VAC from the wall.
The power output vs distortion plots are shown in fig.8, with the traces representing (from left to right) 8, 4, and 2 ohm loads. The 2 ohm trace stops a little prematurely because I shut off the signal generator to prevent the main AC fuse from blowing. Previous attempts at full power output from both channels into 2 ohms caused the 8 amp slow-blow fuse to pop. This occurred beyond the clipping point, however; it's unlikely that this fuse would blow during normal use, even when the amplifier is called on to deliver high power into a low-impedance load. These measurements suggest that the DNA-1 will have no difficulty driving even the most difficult of loads. Further, the manufacturer's power-output ratings of 150W into 8 ohms and 300W into 4 ohms are very conservative.
Fig.8 McCormack DNA-1, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.
Another interesting feature of fig.8 is the distortion curves' shapes. Rather than maintain low distortion until the clipping point and then going into severe distortion, the DNA-1's "soft clip" behavior can be seen to increase the distortion gently before the onset of clipping.
Finally, an unusual incident prevented me from measuring the DC offset at the DNA-1's output. After I'd finished the auditioning and measurements for this review (except DC offset, which I do in my listening room), I was listening for pleasure when the AC power momentarily shut down. It was so short that all the clocks in the house (but one) maintained the correct time. The playback system stopped, but never came back on: two resistors in the DNA-1 burned, tripping the protection circuitry. Often, power shutdowns are accompanied by high voltage surges when power is restored. I don't know if this was a fluke, or if the DNA-1 is susceptible to damage from over-voltage AC supplies.
Overall, the DNA-1's measured performance was somewhat enigmatic; the amplifier's higher-order distortion products didn't correlate to the listening impressions of a smooth tonal balance and natural reproduction of instrumental timbres. In the area of power output and ability to deliver current to low impedances, however, the DNA-1 was exemplary.—Robert Harley
Footnote 1: See my "Follow-Up" on the VTL 225 with KT-90 output tubes in Vol.14 No.10, for example.—Robert Harley
Footnote 2: Stereophile's Test CD 2, which will be available this coming May, includes a series of distortion demonstration tracks which will allow the effect of pure second-, third-, and seventh-harmonic distortion to be auditioned at levels from 10% down to 0.03%. The Test CD also features signals with distortion spectra representing "tube"-type THD, "solid-state"-type THD, and a mixture of harmonic and subharmonic distortion typical of a planar speaker driven at high levels. My experience with generating the signals for the disc using the Audio Precision System One Dual Domain suggests that the situation is more complex than RH describes. While the DNA-1 does have some upper partials present, these are not isolated but are accompanied by the lower partials in an almost regular descending series. This is typical of tube amplifier performance, tending to sound smooth and "fat" rather than grainy.—John Atkinson