McCormack Power Drive DNA-1 power amplifier 1992 Measurements

Sidebar 3: 1992 Measurements

The DNA-1 ran relatively cool, even after the one-hour preconditioning period at one-third rated power into 8 ohms. Although the heatsinks were hot to the touch, they were cooler than most amplifiers subjected to this conditioning.

Input impedance at 1kHz was a calculated 99.5k ohms, consistent with the manufacturer's specification of 100k ohms. The DNA-1's gain was very high, an input voltage of 100mV at 1kHz producing an output voltage of 4.18V. This gain ratio corresponds to a decibel gain of 32.4dB, implying a sensitivity for full output of 828mV. This is one of the highest gain figures I've encountered in a power amplifier, and coupled with the DNA-1's high input impedance, makes it a good candidate for use with a passive level control.

Output impedance, calculated by measuring the rise in output voltage as the load is changed from 8 or 4 ohms resistive to open circuit, was very low at less than 0.01 ohms across the band, rising to 0.03 ohms at 20kHz. S/N ratio was 72.6dB unweighted, referenced to 1W into 8 ohms—fairly good, but not extraordinary. (Again, applying an "A" weighting curve will increase this figure.) Driving the DNA-1 with an asymmetrical raised cosine waveform and looking at the output on an oscilloscope revealed that it does not invert absolute polarity.

Frequency response (fig.1) was predictably flat, rolling off 0.4dB at 50kHz. Interchannel crosstalk, shown in fig.2, was reasonably good (68dB at 1kHz). The separation vs frequency curve exhibits the common 6dB/octave increase in interchannel crosstalk due to capacitive coupling as frequency increases. The DNA-1's reproduction of a 10kHz squarewave (at 5W into 4 ohms) is shown in fig.3, revealing a slight rounding of the leading edge and the very slightest hint of ringing.

Fig.1 McCormack DNA-1, frequency response at 1W into 8 ohms (right channel dashed, 0.5dB/vertical div.).

Fig.2 McCormack DNA-1, crosstalk (10dB/vertical div.).

Fig.3 McCormack DNA-1, 10kHz squarewave at 5W into 4 ohms.

The THD vs frequency plots (fig.4) at 1W into 8 ohms, 2W into 4 ohms, and 4W into 2 ohms, show that the DNA-1's distortion increases significantly as the load impedance drops. When driving 8 ohms, THD was below 0.01% across most of the band. The distortion increased to nearly 0.02% through the midband when driving a 4 ohm load. When the amplifier saw 2 ohms, the distortion rose to over 0.1% between 200Hz and 8kHz, but decreased at the frequency extremes. This is a fairly large deviation in distortion with changing load impedance, but the distortion levels are still relatively low.

Fig.4 McCormack DNA-1, THD+noise (%) vs frequency at (from bottom to top): 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms.

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