McCormack Micro Integrated Drive headphone amplifier Measurements
All of McCormack's measurements were made from the rear panel binding posts. It didn't invert polarity. Its input impedance at 1kHz was 27.4k ohms at full volume (slightly higher at lower settings of the control). Its output impedance was below 0.04 ohms at low and mid frequencies, increasing to a maximum of 0.125 ohms at 20kHz. The output impedance at the line outputs was 99 ohms in the left channel, 101 ohms in the right.
S/N measured an outstanding 108.7dB (22Hz–22kHz, unweighted, ref. 1V). DC offset measured 0.4mV in the left channel, 0.5mV in the right (0.1mV and 0.6mV, respectively, at the line outputs). The tracking of the McCormack's volume control was good, deviating no more than 0.17dB down to half rotation and 0.46dB at one quarter. Voltage gain at the maximum setting of the control was 5.7dB (low gain), 13.7dB (medium gain) and 27.1dB (high gain).
(The remainder of the measurements were taken at medium gain). Since the McCormack is designed to be used not only as a headphone amplifier, but also as a low power amplifier for driving loudspeakers (and additionally as a minimal line preamplifier), I took a number of its measurements into typical loudspeaker load impedances. Fig.1 shows its frequency response into 8 ohms, 4 ohms (the latter not recommended by the manufacturer), and our simulated real loudspeaker load. There is little to comment on here.
Fig.1 McCormack Micro Integrated Drive, frequency response at (from top to bottom at 50kHz): 1W into 8 ohms, 2.83V into simulated speaker load, and 2W into 4 ohms (5dB/vertical div.).
The almost perfect 1kHz squarewave response is not shown. The 10kHz squarewave (fig.2) is a solid result, with very good risetime, minimal rounding, and no overshoot or ringing. The crosstalk is shown in fig.3. The only thing worthy of comment here is the slightly different separation for each channel, which at these levels is unlikely to have any audible consequences.
Fig.2 McCormack Micro Integrated Drive, small-signal 10kHz squarewave into 8 ohms.
Fig.3 McCormack Micro Integrated Drive, crosstalk (from bottom to top at 1kHz): L–R, R–L (10dB/vertical div.).
Fig.4 plots the THD+noise against the output power in watts of the Micro Integrated Drive. The 2W power output into a 40 ohm load will be more than adequate to drive virtually any headphones. Into 8 ohms, the McCormack's discrete clipping point (1% THD+noise, 1kHz) was 4.8W (6.8dBW) into 8 ohms and 4.2W (3.2dBW) into 4 ohms, one channel driven. With both channels driven, the 8 ohm clipping was 2.7W (4.3dBW) left and 2.5W (4dBW) right. Into 4 ohms, the latter figures were 1.9W and 1.8W (–0.45dBW), respectively. (The AC line voltages for these measurements ranged from 115 to 117V.)
Fig.4 McCormack Micro Integrated Drive, distortion (%) vs output power into (from bottom to top at 1W): 40 ohms, 8 ohms, and 4 ohms.
The variation of THD+noise with frequency for an output of 4V into 40 ohms is shown in fig.5. The THD is relatively high into loudspeaker loads, but notably better into a 40 ohm load at 100mW, this more typical of headphone use. The THD waveform at 100mW (1kHz input) into the same 40 ohm load is shown in fig.6. It is primarily second-harmonic, but with some higher-order components also present. At 1W into either an 8 ohm or a 4 ohm load, the primary component becomes third-order but with clearly evident higher-order components (not shown).
Fig.5 McCormack Micro Integrated Drive, THD+noise vs frequency at (from top to bottom at 1kHz): 1W into 4 ohms, 1W into 8 ohms, and 100mW into 40 ohms (right channel dashed).
Fig.6 McCormack Micro Integrated Drive, 1kHz waveform at 100mW into 40 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Driving the McCormack with a frequency of 50Hz at an output of 3.3W into 8 ohms results in the distortion spectrum shown in fig.7. All of the components are below –90dB (0.003%). Fig.8 shows the output intermodulation spectrum for an input of 19+20kHz at 2.3W into 8 ohms—just below the point at which clipping is visible on a 'scope trace. The artifact at 1kHz is at –82dB (0.008%); the largest artifact is at 18kHz, at –65dB or about 0.06%.
Fig.7 McCormack Micro Integrated Drive, spectrum of 50Hz sinewave, DC–1kHz, at 3.3W into 8 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level.
Fig.8 McCormack Micro Integrated Drive, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 2.3W into 8 ohms (linear frequency scale).
The test bench measurements of the Micro Integrated Drive were excellent if your intent is to drive headphones. While it will likely perform adequately in driving loudspeakers for casual listening, its power limitations should caution against unrealistic expectations for that application.—Thomas J. Norton