Crown Macro Reference power amplifier Measurements
A full set of measurements of the Crown Macro Reference was made in its balanced mode, with selected measurements repeated for the unbalanced mode.
Running our standard 1/3-power, one-hour preconditioning test on the Crown proved a real challenge. After about a half hour of the test load absorbing over 500W from the Crown (253Wpc), a strong smell of burning plastic filled the lab. No, the Crown had not self-destructed—its built-in, variable-speed fan kept the amplifier itself no more than moderately warm. But our test load had become hot enough to fry eggs; the smell was coming from the top surface of our test bench, now blistered by the bottom of the test load's metal case. Shortly thereafter, the circuit breaker feeding the outlets serving our test equipment popped. Inspection of the test load revealed internal leads shorted out from melted insulation. At some time during that +30-minute torture test, the test load had turned itself into a 4 ohm load instead of an 8 ohm one, dramatically increasing the power output it was receiving from the Crown. Still, the Macro Reference itself remained totally unruffled, its top almost cool; the only heat of any significance was being blown out the amplifier's side vents by its fan. At this point, I declared the preconditioning test passed, having no desire to again subject our test load to this exercise.
Following repair of the load (the damage was less than I had initially feared), the bench tests continued. The Crown's input impedance varied only slightly with the setting of the input level control, and measured a minimum of just under 9.7k ohms (with the controls at 9:00) in the balanced mode. In the unbalanced mode, level control at maximum, input impedance measured just over 10k ohms. This relatively low value suggests that care be taken in selecting the preamp or other source used to drive the Crown. Most sources should create no problem here, as their output impedance is generally well below 1k, but there are a few—largely early, inexpensive CD players and the occasional tube preamp—which measure higher.
The gain of the Macro Reference was 26dB in balanced or unbalanced mode. The Crown's output impedance (including 6' speaker cables) was 0.02 ohms or less at any frequency—a value which will insure no adverse, frequency-response–altering interaction with any possible loudspeaker load. DC offset measured 1.0mV in the left channel, 0.5mV in the right. Noise (unweighted, ref. 1W into 8 ohms) measured –88dB, either balanced or unbalanced. The Crown is noninverting in the unbalanced mode; in the balanced, pin 2 is configured as the positive leg, pin 3 the negative.
Fig.1 shows the frequency response of the Macro Reference driven both from its balanced inputs and from its unbalanced inputs. Note the slightly more rapid HF rolloff in the balanced configuration, though at –0.5dB at 20kHz, this wouldn't be nearly sufficient to explain LL's comments about the amplifier's dark sound. The waveform of a 10kHz squarewave in the unbalanced mode is shown in fig.2. The gradual high-frequency rolloff is visible as a slight rounding of the leading edges. The textbook-perfect 1kHz squarewave response is not shown.
Fig.1 Crown Macro Reference, frequency response at 2.83V into (from bottom to top at 2kHz): 8, 4, 2 ohms, balanced input; 8, 4, 2 ohms, unbalanced input (0.5dB/vertical div., right channel dashed).
Fig.2 Crown Macro Reference, small-signal 10kHz squarewave into 8 ohms.
The balanced crosstalk shown in fig.3 indicates some asymmetry (a difference in crosstalk from left to right and right to left), small enough in degree to be of little audible significance. The asymmetry was a bit higher, though still inconsequential, in the unbalanced mode (not shown). Interestingly, the low-frequency separation was actually better in the unbalanced mode—not what you'd expect. The increase in crosstalk with increasing frequency is, as always, typically the result of capacitive coupling between channels.
Fig.3 Crown Macro Reference, balanced input, channel separation L–R top, R–L, bottom (10dB/vertical div.).
Fig.4 shows the THD+noise vs frequency for the balanced mode; the unbalanced mode, not shown, is a virtual overlay of this graph. These are very fine results, with a typical but not excessive rise at lower impedances and higher frequencies. Much of the "distortion" visible here at and below 1kHz is simply noise. The 1kHz distortion waveform normally shown is not reproduced here—it confirms that the result was indeed largely noise.
Fig.4 Crown Macro Reference, THD+N (%) vs frequency at 2.83V into (from top to bottom: 8, 4, 2 ohms (right channel dashed).
The spectrum of the Crown's output with a 50Hz input at 800W (that's not a typo) output into a 4 ohm load (2/3 rated power at that load) is shown in fig.5. Despite the very high output level, the distortion components are extremely low here—the highest being the third harmonic at a negligibly low –94.5dB, or about 0.002%. Fig.6 shows the response to a combined 19+20kHz signal—the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies—at 1118W peak into 4 ohms. Again, the highest artifact here is very low in level: –79dB, or about 0.01% at 18kHz and 21kHz. The plot for an 8 ohm load is not shown—the artifacts are even lower in level.
Fig.5 Crown Macro Reference, spectrum of 50Hz sinewave, DC–1kHz, at 800W into 4 ohms (linear frequency scale).
Fig.6 Crown Macro Reference, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 1118W peak into 4 ohms (linear frequency scale).
Fig.7 shows that, at a slightly higher power output—1184W peak into 4 ohms and still prior to clearly visible signs of clipping in the waveform—the distortion spectrum, while still quite low (less than 0.01% at 1kHz) is beginning to get messy. But it may be academic; you're never going to put thios much power at 19kHz and 20kHz into any loudspeaker load and have a tweeter that will live to tell about it.
Fig.7 Crown Macro Reference, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 1184W peak into 4 ohms (linear frequency scale).
The Macro Reference's 1kHz THD+noise vs output power curves are shown in fig.8. (The Crown's discrete clipping levels (at 1% THD+noise) are shown in Table 1.) The curves are self-explanatory, with low distortion up to the curve breakpoint or knee and a rapid rise above that. The power levels here are daunting; I feared again for the safety of our test load, though the output levels shown were only held long enough to get the measurements.
Fig.8 Crown Macro Reference, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms, one channel driven.
Table 1: Crown Macro Reference Clipping (1% THD+N at 1kHz)
|Both Channels Driven||One Channel Driven|
|Load||W (dBW)||W (dBW)|
|8||717 (28.6)||717 (28.6)||791 (29.0)|
|4||1050 (27.2)||1040 (27.2)||1178 (27.7)|
Finally, I measured the THD+noise vs power for the Crown in its two mono modes—bridged (fig.9) and parallel (fig.10). Note that the bridged mode is recommended for loads above 4 ohms, the parallel mode for loads below 4 ohms. While the Macro Reference will put out huge quantities of power in these modes, I doubt if many home listeners will have need of either of these mono configurations.
Fig.9 Crown Macro Reference, bridged-mono mode, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms, one channel driven.
Fig.10 Crown Macro Reference, parallel-mono mode, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms, one channel driven.
Whatever LL's feelings about the sound of the Macro Reference, its test-bench measurements would be hard to criticize.—Thomas J. Norton