Classé Omega line-level preamplifier SIDEBAR 3: Measurements
Unless otherwise noted, the measurements presented are for balanced operation, and were taken with the level control at maximum.
The output impedance of the Omega at its line output measured 96.7 ohms (47.9 ohms unbalanced) in the left channel, maximum gain, with negligible differences between channels and at various level-control settings. The line-level input impedance measured 25.1k ohms in the left channel and 24.3k ohms in the right (21.2k ohms unbalanced, left channel). This dropped to 13.7k ohms at unity gain (36.0 on the level control), and 11.3k ohms at a level-control setting of 20. The output impedance at the tape output is 989 ohms and varies only 0.2 ohms between a 50 ohm and a 600 ohm source impedance, indicating that the tape outputs are buffered.
DC offset at the Omega's outputs measured 0.5mV in the left channel, 0.2mV in the right (and 1.4mV and 2.2mV unbalanced, left and right, respectively). The preamp is noninverting from its line inputs to its main outputs in the unbalanced mode; in the balanced mode, pin 2 is positive. Maximum voltage gain was 7.54dB balanced, 6.9dB unbalanced—lower than specified. S/N (ref. 1V) measured 97dB unweighted over a bandwidth of 22Hz-22kHz, 83.2dB unweighted from 10Hz to 500kHz, and 99.4dB A-weighted.
The Omega's frequency response is shown in fig.1. Since the 50kHz limit of our normal plot would indicate that the frequency response is rising at that point, we have extended the frequency response here out to 200kHz to better show the significance of the ultrasonic peak. Its maximum of just over +1dB (left channel) above 100kHz will be audibly irrelevant. The response is similar in unbalanced mode (not shown), but with the peak above 100kHz approximately 0.2dB lower in magnitude. Measurements taken at other level settings indicate no significant change in frequency response and excellent volume-control tracking.
Fig.1 Classé Omega, frequency response into 100k ohms, balanced mode (0.5dB/vertical div.).
Fig.2 shows the Omega's crosstalk. It increases with frequency in typical fashion, but the separation overall is so good that this will be of no practical significance. The input voltage used for this measurement was high enough—820mV—to keep the effect of noise on the readings to a minimum.
Fig.2 Classé Omega, crosstalk (from top to bottom): L-R, R-L, balanced mode (10dB/vertical div.).