Krell KRC preamplifier Measurements
The KRC's balanced output impedance at its line outputs measured just over 3 ohms (just over 2 ohms unbalanced). Its (line) input impedance measured just over 104k ohms (L) and just under 100k ohms (R), balanced, and just under 47k ohms unbalanced. The output impedance at the tape output was just under 45 ohms, regardless of the source impedance, indicating a buffered tape output.
The DC offset at the KRC's outputs measured under 0.3mV either channel, balanced or unbalanced. The preamp is noninverting from its inputs to its main outputs, and in the balanced mode, pin 2 is positive, pin 3 negative. Line-stage gain (aux input to line output) measured 11.2dB balanced and 5.2dB unbalanced at the low gain setting, 11dB unbalanced and 17dB balanced at the high gain setting.
The KRC's flat balanced-mode frequency response is shown in fig.1. Both high- and low-gain responses are shown; there is little difference between them. The unbalanced frequency response (not shown) is virtually identical. Volume-control tracking is very good, with a maximum channel difference of 0.4dB (at the 12:00 setting).
Fig.1 Krell KRC, balanced mode, frequency response in high-gain mode (top), low-gain mode (bottom) (right channel dashed, 0.5dB/vertical div.).
Fig.2 is a plot of the KRC's crosstalk, both balanced and unbalanced. Though excellent for both modes, one channel's crosstalk in the unbalanced mode is significantly better than the other channel's, showing a greater degree of capacitive coupling at high frequencies.
Fig.2 Krell KRC, crosstalk (from top to bottom at 2kHz): RL, unbalanced; RL, balanced; LR balanced; LR unbalanced (10dB/vertical div.).
The line THD+noise vs frequency for the KRC is shown in fig.3 (570mV input). The distortion is very low across the full range, though, as might be expected, noticeably lower in the low-gain mode.
Fig.3 Krell KRC, balanced mode, THD+noise vs frequency (from top to bottom): high-gain mode, low-gain mode; unbalanced performance virtually identical (right channel dashed).
Finally, the KRC would accept a balanced line-level input of 4.3V (low gain) before reaching 1% THD+noise (with a resulting balanced output of 15.3V). The corresponding figure for the high-gain input was 2.2V input, 15.2V output. In the unbalanced mode, the same inputs resulted in an output of approximately 7.6V at 1% THD+noise.
There were no surprises in the KRC's test-bench results. Its performance is first-rate.Thomas J. Norton