Audio Research Reference Two preamplifier Measurements part 2
Fig.6 Audio Research Reference Two, balanced HF intermodulation spectrum, DC-22kHz, 19+20kHz at 2V into 100k ohms (linear frequency scale).
Finally, the Reference Two can output very high levels of low-distortion signal into high impedances. Fig.7 shows the THD+noise percentage plotted against balanced output voltage into 100k ohms (bottom trace) and 600 ohms (top trace). The preamplifier doesn't clip (defined as 1% THD+N) until 35V into the higher load! And note how the increase in distortion above the "knee" in the traces is quite gradual, indicating soft clipping behavior.
Fig.7 Audio Research Reference Two, balanced distortion (%) vs output voltage at 1kHz into 100k ohms (right) and into 600 ohms (left).
Perhaps more important, note how the THD+N figure drops with increasing output voltage below 2.5V. This reveals that the figure below 2.5V output is actually dominated by noise, and that this noise will be lowest around the voltage where the partnering power amplifier itself starts to clip. This is sensible design. The preamplifier clips earlier into the unrealistically low 600-ohm load, but it should still drive many power amplifiers into clipping before it clips itself.
The unbalanced result is shown in fig.8. The distortion is a little higher and the maximum voltage swing about half that of the balanced mode. Nevertheless, there are still enough low-distortion volts available to drive most power amplifiers to their limits.—John Atkinson
Fig.8 Audio Research Reference Two, unbalanced distortion (%) vs output voltage at 1kHz into 100k ohms (right) and into 600 ohms (left).