Mark Levinson No.38S preamplifier Level Matching
It's essential in listening comparisons to match levels to at least within 0.1dB. A component that's, say, 0.2dB louder than another won't be heard as being louder. Instead, listeners will comment on greater apparent detail or better dynamics.
Stereophile's recommended means of matching levels is to choose a volume-control setting for the comparison, then play a reference tone from a CDI included a 1kHz tone at 20dBFS on all three Stereophile Test CDs for this purpose. An AC voltmeter is then used to measure the voltage present at the power amplifier's speaker output terminals: as long as the levels for the two upstream components are within 1%, the sound-pressure levels will be matched to within 0.1dB (footnote 1).
The difficulty lies in repeatably setting the volume control, which is where the Mark Levinson No.38 and '38S are reviewers' dreams. The ability of the '38-series volume control to be switched in accurate 0.1dB steps, coupled with the fact that each input can also have its sensitivity preset in 0.1dB steps, makes level-matching trivially easy.
For this review, for example, at my chosen listening level for the critical comparisons, the 1kHz, 20dBFS tone gave a level measured at the amplifier terminals of 2.114V RMS ('38 and '38S) and 2.119V RMS (SFL-2) for the first two sets of comparisons, and 2.142V (Threshold) and 2.137V ('38S) for the third comparison. In all cases, the levels at 1kHz were matched to within 0.025dB. I could be sure, therefore, that any differences I perceived between the preamps were not due to a level imbalance.John Atkinson
Footnote 1: Matching at a single frequency assumes that the components have flat, or at least identical, frequency responses. When they don't, as is the case with loudspeakers, we recommend matching the B-weighted sound-pressure levels measured with a noise signal.