ABSTRACT
Some sort of preamplifier or control unit is required in all hifi systems, even if its only function is to select the source and set the volume. Consider also that many power amplifiers have RC filters at the input, not so much for EMC immunity, but more as a gesture against what used to be called ‘TID’ but is actually just old-fashioned slew-limiting and highly unlikely in practice. There is at least one passive preamplifier on the market that controls volume by changing the taps on the secondary of a transformer; this should give much lower output impedances. One possibility is the use of a special low-noise amplifier after the volume control, combined with a low resistance volume pot as suggested above. The noise/headroom compromise is completely avoided by replacing the combination of volume-control-and-amplifier with an active gain control, i.e. an amplifier stage whose gain is variable from near-zero to the required maximum.
