INPUT/OUTPUT IMPEDANCE

Who does it? or did it? Thanks...

@oldhvymec  The best-known example IMO is Ampex in their 351 electronics, which were the tape electronics used to make many early stereo LPs like RCA Living Stereo and the like.

As George alluded to that is most likely to be an issue in the case of tube-based components, many of which use a coupling capacitor at their outputs. The impedance presented by a capacitor increases as frequency decreases, so the output impedance in the deep bass region can be much higher than the specified output impedance, which is usually based on a mid-range frequency such as 1 kHz.

In the old days tubes got around this problem (in balanced systems) by using an output transformer. This enabled the output to also not reference ground, something that is *very* tricky to do without a transformer! We developed a circuit that allows for direct-coupling of the output in the balanced domain but otherwise acts much the same way as an output transformer does, although wider bandwidth and less distortion. So there are tube preamps that can drive loads like this with flat frequency response; in fact can drive loads as low as 600 ohms.