Self on Audio, 2nd Edition

9

A balanced view, Part II

There are only two kinds of input stage – unbalanced and balanced. For interconnection this is the primary distinction. Apart from balancing requirements, a line-level input, as opposed to a microphone input, is expected to have a reasonably high impedance to allow multiple connections to a single output.

Traditionally, a ‘bridging impedance’ – i.e. high enough to put negligible loading on historical 600 Ω lines – was 10 kΩ minimum. This is still appropriate for modern low-impedance outputs. However, a higher impedance of 100 kΩ or even more is desirable for interfacing to obsolete valve equipment, to avoid increased distortion and curtailed headroom.

Another common requirement is true variable gain at the balanced input, ...