Just to, if you=92ll pardon the expression, amplify what David said, there =
are two schemes for the relationship of impedances in an audio circuit.
Originally audio came from the art of telephony, where lines were miles lon=
g and electronic reflections had to be avoided. In that environment the bes=
t scheme was impedance matching. Telephony was the source for the =93600 oh=
m line=94 connection where the source impedance was 600 Ohms and the load i=
mpedance was 600 Ohms. That ensured maximum power transfer with minimum ech=
o. A disadvantage of impedance matching is the voltage delivered to the loa=
d is cut in half (6 dB loss).
The second connection scheme was called =93bridging,=94 where the load impe=
dance is a multiple of like ten times the source impedance and doesn=92t lo=
ad it down significantly, i.e. the load doesn=92t pull down the level of th=
e source. This would be used for splitting off a monitoring feed from a 600=
Ohm line.
Since the 1970s impedance matched lines were no longer used for audio. Impe=
dance matching is still essential for radio-frequency lines, and that inclu=
des digital audio transmission. All analog audio sources now are low impeda=
nce (typically 50 Ohms) and all loads are high impedance (2K to 10K). The e=
xceptions are dynamic mics that are typically 150 Ohms sources, and PIP mic=
lines that are typically 2K-3K sources.
-Dan Dugan
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