> Still, I think the "unclarity" resembles the debate decades ago, when
> there was a lot of talk about impedance matching, preamps and
> transformers. Some combinations between dynamic mic=B4s, input
> transformers and transistors worked, others did not, and noone
> actually knew why, even though "everybody" said they did.
The frequency response and output level of a mic will change depending on w=
hat kind of load it's driving. An understanding of the history of audio con=
nections might help to explain current practice.
The original audio craft was telephony. The talker's mic had to produce eno=
ugh power to drive the receiver at the other end, with no amplification inv=
olved. Experience taught that impedance matching carried the maximum power =
from one point to another. Pro audio adopted the practices of telephony. Th=
us line outputs had 600 ohm impedances, and inputs 600 ohms too, "matching"=
and "terminating."
Very long audio lines, meaning miles, still use terminating impedances to p=
revent the signal from being reflected back. Very high frequency lines, lik=
e video and digital audio, use terminating impedances even on short cables,=
for the same reason.
Where there was a "bus" that had to drive several loads, the concept of "br=
idging" developed, tapping the voltage off a line without loading it. Where=
as a terminating input was 600 ohms, a bridging input was perhaps 15 Kohms.
A generation later, it was acknowledged that maximum voltage transfer was a=
better goal than maximum power transfer, and all connections gradually bec=
ame bridging. In this practice, it's best to have the lowest possible imped=
ance source and a high impedance load.
A practical professional microphone circuit has a source impedance of from =
50 to 200 ohms, and a load (preamp input impedance) of 2000 ohms or more. T=
he factor of ten between source and load insures that the voltage of the so=
urce is minimally reduced by the load, i.e. less than 1 dB of loading effec=
t. I dare say all mics sound their best when bridged.
PIP mics are a higher impedance source than the balanced professional mics.=
They are typically 2 to 3 K ohms. That means ideally input impedances of 2=
0 or 30 K ohms. But there are practical limitations to how high the input i=
mpedance of a preamp can be without increasing noise, and I imagine that th=
ey are generally lower than that. Perhaps someone has measured the actual i=
nput impedances of some PIP mic preamp inputs.
-Dan
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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