On Thu, 29 Jul 2004 14:44:12 -0400, Dudley Chapman wrote:
>The balanced line and he low impedance helped keep noise pickup down,
>especially in long runs of audio cable.
Yes on both counts. But to the extent that the performance is accurately
described by a lumped parameter model (< 3 miles at 3 kHz, a half mile at 20
kHz), the impedance (50-100 ohms typical) of the output stage is the dominant
component of the circuit impedance.
If you don't believe me, look at some output circuits, and measure them --
that's what they are. (Ignore the spec sheets -- nowadays, they're written by
marketing folks, not engineers). Likewise, line level input stages are
typically
10K - 25K ohms, mic stages are typically around 1K ohms (lower to optimize
the noise figure), and the output impedance of pro mics is typically around 100
ohms. In fact, both the IEC and AES Standards for Micropones define the
impedance of a mic as 5X the recommended load impedance!
>I don't know if the phone company uses 600 ohm equipment or that
>terminology, but it's a common expression among broadcast engineers when
>referring to the balanced audio lines.
Yes, it was a common expression. But in the context of transmission line
concepts it is technically incorrect. :)
The only currently valid use of 600 ohms in pro audio (including broadcasting)
is as a minimum performance specification for the lowest load impedance that
an output stage will drive. That is, it defines the minimum current that the
output
stage should be able to supply and still meet its specs. And nealry all output
stages have several dB more headroom and less distortion if they are NOT
terminated by 600 ohms.
Until about ten years ago, there was another valid use -- as a termination for
passive equalizers. One example of these equalizers is the loading coils that
were commonly used to extend the useful working range of long lines. Another
example is the tuned broadband and narrow band (octave band and one-third
octave band) equalizers used to tune sound systems. Both of these uses,
while entirely valid and useful, are based upon lumped parameter circuit
models, NOT transmission line models. For all practical purposes, both have
been replaced by newer technologies.
Jim Brown K9YC
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