On Tue,8/26/2014 6:33 PM, Bob McGraw - K4TAX wrote:
While the radio may indicate an input impedance of 10 k ohms at 1 kHz,
then one might ask what is the input impedance at 300 Hz. or some
other frequency? Another model radio specification states "Microphone
Input: 200 - 50K ohms" and then another model radio states
"Microphone 2 K ohms - condenser". These numbers range from 200 ohms
to 50 K ohms.
Bob,
Hams do WAY too much handwringing about impedances in audio circuits.
Also, you're assuming more knowledge of audio by the folks who write the
data sheets for ham gear. MANY mfrs still think that all pro audio is
600 ohms in and out, and that impedances are matched between stages.
That has not been true for nearly 40 years (although I've talked to
designers of broadcast transmitters who didn't know that)! The only
ways to know the loading Z of an input are to study the schematic or
measure it. What mfrs often mean is that the product is designed for
mics of a given range of impedances. And often they don't even have a clue.
As to Z and 300 Hz vs. 1,000 Hz -- the load Z of virtually audio stages
is pretty flat in the audio spectrum, certainly from 100 Hz to 10 kHz
unless some effort has been made to modify the frequency response by
loading (not generally a good idea). Now, we SHOULD consider impedances
above and below the audio spectrum, but that's a circuit designer's
problem, not ours.
The output Z of nearly all dynamic or electret mic used by hams is on
the order of a few hundred ohms, and the mic wants to see a minimum load
Z of 5X that value. So as long as the loading by the input stage is more
than about 800-1,000 ohms, the mic is quite happy. Remember that the
output stage of an electret is simply a FET follower that transforms the
high Z of element to that few hundred ohms range. When we add the 5K
resistor from V+ to the mic output, we are simply providing operating
voltage for the FET!
There's a tutorial on my website about this. It's called something like
Ham Interfacing.
73, JimK9YC
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