[TenTec] Mike & Eagle

[Jim Brown]

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