[Elecraft] LINE OUT output too low?

[Jim Brown]

On Wed, 17 Dec 2008 19:38:34 -0000, David Cutter wrote:

>600ohms still seems to be the practice for ship and aircraft intercoms that 
>I've come across recently.

Maybe, maybe not. Often the people the write the data sheets are marketing 
dweebs, and have no technical background. 

I don't know anything about aircraft electronics practices. My observations 
are based on actual measurements of input and output impedances, and/or 
studying the schematics. 

Often, a data sheet says 600 ohms and all that they mean is that the output 
stage can be loaded with 600 ohms and meet its specs. When you measure it, 
you find a 100 ohm output impedance. Often, a data sheet will say 600 ohms 
because the marketing dweeb (or even the engineer) thinks that pro stuff is 
600 ohms. It is not, and has not been since the days when tubes ruled. 

It's also common for engineers in industries unrelated to audio (like RF and 
vdieo) to apply transmission line methods to audio. That, of course, is 
completely inappropriate. The only audio circuits that are long enough to 
behave as transmission lines are VERY long telephone lines. At audio 
frequencies, the characteristic impedance of ANY practical transmission line 
is complex (that is, R + jX), and varies widely with frequency. There's a 
tutorial about this on my website. 

73,

Jim K9YC