[Elecraft] Mic and headphone

[Jim Brown]

On 4/14/2014 4:46 PM, Harry_Yingst wrote:
> Phone lines are Typically 600 ohms

Actually, the Zo of phone lines at radio frequencies is about 80 ohms, 
and varies widely with frequency in the audio spectrum. That 
characteristic is determined by the diameter, spacing, and insulation of 
the twisted pairs used to carry the signal. They have not been 600 ohms 
for at least 75 years. More than 40 years ago, it was standard practice 
to equalize broadcast lines for extended frequency response over 
relatively short paths, and transformers were used to step that 
impedance up to the 600 ohms used by older broadcast equipment. But the 
broadcast and pro audio world abandoned 600 ohms as a standard about 50 
years ago.

Modern audio circuits (since we converted from hollow state to solid 
state) use low impedance sources to drive high impedance imputs, there 
is no impedance "matching" except to the extent that the load impedance 
must be high enough that it does not draw any more current than the 
output stage is designed to provide.

73, Jim K9YC