[TenTec] Ear Phone Audio

[Jim Brown]

On Wed,8/24/2016 10:08 AM, Robert P. Santella wrote:
> Impedance needs to be 30 ohms.

NO! Headphone impedance is NOT critical. Audio power amplifiers are VERY 
different from RF power amplifiers. Virtually all audio amplifiers are 
low impedance sources, and are designed to drive any impedance greater 
than a design minimum. For example, the source impedance of a 
loudspeaker output stage is a small fraction of an ohm, and is designed 
to drive any loudspeaker load greater than 4 ohms. The relationship 
between these two impedances is called the "damping factor," and 100 is 
a typical value. A well-designed headphone amp will include a low-value 
resistor in series to protect the amplifier from being shorted when 
headphones are being plugged and unplugged.

Most modern headphones are in the range of 20 - 300 ohms, and a 
well-designed headphone amp should drive all of them just fine. What 
matters a lot more is voltage sensitivity, and headphone mfrs generally 
get that right too. Many years ago, I carried around a set of crystal 
headphones in my toolkit when I worked on troubleshooting audio systems 
in buildings. They were sensitive enough that I could hear (weakly) the 
signal of an unamplified microphone, yet the impedance was high enough 
that they didn't load most circuits!

73, Jim K9YC