[Elecraft] headphone impedance

[Jim Brown]

On Mon,12/28/2015 5:10 AM, Barry N1EU wrote:
> I've got some 300-ohm headphones and wondering if the impedance alone makes them a poor choice for my K3s?

As others have told you, it's not an issue. For at least 50 years, audio 
equipment has low impedance outputs that are designed to work into 
relatively high impedances. In audio, we don't "match" impedances like 
we would in RF. Power amplifiers, of which headphone amplifiers are a 
special set, are typically designed to provide 1V or so into any Z 
greater than about 100 ohms.

Think of it this way. High Z phones are designed to require more voltage 
but less current, while low Z phones require less voltage but draw more 
current. The IR drop in the output stage will reduce the voltage to 
low-Z phones, but they don't need as much voltage to be loud enough. And 
there will be little or no IR drop to higher Z phones, so they see the 
full 1V output.

Bottom line -- nearly all modern headphones are designed to work with a 
1V source.

There's discussion of these issues in

http://k9yc.com/GroundingAndAudio.pdf

73, Jim K9YC