Steve wrote:
"I'm not sure I understand this. If the audio amp has an 8 ohm impedance
output and we put a 100 ohm resistor in series with it, why would 2000 ohm
headphones work unless they are just more sensitive. There certainly
wouldn't be an impedance match here."
Guy's,
Modern audio amps don't have an output impedance per se which requires
matching. Instead, they amount to being a voltage source, with nearly zero
impedance. The Z of the load they work into determines the current which
will flow into that load. This is why hi-fi amps specify a lower limit on
the speakers' Z which they will work into without damage to the amp from too
high current output. Usually the lower limit is 4-ohms. So, the Z of the
headphones determines the power output for a given volume control setting
on the rig. Again, the concept of "impedance matching" here is
inappropriate. And, again, same for modern hi-fi amps which do not use
output transformers. 'Open circuits' don't harm such amps as they did in old
tube power amps with output transformers, which could arc internally. But
shorts do harm the new amps, because of their near zero internal impedance.
The effective Z at the headphone output jack is set by that series resistor.
73, Roy K6XK
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