Quote:
Originally Posted by griff2 /img/forum/go_quote.gif
... you only have to drop the voltage down from the speaker outs to feed headphones - i.e. a simple resistor in series would do the trick.
|
It is best to drive any dynamic driver with the lowest impedance you can. Thus, it really isn't a good idea to add a resistor in series with any amplifier output. Look at "damping factor."
Since most headphones designed for home listening (as opposed to portable types) are fairly inefficient, and can are perfictly happy being driven by the speaker outputs of most moderate power amplifiers, it is generally fine to do just that.
When you first crank things up, turn everything on with the volume all the way down. Then carefully and slowly advance the volume control. Don't fry your 'phones or your ears!
If you DO need to pad down the level, put a voltage divider between the source and the input of the amp.
We use Crown D-75 amps as headphone amps in radio stations all the time, and that is to the speaker output, not the headphone jack. Since both "black" terminals are ground referenced, that is fine and doesn't require rewiring the headphones.
WHY, you might ask? Crown amps are just bullet proof. A broken headphone amp in a radio station is a real problem as the jock cannot hear himself or the music once he turns on the mic as that mutes the speakers.
Back to the issue of damping factor: Dynamic drivers are braked (stopped) by the low impedance of the amplifier stage. That low impedance sinks the current generated by the moving driver, making it stop. This makes sure the driver generates an accurate representation of the music. If there is resistance, the amplifier output stage cannot sink the current from the driver and can continue to move when it should stop, or even continue in the wrong direction for a brief time.
Instead of having the current be absorbed by the output stage, it is turned into heat in the resistor.
Sadly, many receivers and integrated amplifiers DO put resistors in series with the headphone outputs.
