HOW TO UNDERSTAND DIFFERENT IMPEDANCE RATES?!!!

[Arthur Kloc]

Luc.vanHoye at ping.be wrote in article <33935466.1CC at ping.be>...
> I am trying to figger out if the impedance of an audio device has
> something to do with how loud its signal can go...
> 
> for example:
> 
> I have a Roland XP80 (the audio device) which allows headphones with an
> impedance between 8-150 Ohms.
> 
> Now, is it so that if I should use an 20 Ohms headphone I will probably
> get a louder signal then if I should use a headphone with an 150 Ohms
> impedance?!
> 
> Could anyone explain me how this works and what people mean if they say
> that a headphone is 'hard to drive'?! 
> 
> Second, if I have the option, is it always better to chose a headphone
> with low impedance?
>  
> I would really appreciate it if somebody could explain this to me!
> Thanks in advance!
> 
> Regards,
> 
> Luc van Hoye.
> 
It all boils down to simple Ohm's law:
- your audio device (or amplifier) is a voltage source with (usually) low
impedance, which depends on output power (and its internal supply voltage)
- so lower the headphones impedance - higher the audio output (P =
E*E*R/(RS+R) where	P is output power (delivered to your headphones)
	E is your source voltage
	RS is output impedance of audio device and
	R is headphones impedance
- there are limiting factors as maximum power that your headphones can take
( thats why headphones output in high power audio amps in connected trough
series resistor) and maximum output power that audio device can deliver
(ususally not a problem, unless you trying to connect to line output)

Hope that helps,

Arthur