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Questions on Output Impedance

post #1 of 2
4/1/11 at 1:20pm
Thread Starter 

I’ve tried educating myself on the science of amps and headphones, as I’d like to get an amp in the near future.

But I was confused as to output impedance and its effect on the quality of headphones and amps.

 

Is it desirable to have the lowest impedance? Or is there a cutoff point where too little impedance is bad?

 

If anyone could provide some links to amps and how they work, that’d be great. I’ve only started reading the wiki page on headphone amps, and it has these short 2 sentences on how amps work.

 

Thanks

 

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post #2 of 2
4/1/11 at 2:20pm
Ideally the output of an amp is input times gain, in other words 'wire with gain'. This means that for example with 1 V input and a gain of 2 the amp should output exactly 2 V into any load that is within specs.

This doesn't work if there's an output impedance > 0 ohms because the amp only sees the total load. For example with 16 ohm output impedance and 16 ohm headphones (both at 1 kHz) the 2 V will split about evenly. Since impedance is complex and changes with frequency this 1V : 1V ratio will also change with frequency. Depending on the impedance curves you might get bass roll-off, boosted bass, etc. ranging from inaudible up to a couple dB of deviation.

0 ohms ... won't work in practice but you can get the output impedance down to less than 1/10 or even 1/100 ohm.

It should be noted however that some manufacturers don't take this too serious or are more concerned about the volume difference you'd get if you switch from full-sized headphones to low impedance in-ears (at the same volume knob position) and therefore install ~120 ohm resistors at the output.
Edited by xnor - 4/1/11 at 2:32pm
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