Quote:
Originally Posted by
stv014
I have already explained this, but you are wrong. If you still do not believe, here is a simple test with a source that has 100 Ohm output impedance, and a 250 Ohm headphone. The left channel was connected directly to the headphone, but its level was reduced by 20 dB. On the right channel, I used a 680 Ohm potentiometer as a serial resistor, and a 27 Ohm parallel resistor. I adjusted the potentiometer so that the levels were matched. At that setting (~220 Ohm), the source "saw" a roughly similar impedance load on both channels. I have then created a frequency response and 40 Hz distortion graph, recording the voltage from the headphone. The result is:
As you can see, the frequency response is nicely flattened out, and the THD is reduced by a factor of about 4. This is consistent with what would be expected from an output impedance reduction from 100 Ohm to 25 Ohm.
Your reply is a bit rude.
We agree that adding a 220 resistor reduces gain and reduces efficiency.
You haven't listed you headphone amplifier (or source) or your headphone.
What you have proven that adding a 27 ohm damping resistor in parallel with the headphone adds damping, therefore decreasing distortion. It is not the optimum damping value for every headphone. Some headphones don't even require damping.
You have also proven that adding a 27 ohm resistor in parallel with that headphone swamps out the effects of the impedance of the headphone (assuming the impedance varies with frequency) therefore smoothing out the frequecy response.
You have proven that your resistive network mitigates the frequency and damping effects of high output impedance on that headphone, but you can't make the actual output impedance go away by adding more series and parallel resistance.