At that particular point in the circuit (C
i) the "load impedance" consists of R
i.
That is because the right end of R
i is "virtual ground". Hopefully you learned that from studying op-amp circuits.
Since we have a different DC reference in the source AC signal (the audio from your TDA7439) than the LM4880 wants, we must use a DC-blocking AC-coupling capacitor (C
i)
The series capacitor C
i and the parallel resistance R
i form a high-pass filter.
So we must chose a value for C
i which is large enough to pass-through the lowest frequency of interest.
"Roll-off" refers to the decrease in the signal because of a filter function.
Most of these terms are adequately defined and illustrated on Wikipedia:
http://en.wikipedia.org/wiki/Roll-offIt is good practice to attempt to look up unfamiliar terms on Google or Wikipedia and study the explanations for yourself.
Then if you don't understand the explanation, tell us what you don't understand and where you found it.
We will be happy to help you understand the terms.
But simply asking for an explanation without first trying to discover it yourself makes people think you are lazy and wanting us to do the work for you.
Standard "audio frequency band" is 20Hz to 20KHz.
So we would like to have the circuit pass everything from 20Hz to 20KHz without reducing the audio ("roll-off")
The series capacitor C
i and the parallel resistance R
i also form a "voltage divider".
But the "voltage division" depends on what frequency you are talking about.
A very simple voltage divider with two equal resistors will divide the voltage in half.
So, at a single frequency, if we select a capacitor whose reactance is equal to the load impedance,
then we have a 50% voltage divider at that frequency. 50% is 3dB in terms of voltage.
If I go to that RFC calculator page and plug in 20000 ohms, and 20Hz, it says that a 0.397885 uF capacitor is 20K ohms at 20Hz.
So If I go back and select 20000 ohms, and use a "standard" capacitor value like 0.5uF, it says that at 15.91Hz, a 0.5uF capacitor = 20K ohms.
That means that your frequency response will be down 3dB at 16Hz which sounds very good to me for a headphone amplifier.
If you used a 1.0uF capacitor, you would have a 3dB-down frequency of 8Hz. And it is unlikely that your headphones will reproduce 8Hz.
And you can use the same technique for selecting the output capacitor (C
o) assuming you know the impedance of your headphones.