Originally Posted by UnknownAX
my headphones don't even need it, but I still want to make one
If nothing else, a headphone amp gives you a volume knob at arm's length. That's not nothing.
The direct alternatives to that have their own problems. Cable-mounted passive volume controls hurt the sound, and the newer digital sort popularized by the iPhone require special headphones and compatible sources.
What gain should I use for 40-60ohm headphones?
Headphone impedance only loosely correlates with required gain. Before I broke one of them, I had a 64 ohm set and a 300 ohm set that required exactly the same gain to achieve the same loudness level. I also had a 120 ohm set that also wanted the same voltage!
You probably don't actually need any gain, but if you make me give a wild guess, I'd go with g=2.
Is there any point at using big capacitors rated for high voltages, does it improve sound?
Voltage tolerance has nothing to do with sound quality.
In electrolytics, using a too-high voltage tolerance merely risks breakdown of the electrolytic barrier between the conductors, eventually causing the cap to fail. Since low-voltage electrolytics are widely available, there's simply no reason not to pick as low a voltage as you can get away with.
Film caps are a different story. Again, using too high a voltage won't help, but because plastic is such a good dielectric, getting a cap with a voltage tolerance anywhere near your actual signal level is basically impossible. Someone could make it, but no one does. Still, given a choice between a 0.22 uF 50V cap and a 0.22 uF 100V cap in the same product line, go with the 50V cap. All you'll get by going with the other is more bulk for no gain.
Edited by tangent - 3/27/13 at 8:56am