I have headphones that don't have volume control built in on the cable, so I thought why wouldn't I try to make one? However, I don't know where to start.
Is it enough to just use a logarithmic potentiometer, or do I need something more?
If just a potentiometer is enough, what should be its ohmage?
I also use FIIO E3 headphones amplifier. Is it better if I put volume control before or after the amplifier?
At what should I pay attention to avoid creating hum or other unwanted noises?
My headphones are: http://www.sennheiser.co.uk/uk/home_en.nsf/root/private_headphones_dj-headphones_500156
Best Answer
The writeup you linked to doesn't say much useful, but it appears these are bare speaker-type headphones (guessing from the size and shape). That means they are probably small speakers with 4 or 8 Ohms impedance.
You don't want to put a pot in line with speakers. That wastes power, doesn't present the right load to the amplifier, and probably messes up the frequency response due to the impedance change. The best place to put a volume control is in the signal path, not the power path. This means put it at the input of the amplifier that drives the headphones.
Whatever is driving the amplifier input is probably a "line" output. These usually have a few 100 Ohms impedance with a nominal 1V signal. A 1 kΩ logarithmic taper pot would be about right. Nowadays logarithmic tapers are harder to find because old fashioned analog pot volume controls aren't used that much. Nowadays the signal is handled digitally somewhere anyway, so the volume control is done by a digital multiply. If you can't find a logarithmic taper pot, it's not that big a deal. A linear taper will have a lot of change at the low volume end and not much at the high volume end, but for just setting a comfortable headphone volume it's probably good enough.
You can make a linear pot non-linear by putting another resistor accross the output. This doesn't make it logarithmic, but should spread the volume range out a little. Personally I wouldn't bother with this unless you've tried it directly and really didn't like the result.
As for avoiding hum and noise in audio, make sure everything is shielded. If necessary, mount the pot in a small metal box with the box grounded to the bottom lead of the pot, which should also be the ground for the input and output cables.