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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

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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.

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    \$\begingroup\$ The Technical Data section of the page linked to indicates the headphones have a 32 ohm impedance. \$\endgroup\$
    – tcrosley
    Jan 1 '12 at 16:22
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    \$\begingroup\$ @tcrosley: OK, that's more than 4 or 8 Ohms, but still "low". What I said then still holds true, which is to put the volume control in front of the amp, not between the amp and the headphones. \$\endgroup\$ Jan 1 '12 at 16:32
  • \$\begingroup\$ @OlinLathrop: Thanks !! Soundcard output is driving the amplifier input, and it's configured as "Line output" jack. I can get 1K logarithmic taper for about $2, so I'll try that :) \$\endgroup\$
    – xx77aBs
    Jan 1 '12 at 17:17
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    \$\begingroup\$ @xx77abs: That sounds like it should work exactly as you want then. \$\endgroup\$ Jan 1 '12 at 18:05
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I know you said you wanted to make your own, but FYI RadioShack makes a Volume Control Cable for Stereo Headphones, and there are probably others out there as well. It would probably work best placed before your FIIO E3 amp.

enter image description here

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  • \$\begingroup\$ Thanks, but I'm from Croatia, and I'm not sure if I could get that here ;) \$\endgroup\$
    – xx77aBs
    Jan 1 '12 at 17:08
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Keep in mind there are two ways of connecting the terminals of a wiper pot. Method #1 puts the resistance in line with the ground while the second method puts it in parallel, dumping the extra unused power back to the ground. This is called wiring it in a voltage-divider mode and is the correct method of doing things for an audio dial.

Refer to the following schematic to get briefly acquainted with the idea on how this works: potentiometer basic schematic

Moving the wiper as close to the ground as possible gives minimum resistance and thus maximum signal. While in case 1, having the ground at the end means you're adjusting the signal by just adding inline resistance, this is terrible and creates distortion by selectively modifying the output signal. As shown in this online calculator. It also requires huge potentiometers values to bring the signal's volume down below audible ranges.

Method 2#: Connecting it in voltage-divider mode solves all of these problems and can work just as well with a lower value pot, meaning minimal added distortion to the signal path, with most of the driving potential going towards moving the diaphragm membrane on your headphones/speakers.

Also, there is another thing to consider in Audio which is that you need the difference between the amplification's circuit internal resistance (which should ideally be as low as possible) and that of the headphones as high as possible in order to avoid distortion. This becomes tricky with speakers as they usually have impedances of around 4 or 8. For this reason, many prefer huge value pots for speaker amplifiers, to try and offset this effect, but as I hope I've demonstrated, this is the wrong approach to take.

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