rvan
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I have a cheap(ish) external soundcard with a headphone output but no volume control of its own.  If I connect my headphones directly, the output is unbearably loud.  I'm thinking of building a fixed attenuator to bring the output down to a listenable level (softvol isn't a viable option as sometimes it fails and I am blasted by extremely loud output).  I've got a few questions about attenuator design which I'm hoping someone can help me with:

  • Am I right in thinking that I need an unbalanced circuit?  Are balanced attenuators for balanced audio and unbalanced attenuators for unbalanced audio?

  • Which configuration (T, Π, etc.) should I use?  What are the differences between the different configurations?  (I have searched for the answer to this but haven't found much information.  Is the choice essentially arbitrary?)

  • Given that the easiest way to find the right amount of attenuation is by trial and error, how important is it to know my soundcard's output impedance?

  • Will regular 1/4 W resistors be sufficient?

Thanks in advance,
rvan

nitro2k01
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Sweeeeeeden

A headphone output is an unbalanced circuit. Using a T or a Π type attenuator would likely be way overkill for your application. The point of those topologies is to match the impedance of both the input and the output in order to avoid signal reflections, which is mostly a problem for high frequency transmission lines.

You should use an L type attenuator, that is a two resistor voltage divider, but you may be able to get away with just a series resistor. The downside of just putting a resistor in series is that the attenuation will vary with the impedance of the headphones, if you have multiple different pairs of headphones that you wish to use with the same soundcard. Just shooting from the hip, you should aim for a resistor value that is slightly lower than the headphone impedance. Lower value = less attenuation. This could be done externally without modifying the soundcard, if you get a siutable stereo plug, and jack that you can use for an adapter. Standard 1/4 W resistors should be sufficient for this task.

Another option is to modify the soundcard internally. There will probably be a headphone amplifier. If you can attenuate the signal before going into the amplifier, you could apply a constant attenuation regardless of the headphone impedance. I could probably help you with that, but then I would need something to go on, preferably pictures of the internals of the soundcard around that area of the board.

rvan
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Thanks for your reply.  I'll set about trying out resistor values in an L attenuator when I get the time.  I think I'll stick to the external option for now; I don't really feel like doing any SMD soldering for this.  I generally only use the one pair of headphones anyhow.