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I'm trying to come up with a schematic for building a passive audio mixer for an aviation headset, but I'm having some questions about resistor values. Figuring out the general circuit is easy enough, but I'd like to bounce this off people who know a lot more than I do so I can make sure I'm doing this right.

The idea here is to combine the audio from the aircraft intercom system with the audio from a MP3 player and send it to the low impedance headset (10 ohms). I'm not that worried about sound quality because the headset itself has a small frequency range (100Hz - 5.5kHz).

Here's what I have for a schematic so far: Aviation passive audio mixing circuit

My question is, what should I be using for resistors here? Would 10 ohm resistors be sufficient for this? I feel like this is pretty basic stuff, but my Google Fu is failing to help me find the answer I need.

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  • \$\begingroup\$ I would just experiment with 3 potentiometers until you get what you want \$\endgroup\$ – BeB00 Apr 25 '17 at 18:52
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I'll assume this is not actually for use in an aircraft- if you are actually mucking with stuff in the cockpit, see your DAR for official advice that conforms to FAA etc. airworthiness requirements.

If you use 10 ohm resistors and the impedance of the phones is really 10 ohms (often that's pretty variable) your av audio power will be reduced by about 12dB, which is quite a bit. Also the headphone impedance your MP3 player is expecting is typically more like 35 ohms.

I would suggest trying more like 27 ohms on each MP3 output and 5.1 ohms on the av output. That will reduce the av power by less than 6dB. The MP3 sound may not be loud enough, but it's probably not really designed to drive a 10 ohm headset.

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I would not worry about the sound quality because of the limited frequency range of the headphones. Since the headphones are 10 ohms, using 10 ohms resistors would mean the volume of the sound would be halved. This probably sounds worse than it is. Human hearing works in a logarithmic way, so it will be lower volume but you will not perceive it as half the volume.

I think 10 ohms for all resistors should be a good starting value. Anything higher and the volume loss would be too much I guess.

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