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I'm trying to inject additional sound into a phone call. Therefore i modified a basic cable headset to use audio from my iPod.

This is my current prototype circuit:

circuit

That circuit is already working, but has some disadvantages due to the very basic passive mixing. When i turn up the iPod's volume it get louder and louder, but it also starts to decrease the mic volume.

Basically I'd like to be able to increase the iPod's volume without affecting the microphone level.

I think it has something to do with the iPod being a low impedance device (around 32Ω i think) and the electret mic being a high-Z device (around 1.5 k – 10 kΩ). Maybe there's also a problem with the power? The mic will only produce several millivolts, where as the iPod can supply more than 1 volts?

How could i improve my circuit, but still keet the headset recognized as a high impedance device. (if it's low-Z, the phone won't detect it properly).

Thanks in advance & bye! Hans

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    \$\begingroup\$ Are you sure the phone isn't reducing it's overall input gain as a result of feeding too much iPod audio into it? \$\endgroup\$
    – Dave Tweed
    Commented Aug 27, 2012 at 18:32
  • \$\begingroup\$ I don't think so, but i was also wondering how much i could turn the volume up until it started to distort. \$\endgroup\$
    – superno
    Commented Aug 27, 2012 at 19:24

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You could do something like this if you're not afraid of op amps. The microphone gets its high impedance, and if it needs higher impedance, increase R1. I have the iPod a 330 Ohm load because I seem to remember reading somewhere that's a good starting point. The output impedance is 10k, and can be increased if necessary. This circuit has a gain of 2, so if you need less, decrease R6, and if you need less than 1, switch it to an inverting configuration. Trying to sum these inputs passively is going to require a matching network.

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  • \$\begingroup\$ Hey Matt, thank you so much! I'll give that a try with some LM358. I think I have to add two more parts (R14, C1) to supply the electret mic with phantom power. (circuitlab.com/circuit/a52jsm/opamp) Will try this tomorrow and give you some feedback. Thank you again! \$\endgroup\$
    – superno
    Commented Aug 27, 2012 at 19:39
  • \$\begingroup\$ Tried it today and: It is working – not perfectly, but: working. I have heavy noise on both mic and line input. I also had to make some adjustments, that the phone detected my setup as a headset (added C2 + R15). Maybe the noise is because i'm using the LM358? circuitlab.com/circuit/a52jsm/phone-headset-line-source-inject \$\endgroup\$
    – superno
    Commented Aug 28, 2012 at 11:51
  • \$\begingroup\$ What needs to change? \$\endgroup\$
    – Matt Young
    Commented Aug 28, 2012 at 11:54
  • \$\begingroup\$ I don't know. The distortion seems to be present in only some frequencies, but more or less independent from volume. I'm wondering if the power supply is okay, the way I set it up… \$\endgroup\$
    – superno
    Commented Aug 28, 2012 at 13:31
  • \$\begingroup\$ The more likely scenario is the mic input of your phone does not like the full range music. I don't know the exact spec off the top of my head, but I think phone bandwidth is only 100Hz-5kHz. Not sure what kind of filtering the mic input has, but you may need to filter the output of the circuit a little. It's hard to say without hearing it. \$\endgroup\$
    – Matt Young
    Commented Aug 28, 2012 at 13:35

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