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XLR microphone input to audio codec

Amir Kamzel
Prodigy 70 points
Other Parts Discussed in Thread: TLV320AIC3105, TPS40210, INA163

Hello,

I'm working on camcorder application and we intend to implement an on-board microphone + a pro XLR micropohne input (for both dynamic and condenser microphones)

The on-board microphone is quite standard and it will provide me a single ended line which will go directly to the audio codec (TLV320AIC3105).

The main issue is the XLR input which probably requires a pre-amp amplifier (INA163 powered from +-5V) and 48V phantom power  (probably will be implemeted by TPS40210).

My questions are below:

  • Do you see any problem with using TLV320AIC3105 for such application, or do you have any alternative to suggest, maybe with differntial input?
  • The maximal input swing of this audio codec is 0.7Vrms. Do I really need the pre-amp for that?
  • In this application, can I avoide the -5V negative voltage of the pre-amp by lifting input DC reference?
  • What should be the gain in the pre-amp for such application? It has to be quite low, isn't it?
  • Do you have any recommandation for the 48V boost, different from my suggestion?

If you have a complete reference design for XLR micropohne input, it could help me a lot. Since it's a standard input in the pro-audio industry I don't think it should be a problem.

Thanks in advance,

Amir Kamzel

 

  • 0
    Guru 21975 points

    Amir,

    The INA163 sounds like the correct choice for the application. You would certainly need gain prior to the codec as the typical output of microphones is a few mV.

    A full pro type mic input would include a gain trim adjustment for the INA163 to allow for different mic sensitivity and sound level environments. Simple equipment like you describe often has a switch with two gains for the INA163 that differ by 20dB or so. You also need a level control, 0% to 100%, after the INA163 leading to the codec.

    You should have a switch to turn the 48V phantom power off for mics that don't need it as the voltage boost converter will affect battery drain.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 70 points

    Hi Bruce,

    Thanks a lot for the answer.

    About the two gains you suggest, is it better to implement it by a changable resisotor on Rg gain resistor of INA163 or just by placeing a serial audio switch with two different serial resistances on the audio lines? The issue is that I prefer not use a standard analog switch due to different resistances it provides on different frequencies. What is your suggestion in this case?

    About the level control, I thought of implementing it inside the codec. Do you have another alternative to suggest?

    Phantom pwoer is clear.

    I'd appreciate your help.

    Thanks,

    Amir

     

     

  • 0 in reply to Amir Kamzel
    Guru 21975 points

    Amir,

    I should have looked at the TLV320AIC3105 specs before answering. I see now that it has a PGA that serves as the gain control after the INA163. It also has substantial gain that allows direct connection to an electret microphone capsule. This presents an issue:

    The codec probably has nearly enough gain for some situations without additional gain from the INA163. Still, I believe that many situations would require more gain, thus the INA163 would be needed. It may be best to over-amplify in the INA163 and attenuate with voltage divider (maybe -6dB) prior to the codec. Then switch the gain in the INA163 between 10dB and 30dB, or so. Switching the gain at the gain set resistor is probably easier than creating an input divider due to the complications of phantom power. Switch a resistor in parallel with the low-gain resistor to make the high gain--you will probably get less of a pop.

    You should experiment with the whole gain configuration with the codec to be sure you get an appropriate gain "feel" with both dynamic and phantom powered mics and switched gains.

    Hope this helps,

    Regards, Bruce.