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I've got a cell phone, gopro, and DSLR that I want to record audio into. I don't want to record audio into a separate device.

The problem with analog mic-level (millivolt) signals is that they require amplification inside the recording device before being converted to digital and recorded. The preamp inside the aforementioned devices are all very noisy and all introduce a background hiss into the audio (not difficult when the mic input signals are in the millivolts).

I have a device that can take a microphone and output clean line-level signals of what the mic picks up.

I can rewire an AUX cable so that the mic contacts on the plug output line-level signals from the device above, which means the mic input on the camera would receive line-level signals from the AUX cable.

Would this allow me to then set the camera's internal microphone gain to zero, thus utilizing the noisy preamp to a lesser degree, and result in a cleaner recorded audio? Or do I have a high risk of overloading and damaging the circuitry of the camera?

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Audio line level is 1.23V RMS or 1.74V peak, this should not be dangerous for the mic input.

If you're feeling extra paranoid, you can add a resistor, about 1-10kOhm in series with the signal wire, to protect the input. Then check if your line level clips the DSLR's input amp when its gain is set at minimum. The resistor will ensure that current flowing into the input is low even if the input protection diodes in the input circuit start conducting. After confirming that it works, you can remove the resistor.

I'll bet that if you check in the manual, you will see it is actually a "mic/line" input, because this feature makes sense. The manual should specify a max input level either in volts or in dBu. I mean, it's a pretty common use case, every soundcard does this, so it would be surprising if the DSLR manufacturer didn't implemnt it.

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Milivolt level signals do not necessarily mean large noise. It is perfectly possible to deal with milivolt signals and still have more than enough noise-free dynamic range for most applications (keep in mind that professional audio microphones also deal with milivolt signals or lower). The fact that you have an amplifier that you want to put in between the microphone and the DSLR is proof of this.

I have never really had issues with line-level sources, but your mileage may vary. You should consult the user manual of the devices to see what the allowed levels are. Many devices do in fact allow line-level inputs, and if they do not, will likely have protection that prevents you damaging them (you would likely just heavily distort the signal as you clip the amplifiers).

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