Using iPad/iPhone headphone jack for audio input
Why the iPad (or iPhone) jack audio input does not always work?
The iPad headphone jack can act as an audio input. However, it's important to understand that this input is not a standard line input. Here is what makes this input a bit special:
- It is available via a TRRS jack.
- It works with condenser microphones which need a bias voltage and an high gain.
- The iPad enables the input only when a microphone or similar audio source is detected.
- There is only one input channel (mono).
To connect an audio mixer to the iPad, the good technical solution would be to use a cable having the following schematic:
In the schematic, there is a circuit composed of a capacitor and three resistors (C1, R1, R2 and R3). Sometimes, this circuit or equivalent is present in the device that provides the audio (mixer, wireless microphone receiver, ...). In this case, you don't need these components on the cable and a simple adapter like the StarTech on Amazon just works.
If you are not so lucky, or if you need a solution working with any line-level source, you probably have to buy a special adapter (from AV Resilience, for instance) or build your own.
The iPad (or iPhone) enables the jack audio input only when a microphone (or equivalent audio source) is connected. To detect that, the iPad looks at the input DC impedance. In the schematic above, the DC impedance seen by the iPad is defined by R1. If its value is too low, the iPad thinks that an headphone has been connected. In fact, the headphone TRS jack short-circuits the mic input. If its value is too high, the iPad thinks that nothing is connected. The good value is R1=1kohm.
The iPad jack input is for microphones which normally provide very low level signals. If you provide a standard line-level signal instead, you easily saturate the iPad input. In the schematic above, R2 is in charge of reducing the line-level signal down to a low level mic signal. If R2=47kohm, for example, you have a reduction of about 33dB.
The C1 capacitor is needed to remove the bias voltage and to avoid that the audio source alters the DC impedance seen by the iPad. If R2 is very high compared to R1 (which is the case with R1=1kohm and R2=47kohm), C1 becomes optional (can be replaced by a short-circuit) and R3 can be removed as well.
If your audio signal is weak, you can reduce R2 in order to provide an higher level signal to the iPad. However, with low values of R2, C1 become important and R3 is also needed (C1=47uF, R3=10kohm).