Getting line-level audio into an iphone

Now that more musicians are live streaming music during Covid-19 stay home orders, I thought this design might come in handy. If you are looking to get stereo line-level audio directly into your iphone (and your iphone still has a headphone jack), this is a pretty straight forward DIY hack that can get you there for under $20 if you have a soldering iron. Last year I wanted to do some live streaming of our local synthesizer collective and didn’t want the audio to be degraded by the iphone’s microphone, so I put this device together using parts I had in the shop.

If I knew I was going to be putting this project online, I would have called it something better than “STLN2IPOO” – which stands for “Stereo Line-level audio to iPhone”, and that’s exactly what this does. By cutting some traces and adding a few resistors on a cheap off-the-shelf D.I. Box and attaching a 3.5mm TRRS plug onto a microphone cable, you can feed a stereo or mono line-level audio signal from a mixer or other sources directly into your iphone without any signal loss. A volume knob is also included in the design so the level can be set to avoid clipping.

parts

Full size instructions here (open in new tab to get out of modal window)

instructions

Open the PDC21, cut the traces shown and test to make sure they are cut with a multimeter if you can. Wire up the resistors and potentiometers, drill a hole in the chassis for the potentiometer. Reassemble and attach knob. For the cable, you can either attach the adafruit 3.5mm TRRS plug onto a regular microphone cable by cutting off the female end and wiring as shown, or you can cut the female end off of a TRRS extension cable and add the XLR Male connector.

how it works

This hack rewires the “input” and “output” 1/4″ jacks of the DI for our stereo inputs, the line-level signal gets summed into a mono signal through the two resistors before going into the potentiometer. The potentiometer acts as an attenuator, so you can turn down a loud signal to avoid clipping. The signal then goes into the DI’s transformer that converts it to microphone level and impedance. The 3k resistor lets the iphone know a microphone is plugged in and our special cable gets the microphone signal and ground to the right pins on the 3.5mm TRRS jack.

using it

Take line-level audio out from a mixer or musical instrument into your STLN2IPOO into one (mono) or two (stereo) 1/4″ inputs and run the mic cord into your iphone. Run the “voice memo” program and start recording to get a visual representation of the signal levels, if it is clipping turn the volume knob down. The DI box’s ground lift switch still functions if you find that you have a ground loop somehow between your gear and phone. That’s it! Now stream some video with good audio!