How to connect an external mic or mixer to a smartphone
by Bruce Bartlett

When you use a smartphone to capture a video of your band, does the audio sound distant and muddy? That’s because your smartphone’s built-in mic is too far from your band. Place a high-quality mic about 2 feet from your band and connect the mic to your smartphone. Your audio will sound more close-up and clear with this method.

Some condenser mics are available that connect directly to a smartphone. An example is the Rode smartLav+ and a TRRS extension cable.  

This article tells how to connect any professional mic with an XLR output to a smartphone. You’ll need a female-XLR-to-TRRS adapter.  If you have a condenser mic, connect the mic to a phantom power supply. Then connect a mic cable from the supply output to that adapter (Figure 1). 


Figure 1. Connecting an external mic to a smartphone.

Some XLR-to-TRRS adapters are made by LyxPro and Movo (Figure 2). They mate a professional mic to the mic input of a smartphone, which is a TRRS jack. Recent Apple iPhones come with a TRRS-to-Lightning adapter dongle, also called 3.5mm-to-Lightning adapter. You’d plug the XLR-TRRS adapter into Apple’s dongle.

Figure 2. A Lxypro XLR female-to-TRRS adapter. It includes a headphone jack.

Readers who are handy with a soldering iron can build their own XLR-to-TRRS adapter. This adapter works with an iPhone 7, and should work with most recent iPhones and Androids.  I’ll give the technical details here.

Since a smartphone requires an unbalanced mic signal, you need to convert the balanced mic signal to unbalanced. If you try to do that by connecting a device to a dynamic mic’s XLR pins 2 and 1, there is no signal. Those pins are not connected inside the mic. You must wire pin 3 to pin 1 to get an unbalanced signal from a dynamic mic.

Now we need to consider the special configuration of a smartphone mic input. The iPhone produces a voltage of 1.5V to 2.7V at its mic input TRRS connector. That voltage sends a current through an external mic to determine if an external mic has been connected. 

The iPhone 7 mic input needs to see a mic impedance of at least 400 ohms to recognize that an external mic has been plugged in. Most professional mics have an impedance of 150 to 250 ohms. So an XLR to TRRS adapter needs an internal resistor in series with the mic output to ensure that the phone sees at least 400 ohms. A resistor of 470 ohms to 1.6 kilohms works well. The higher the value of that resistor, the lower is the output signal level. So keep that resistor value low to get the best S/N in your recordings. 

Here are the wiring instructions (see Figure 3):

  1. Inside the female XLR connector, solder Pin 1 to Pin 3. Solder the cable shield to Pin 3.
  2. Inside the female XLR connector, solder Pin 2 to a resistor (470 ohms to 1.6 k ohms, 1/8 watt).
  3. Solder the cable center conductor to the other end of that resistor.
  4. At the plug end of the cable, solder the shield to the ring tab. Use an ohmmeter to check for continuity between the tab and ring.
  5. Solder the cable center conductor to the sleeve tab.

Figure 3: A female XLR-to-TRRS adapter.

The ShurePlus Motiv recording app for smartphones indicates whether the phone is using an external mic or its own internal mic. When you connect a mic to the XLR-to-TRRS adapter, and connect the TRRS plug to the smartphone, the Motiv app should show that the phone has detected an external mic. 

I connected a Rode NT1 condenser mic to an iPhone 7 using an XLR-to-TRRS adapter. The Rode NT1 produces 35mV at 94 dB SPL and 70 mV at 100 dB SPL. So it’s easy to clip the iPhone when close miking a loud band. Blocking occurs, not just clipping. You might want to insert a mic pad between the mic and the adapter.

A dynamic mic through the adapter is very low level and gives a noisy recording. The noise is kind of a boiling sound rather than hiss. You’ll need a mic preamp or mixer to boost the signal level before it goes into the smartphone.

CONNECTING A MIC PREAMP OR MIXER TO A SMARTPHONE

 Rather than using a single mic to pick up a band, you could use multiple mics for the best sound, and run those mics through a mixer. Do a trial recording and listen to the playback to check the mix.

Connect the mixer output to a smartphone using the adapter just described. A mixer output is line level: about 1.23 volts balanced or 0.316 volt unbalanced. But a smartphone needs to be fed a mic-level signal (about 2 to 4 mV). You need a pad to bring the mixer output level down to mic level. The Shure A15LA is an example of a 50 dB pad. You can build your own pad as described in this website: http://www.uneeda-audio.com/pads/ .

CONNECTING A USB MIC TO A SMARTPHONE

A USB mic close to your band also will give a clearer, more closeup sound for your videos than the mic built into a smartphone. 

Figure 4. Connecting a USB mic to a smartphone.

To connect a USB mic to an Android:

1. Find out what kind of USB connector the mic has: micro USB or mini USB or USB-B.
2. Search Amazon for a 10 ft micro USB to USB cable, or a 10 ft mini USB to USB cable, or a 10 ft USB-B to USB-A cable.
3. Search Amazon for a cablecreation micro USB 2.0 OTG cable. It’s a female USB to micro USB adapter.
4. Connect the 10 ft cable between the mic and cablecreation. Connect cablecreation to your Android phone.
5. Download and install Open Camera. Make sure your phone is Android 5.0 or higher and supports OTG USB. In Open Camera settings, select External Mic as the audio input.
6. Shoot a video using Open Camera with the USB mic.

To connect a USB mic to an iPhone:
1. Search Amazon for a lightning-to-USB adapter.
2. Search Amazon for a 10 ft micro USB to USB cable, or a 10 ft mini USB to USB cable, or a 10 ft USB-B to USB cable — whatever mates with your USB mic’s connector.
3. Connect the USB mic to the 10 ft cable. Connect the 10 ft cable to the lightning-to-usb adapter.
4. Shoot a video using the USB mic.

SINGLE-MIC MIXING TECHNIQUE

You might need to use a single mic to capture your band, rather than multiple mics and a mixer. Adjust the balance among instruments and vocals by varying their distance to the microphone. Again, do a trial recording and listen to the mix. If an instrument or vocal is too quiet, place it closer to the mic and vice-versa. It’s the single-mic method used by some bluegrass and old-time bands for sound reinforcement. You could use two mics into a mixer, then group a few musicians around each mic. 

By following these tips, you can create a clearer, more professional sound in your band videos.

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Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer (www.bartlettaudio.com). His latest books are “Practical Recording Techniques 7th edition” and “Recording Music On Location 2nd edition”.