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DACs convert your phone's digital sounds to analogue; while every phone has a built-in DAC, purchasing an external one will both boost the power of the analogue sound and enable you to attach otherwise incompatible hardware-- in this case, a standard set of 3.5 millimeter headphones. Some popular DAC options include the following:  Chord Mojo - A large DAC with a secondary headphone input that plugs into your phone via USB cable ($599 USD). While considered fairly high-quality, the size of the unit and overall price are common complaints.  AudioQuest Dragonfly - Another USB DAC with a headphone jack. Comes in standard Black ($100 USD) or the higher-quality Red ($198 USD) models. Common complaints include poor volume control and less refined audio than more expensive counterparts.  Arcam MusicBoost S - A DAC built into an iPhone 6 and 6S case ($190 USD). Common complaints include it's limited compatibility (it won't work with a 6 Plus or 6 SE), mandatory charging and limited improvement to sound quality.  Make sure your DAC supports a 3.5 millimeter set of headphones before purchasing it-- while most do, you don't want to order an expensive piece of hardware only to find that it doesn't work with your technology. Amazon is a reliable source from which to purchase technology if you plan on ordering online. This should go into the Lightning port at the bottom of your phone. Depending on your model, you may have to complete an installation on your iPhone's screen. The headphone jack location will vary depending on your DAC model. You'll need to fine-tune the DAC's volume since DACs tend to have higher-quality audio output than standard 3.5 millimeter ports. This will open your iTunes library. This should begin playback; if you can hear your music, you've successfully used headphones with a DAC on your iPhone 7! If you can't hear anything in your headphones, try adjusting your phone's volume. You should also check your headphones' connection to the DAC, the DAC's connection to your phone, and any volume options on the DAC itself.
Research digital-to-analogue converter options. Purchase your desired DAC. Plug the Lightning end of the DAC's cable into your phone. Plug the USB end of the DAC's cable into your DAC. Plug your standard headphones into the DAC's other end. Place your headphones in your ears. Unlock your phone, then tap your "Music" app. Tap a song.