The LG G5 is here, bringing with it a concept that's both old and new: expansion slots. It's been well over a decade (Handspring Visor of 2001) since we last saw a major handheld device with an expansion slot, but LG's bringing them back in a big way with the G5. In addition to a physical camera controls module , LG also teamed up with Bang & Olufsen's B&O Play division to develop a module that adds high-end sound to the LG G5.

The LG Hi-Fi Plus with B&O Play (that'd be the name) sticks onto the bottom of the LG G5 like any other module: unlatch the bottom bezel, pull it out with the battery, swap that battery off onto the Hi-Fi Plus module, and slide the combined unit back into the G5. Doing so replaces the matte metal finish of the G5 with the matte black of the Hi-Fi Plus unit (in addition to replacing the only LG logo on the phone with a B&O logo), and extends the bottom of the phone by roughly 1/8-inch to the length of the phone.

Inside that module you'll find the standard USB-C port and loud speaker you expect to find on any G5 module (we have no indication that this speaker's any better than the default speaker), along with a new 3.5mm headphone jack powered by an integrated digital-to-audio converter.

That DAC is a 32-bit unit plus a dedicated amp, both developed by Denmark's Bang & Olufsen. The Hi-Fi Plus supports native direct digital streaming for high-resolution aduio files and will up-sample lower-bit-rate audio from all source. It'll output 32-bit audio right through that new headphone jack (the top-mounted built-in headphone jack still outputs 24-bit audio, and the two cannot be used concurrently). Of course, you'll need a pair of high-end headphones with quality drivers to really notice the difference, and it'll be better with an already-high-resolution audio source versus an upsampled one.

Interestingly, LG says the Hi-Fi Plus module was designed to work with more than just the G5. Of course, it won't dock up to other devices as it does on the G5; instead it ships with a cap that covers the open end and adds on a second USB-C port. Though LG hasn't elaborated on how yet, they claim it will be compatible with Mac OS X, iPhone and iPad, and other Android devices.

We're reserving final judgement on the LG Hi-Fi Plus with B&O Play until we've had a chance to give them a listen on the requisite high-end headphones in an environment that's more conducive to the nuances of high-resolution audio outputted at 32 bits. But in the meantime, color us intruiged and maybe even a little optomistic about the potential — high-end audio on a consumer-grade mobile device like this is an interesting proposition.