I'm praticing the guitar with the Yousician app installed on an android phone. Yousician provides a lot of tracks and other exercises. The app can use the phone's microphone and loudspeaker, without any connection, for instance for an accoustic guitar. But it can also make use of adapters connecting an electric guitar to the smartphone. I'm usually using the latter, for practicing with an electric guitar, which allows me to practice in a rather quiet way (family, neighbours, etc.).
Yousician helps you correct your practice, as it will listen to the notes played, and display green notes when you're on time with the "tablature", or red notes if you're out of tune. Thus it needs to listen to the sound of the guitar.
I'm usually praticing in the evening, from an armchair, and not necessarily close to a guitar amp, or to an electric plug. Hence the need for some portable gear (mini amp, batteries powered). As an aside, I'm usually practicing on a travel guitar, which is more suitable for use in an armchair, as there's less volume to fit next to me, since the guitar doesn't have a proper body... but that's another story to tell.
Let me describe the setup I'm using. A picture follows.
First I want to listen, in a headset, to the mix of my guitar's sound with the backing track played by Yousician. For such a basic setup, I could be using a mini amplifier suited for headphones, like my Vox Amplug 2. Praticing over Yousician's backing track is much more enjoyable that with just a metronome, or the full original tracks, in particular when you're a beginner, since it allows to adjust the tempo.
But I'd also like to be able to use a more precise detection of the sound of my guitar that what's done with the smartphone's regular microphone. Hence the need to not only plug the output of the phone to my headphone, but also to plug the guitar's as a sound input to the phone. I'm using a iRig 2 for that.
The iRig 2 is supposedly supported by Yousician so that you can use the iRig as a virtual pedal system, offering a lot of sound modulation, just like with other apps like ToneBridge... but there's a big issue on Android : the kernel doesn't seem to be optmized for real-time, and there's a delay between what I'm playing and the output modulation. Not nice.
I've then ended up plugging the headphone to the Amplug, the amplug to the iRig2, then the guitar to the iRig2. The iRig 2 listens to my notes, without interferences of the phone's microphone : the notes recognition is more precise: no more red notes whereas I played them right. The Amplug is plugged on the iRig2 pass-through jack plug, and allows me to change distortion and other virtual amp settings. And the sound is played without any delay. Finally, I'm plugging a male-male mini-jack cable between the iRig's headset output and the Amplug's aux input, to be able to route the backing track from the smartphone to my headset.
A bit of explanation of what's in the picture: the Amplug is plugged to the iRig2 (guitar passthrough jack)... black plastic in black plastic... not really clear on the picture. The metallic "guitar in" (top-right) is a regular guitar jack cable. The tiny white male-male jack cable is womething I managed to reuse, or buy (forgot of where it came from), which is fortunately to the exact length allowing me to minimize the spaghetti ball ;-). The black (again) cable of my headset is plugged to the Amplug's headset output, at the bottom. Fianally, the smartphone is plugged to the iRig2's jack cable (top left).
In the end, this is quite handy. It's not using the full versatility of the iRig pedal emulation capacities, but I don't care so much as I merely need to practice, and the Amplug's amp settings are sufficient (clean version in my case: AP2-CL model). I have other gear to play louder or record, on a "real" amp or computer, which I'll describe elsewhere. The end setup is quite costly: 2 devices, several cables, the smartphone, the Yousician subscription (not mandatory, but I'm gladly paying for the full library of tunes).