The Korg Pandora PX4 is pocket-sized multi-effects processor and practice tool for guitar and bass.

Full disclosure the PX4 is discontinued. Power supply not included. It runs on 4 AAA batteries.

Here's Korg's description:

The Korg Pandora PX4 (released by Korg ToneWorks) is a highly capable, pocket-sized multi-effects processor and practice tool for guitar and bass. Originally debuting in the early 2000s, it measures roughly the size of a pack of cards and remains a cult favorite for its highly portable silent practice and recording features. 

Key Features

REMS Modeling: Uses Korg's proprietary modeling technology to replicate dozens of classic guitar/bass amps and speaker cabinets. 


Massive Effects Library: Packs over 120 different effects, allowing you to run up to 7 effects simultaneously (e.g., compression, reverb, delay, and modulation). 


Rhythm & Bass Machine: Features roughly 40 onboard drum patterns and bass lines that you can chain together to create custom backing tracks in any key. 


Phrase Trainer: Offers 30 seconds of recording time, allowing you to loop short licks to jam over or practice improvising. 


Aux In with Key Transpose: Allows you to plug in a smartphone or media player to jam along with your favorite tracks, with the ability to adjust the pitch of the aux audio by up to an octave. 


Portability: Can be powered by 4 AAA batteries and features a handy belt/strap clip for hands-free playing.