The implementation of digital musical instruments is often opaque to the performer. Even when the relationship between action and sound is readily understandable, the internal hardware or software operations that create that relationship may be inaccessible to scrutiny or modification. This paper presents a new approach to digital instrument design which lets the performer alter and subvert the instrument’s internal operation through circuit-bending techniques. The approach uses low-latency feedback loops between software and analog hardware to expose the internal working of the instrument. Compared to the standard control voltage approach used on analog synths, alterations to the feedback loops produce distinctive and less predictable changes in behaviour with original artistic applications. This paper discusses the technical foundations of the approach, its roots in hacking and circuit bending, and case studies of its use in live performance with the D-Box hackable instrument.