Measuring distortion from a mass market bluetooth receiver

test setup
Belkin F8Z492 connected to Analog Discovery

For my cigar box bluetooth speakers, I’ve been taking the easy route for bluetooth connectivity. I simply took the housing off a Belkin F8Z492 audio receiver and plugged it into a cheap class D amp. It worked really well and got me to a finished product quickly. Initially, it sounded great. The Dayton Audio ND65 drivers alone provide a big upgrade over the tinny laptop speakers and earbuds I’m used to. But after a few weeks, even my untrained ears could hear some distortion. It sounds like a subtle reverb, usually affecting only vocals. Initially, I suspected the front end: the cigar box enclosure, or the drivers. Or, just as likely, something wrong with the wiring. This is a lesson in engineering fundamentals. Armed with only a primitive knowledge of audio and electrical distortion, I needed a plan to isolate and quantify the problem.

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