You can't transmit a digital signal from the AD-20's coax SPDIF output to the JB3's digital input without using an optical cable - the JB3 only accepts optical SPDIF. You need a digital format converter that converts coax SPDIF to optical SPDIF. The Hosa ODL-276 (and new -276A model that supports 24-bit) will do the trick. They should run around $60 shipped (though many sites list them as ~$100 retail, you should never have to pay that much.) You'll need a mini > RCA adapter since the Hosa has an RCA coax input, not a mini coax input. And you'll also need an optical cable from the Hosa to the JB3 - standard toslink on the Hosa side of the cable, and a mini connector on the JB3 side. I recommend an optical cable with a right-angle mini connector for the JB3. While preferable, optical cables with integrated mini right angle connectors may be hard to find, so some people use a standard toslink-toslink cable with a right-angle toslink > mini adapter.
Wait wait wait...doesn't the AD-20 also have an optical output? If so, you could simply pick up an optical cable with the proper connectors and skip the digital format conversion.