AC-3 can come out of either the optical output or a coaxial (RCA) digital output, depending on which your receiver has. In either case, it is a special digital signal that is compressed and requires an AC-3 (AKA Dolby Digital) decoder. Even if your receiver has optical outputs, it won't necessarily output compressed digital audio (AC-3/Dolby Digital). Some only output PCM on the optical outputs, which is uncompressed.
So you need a FTA receiver capable of outputting compressed Dolby Digital (AC-3) AND a stereo amp (or receiver) capable of decoding Dolby Digital with the appropriate type of input (either optical or coaxial digital).