A small dedicated preamp with an appropriate output cable is the best bet. If you want clean, something like the FMR Really Nice Preamp would work. Also, a lot of audio interfaces have pres and can route inputs to outputs. The cheapest option would be something like an ART tube pre; however, those are not the cleanest and if you have a mic that requires a lot of gain, it is less than ideal.
A small dedicated preamp with an appropriate output cable is the best bet. If you want clean, something like the FMR Really Nice Preamp would work. Also, a lot of audio interfaces have pres and can route inputs to outputs. The cheapest option would be something like an ART tube pre; however, those are not the cleanest and if you have a mic that requires a lot of gain, it is less than ideal.