The Audioengine D2 Wireless DAC works very well over that distance.
It has a transmitter that plugs into your Mac usb port and a receiver that you connect to one of your preamp's RCA inputs.
Of course, the DAC onboard the receiver obviates the need for your own DAC at the receiving end of the chain. [Actually, it is a pretty good DAC..... the sound quality of Spotify+ is excellent with this set-up.]
Sorry if I have changed the OP's subject of concern, since he evidently wants to transmit over the 20 feet in question from his Mac to his own DAC.
BTW, before getting the Audioengine wireless I did use a 20 ft USB cable to cover the distance.... and, contrary to one of the earlier comments, there was no deterioration of signal quality.... The set-up was simply too clumsy to live with.... thus the Audioengine.