Convert USB to SPDIF

yeah, I don't want to spend more on the converter than the price of the AVR.
djones51 suggestion might work, although that is getting towards the price of buying a budget streamer.  I just wanted a quick fix to replace my Node2 in this system which is dead in the water after the last update and since I am not using this MacAir with Audirvana loaded at the moment I thought I might try to repurpose it.  There is a bargain basement convertor out there for about $20 but that can't be much good.  I have a first gen Audioquest Dragonfly that I am not using maybe I can stick an RCA cable (are they called mini cables?) that is single ended on one side and double ended on the other and attach to the analog inputs of the AVR while I try to get the Node fixed

You can’t.  You need to add a DAC, and one that will accept usb.  And you probably need a “Player” and that can read the files on the flash drive and send them to the DAC

The laptop-Dragonfly-mini cable into the analog inputs of the AVR seems to work o.k.  I had bought a mini cable from Best Buy late yesterday but then didn’t have much time for comparative listening.  I don’t use this system for streaming very much, it’s primarily for video and surround sound music.
  Searching through my pile of unused equipment I came across a Musical Fidelity VLink.  I had bought one when it came out because I had a DAC that didn’t play asynchronous USB.  If I remember it worked for a few days then would sputter during replay.  I think it was a demo unit from a dealer who was going out of business so no refund.  Ultimately I bought an Oppo 105 which had asynchronous USB

a further update...I have an Audioengine Bluetooth DAC in the basement system and discovered that it has a usb input, so I run the usb 2 from the MacAir to the Audioengine.  I hardly stream in the basement and frankly mile files streamed from my phone to the bluetooth part of the Audioengine DAC don't sound that different from the PC directly connected to the DAC, so I do either.