Wavendon:
If you are looking to go from a steaming service on your smartphone, to your amp, you have some options. Like a form member stated you can buy an Audioquest Dragonfly ( Black. Red, or the new Cobalt). With the Dragonfly....You will also need to buy (if you have an iPhone), the camera adapter kit as well as a 3.5 to RCA Converter cable. Depending on your budget, you can buy the Black, but honestly I would at least swing for the Red.
But...
If your amp has an available optical input, then you are in luck.
Go out and buy a Google Chromecast audio. It already has a decent DAC built in, and the audio output has a combo mini Toslink/3.5 mm jack. The Google device was just recently discontinued, but you can grab them on eBay for like $100 bucks new and get them now....or you can get them from a China distributor on eBay and pay $15 - $50 bucks. (You just have to wait for it 2 months). [ I just bought 2 more as a backup].
Anyway...just buy a miniToslink to optical cable, plug it into your amp, and you are off to the races.
To save some money, buy a regular optical cable, and buy the Audioquest mini Toslink adapter.
This configuration is nice...because you are not tethered like you would be with the Audioquest Dragonfly setup. Plus....it’s WiFi based..and sounds way better than Bluetooth. Download the Google home app, and enable the High Fidelity setting with the app.
Only limitation...the Google device cuts out at 24/96 Hi Res files. If you streaming Spotify at mp3 quality (not desirable) or Flac up to 16 bit, you are golden.
I run a Chromecast audio on e bedroom device, and it serves it’s purpose on the cheap. Best part ...buy a few...add them to each room to like a Bluetooth speaker ( as long as it has a 3.5 mm input), create a speaker group in the Google Home app, and now you have a whole home audio setup that you can sync together, since it’s WiFi based.
Here are 2 links that will help...
https://youtu.be/4m8v7xjWu60https://youtu.be/qX2LbNfMAS4Hope this helps
Dom