If you paid for the Premium plan, you can easily download the Spotify playlists and songs quickly offline at anytime. But if you are using the free subscription and don’t want to upgrade to premium, you are suggested to try some Spotify music converter software like TunesKit Spotify Music Downloader.
It’s able to help you easily download any Spotify tracks and playlists as MP3 or other formats without losing any music quality. For more other Spotify downloading tools, you can visit this review: 5 Best Spotify Music Downloader of 2018
Check this open-source tools from Github that makes it easy to download your Spotify playlists
https://github.com/luisvonmuller/spotify-playlist-download
The list goes on, you just need to search spotify playlist download github 
I recommend a software called Cinch Streaming audio recorder (Search it in google.). It is most convenient for me to save all my Spotify playlist or iTunes music or YouTube music or any songs you can find in web, you can save them in mp3 format with just one click. And all the downloaded music files come with ID3 tags such as albums, singer, and title .
Here is what I did:
I installed and open it, press “Start” button to get ready music recording, and then playing a Spotify playlist. The recorder will start to record music and split songs automatically. Then I just left my computer and went to sleep. When I woke up, the Spotify playlist with more than 100 songs was downloaded and in the format of MP3. That’s great, so I can put these songs on any phone or player.
What’s more attractive is that this software can get the original sound quality song, which means that if the song itself has 320kpbs , then the music I get will have such high sound quality, which is not possible with other similar software, highly recommended!
The shortcoming is that the recording time is longer than music downloading. It is recorded in the sound card that means you can record music with muted sound and without disturbing anyone.
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