It sets up just like an Amazon Echo via the Alexa app. The major difference is that you need to connect it to a speaker in order to be able to use it.
If you have an external speaker, plug it in. If you are connecting it to the receiver on a stereo system, you will need to connect it to the left and right line out ports (e.g., AUX or some other available lines out of the audio receiver) with the included cable or a separately purchased one if it is not long enough.
For connections to a receiver or to a speaker that has it's own volume control, make sure you set the volume on the Echo Input to 10 (e.g., "Alexa, set volume to 10"). Then control the volume from the volume control of your receiver or speaker. Otherwise, if connecting to a speaker that does not have it's own volume control, you would control the volume from the Echo Input.
Of course, whatever you connect it to needs to be turned on. If connecting to a receiver, not only does it need to be turned on, you also need to remember to select the associated output (e.g., AUX) that you connected to the Echo Input.
The Echo Input also connects to Bluetooth speakers by using the "pairing" option in the Alexa app. I did not test that feature, but here is a link to compatible speakers: https://www.amazon.com/b?node=18007819011.
Once you are set up, just use the device like any other Amazon Echo device. You can ask it for the time or the weather, for example, but the point for such a connection is to enjoy music through better speakers/sound systems, and if you've connected to stereo speakers, then of course you will hear stereo - not something you can do with a standard Echo device.
The Amazon Echo Input can typically be found on sale for less than an Amazon Echo Dot. For example, during Amazon Prime days, it was on sale for $15.00.
Enjoy.
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In the event you missed previous articles about Google and Amazon devices, you can find them here:
https://www.thekuperreport.com/2018/11/review-google-home-mini-vs-amazon-echo.html
https://www.thekuperreport.com/2018/12/part-2-review-google-home-mini-vs.html
https://www.thekuperreport.com/2019/01/google-home-adds-feels-like-temperature.html
https://www.thekuperreport.com/2019/04/alexa-why-arent-you-doing-what-i-ask.html
https://www.thekuperreport.com/2019/07/real-youtube-app-added-to-amazon-fire-tv.html
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Richard L. Kuper
The Kuper Report