I have a small tripath amp driving 2 speakers. There are 2 inputs types that are common. I am using both inputs which weakens the signal input on both. I have a TV and an Amazon echo for inputs. When I disconnect one of them both the volume and the clarity increase significantly for the unit that is still connected. Is there a way/device , besides getting a manual switch, that I could use to resolve this?
You have asked a complex question.
Your best and simplest solution is a switch box, to switch sources.
A passive mixer needs to be designed knowing all the source and input impedances as well as the output voltage of the two sources. There is no off the shelf solution.
In addition a passive mixing circuit will substantially reduce the voltage of the sources available at the input of the amp.
These sorts of circuits work best when you are mixing two sources with gain/voltage to spare, and they will not vary in gain structure. For instance I designed one for blending my sub woofer outputs with the Baffle Step Compensation signal I created for my triamped main speakers, and it works very well.
For your application a passive mixer circuit is not a good solution. For your issue you need a switch or an active mixer. Those are your only two realistic options. The switch will be the cheapest solution.