I have a couple of setups which are always On and hooked to Alexa (Echo dot line out) and Airport Express (For airplay)
The setups are of moderate to high-ish resolution (KEF speakers and HK/Marantz amplifiers) and have reasonable amount of imaging present if I play back directly from a good source hooked directly to the line-in
The problem I have is that I need to have both the Alexa and Airplay output plugged into a single line-in at all times.
I am currently using a basic passive mixing circuit (Common ground, 1K resistors on L/R channels on both inputs terminating on line feeding into Amp)
Obviously there is some signal attenuation but I have enough head-room on the amps to balance that out.
However I am not sure if this is the right way to do it.
I also perceive a loss in quality of stereo imaging (don't expect it from the echo dot but do on Airplay) - although it could just be a placebo effect
Question: Is this the right way to do this ? Or should I be looking at active mixers instead - and if yes, any recommendations?
You could try an "auto" sensing/switching type box such as this https://www.amazon.com/5-way-Tilting-Auto-sensing-Stereo-Selector/dp/B00XQL6UEK (https://www.amazon.com/5-way-Tilting-Auto-sensing-Stereo-Selector/dp/B00XQL6UEK)
Whenever either output device (Alexa/Airport) becomes active, it should switch to them. I have neither of those nor this device, YMMV.
I do have a similar Sony switcher from a decade ago buried somewhere that did exactly that and worked fine.
I'd want to try an active setup, because with passive, matters of input and output impedance and cable capacitance can make or break your sonics.
If you are building circuits with op-amps it's super easy to make a
summing amplifier (https://www.electronics-tutorials.ws/opamp/opamp_4.html). (Analog mixers are built-around summing amplifiers.)
1K is a little low for a line-level output. It's probably OK but I'd use 10K.
I also perceive a loss in quality of stereo imaging (don't expect it from the echo dot but do on Airplay) - although it could just be a placebo effect
That shouldn't happen. You might want to disconnect the left & right, one at a time, to see if there is any left-to-right (or right-to-left) leakage.
There are lots of mixers and as long as it's stereo (most are) and as long as it has at least 4 line inputs (or two stereo line inputs) it should be perfectly adequate.