|Not trying to argue, but with a properly implemented opamp circuit using a low noise device, the resistor noise dominates, so a "passive" solution is not necessarily any less noisy. An active opamp circuit also has the advantage of low output impedance which may or may not be an important factor here.
also not looking to argue but have to state my side.The "why" of my advice to be taken or not,no matter either way
it is just not rational to me personally to use an active device where one is not only not need but is in fact the less technically meritorious method when the entirety of what you are attempting is weighed.That "attempt" is to combine two signals into one.
Using an opamp for a simple stereo-to-mono mix and with that simple ciruit a power supply,line operated so AC line noise suppression,RFI protection at the entry points to the device (power supply pins,input pins,output pin,feedback loop) in place of even a simple two resistor mixer is like using an opamp as an "anti-gain" device because you have too much gain when a resistor pad or pot would do the same job or like using a X5 opamp stage going to a X5 opamp stage because you need X10 gain.
Downsides ? possible comb filtering from mixing the signals 1:1 but the added resistor to ground loading helps here a bit by adding a "preload" split so a path to grund for each at a load potential of 10K.The transformer totally eliminates this by actually using 100% isoltated inputs for each leg which are then combined without actual physical contact of those two signals into one single audio output signal.
Upside is obviously sonic purity because ALL active stages no matter if just a single transistor have an audible impact or we would not use various opamps/topologies to tweek system sound and way more
audible than any single resistor would or could be.
Any active stage also has a higher cost factor and that not a small one. Circuit cost+power supply cost before even thinkng about connectors and enclosure or chassis.My opinion is even a simple power transformer from the cheapest outlet will cost more than the three resistors (and likely more than the cased trafos combiner).
In audio less really is more bcause everything adds a sonic imprint so the less you have the less imprint.
Just my personal opinion but just like most here would use an opamp as a simple first order filter i would use a single cap and a single resistor to accomplish the same thing and have better sonics at 1/4 or less the price of the iffy active stage.
BTW-I personally if attempting the same thing would shoot right to Jensen and buy one of their raw line splitting trafos but that would totally toss any cost savings in the active vs .passive argument straight to the trash can.
The splitting transformer would whip any
active stage butt hands down on the sonics but cheap ? Not