I'm wondering if there's some circuit configuration (presumably an op-amp based design) that would subtract two signals (say, two audio signals) in a way that is *100% and absolutely unaffected* by components tolerance.
I do not mean using 0.1% tolerance resistors, or matched resistor networks, or adding a potentiometer for fine adjustment. Those solutions *minimize* the effect of components tolerance.
For example (more like an analogy): if I need a non-inverting buffer/amplifier with absolutely precise gain, say 2, then I could try the standard op-amp circuit, and use two identical resistors, for a gain of 2. Problem is, gain *can not* be exactly two (well, it can not be *expected* to be exactly two); I *can* obtain an absolutely precise gain of *1* ... Connect output terminal*directly* to the inverting input, and voil=E0 --- this will give, from any conceivable point of view (at least for every practical purposes), an *absolutely exact* gain of 1; where I'm trying to get is: the solution in this example goes beyond the highest available precision components; it goes beyond the most expensive and most precise matched pairs of resistors, etc.
So, my question: what about for a circuit that subtracts two signals? Or, equivalently (and even better for audio signals), a circuit that adds two signals + an inverting circuit with gain 1 ? (the standard solutions I know for these two rely on components precision/tolerance)