Hi,

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)

Thanks,

-Zico