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I noticed that 10 pf across 22k ohms sets the bandwidth very high, so high that stray capacitance from the sum resistors could cause ringing or even oscillations with the sum opamp. I would increase that cap to either 47 or 68 pf to maintain stability. That would also eliminate the need for the 100 ohm buffer resistors which will lower crosstalk. You can fine trim it by feeding a 2 k square wave and adjusting that feedback cap for best symetry.
If you want lower noise and THD, sub the OP-275 with a National LME49720NA or a AD8599 on an adaptor. Those are about 20 db cleaner. They are also precision devices that can be used without the 100 uf coupling caps.
Jim Williams
Audio Upgrades
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