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When I first built it the amplifier was unstable - oscillating at about 250kHz I think. However it only oscillates if the input capacitor C5 is in-circuit but with Vin left open. It becomes stable if C5 is removed entirely and also if Vin is tied to ground, to a sig-gen or any source really. So why is the circuit unstable with C5 left open ended? Why does it become stable when C5 is removed? I'd have thought that C5, if left open circuit on Vin, would have no impact on the circuit since it's essentially an open circuit.