Opamps for Active Crossovers 383 If an input is fed through a significant resistance then the input pin will be more negative than ground due to the voltage-drop caused by the bias current. The inputs are connected together with back-to-back diodes for reverse-voltage protection, and therefore should not be forcibly pulled to different voltages. The 5532 and 5534 type opamps require adequate supply-decoupling if they are to remain stable; otherwise they appear to be subject to some sort of internal instability that seriously degrades linearity without being visible as oscillation on a normal oscilloscope. The essential requirement is that the +ve and -ve supply rails should be decoupled with a 100 nF capacitor between them, not more than a few millimetres from the opamp; normally one such capacitor is fitted per package as close to it as possible. It is not necessary, and often not desirable to have two capacitors going to ground; every capacitor between a supply rail and ground carries the risk of injecting rail noise into the ground. The 5532 is a robust opamp but it is possible to permanently damage it so that it keeps working but shows high distortion. This seems to be associated with faults where one supply rail fails; see Chapter 18 for ways of guarding against this. Obviously such specimens should be disposed of at once to prevent confusion in the future.