Other Parts Discussed in Thread: NE5532, OPA1642, OPA1678, , OPA1662, RC4580, TL074
I'm searching for the best reasonable practice of protecting input amplifiers of audio/studio equipment. When connecting or disconnecting cables between units that are not properly grounded (a common usecase), the grounds of the units are floating and a unit can experience a 50/60 Hz line voltage hum in the range of 100 or 200 volts on the incoming signal when a cable is connected and the tip of a tele plug first touches the floating ground of a jack.
1) When using an op-amp as input buffer, a typical simple input overvoltage protection scheme can look like the first example in the attached image below. Schottky diodes clamps any overvoltage to the rails, limiting the op-amp input voltage to under half a volt, thereby (hopefully) protecting the op-amp.
2) In a real-world scenario it's likely that it looks more like the second example below. The 47u electrolytic capacitor blocks DC, and the 330p caps protects from having RFI coming into or out of the unit. The op-amp input is still protected, but the electrolytic and the components to the left of it are not protected. The 330p caps can get large voltages, and there could potentially be a large voltage over the electrolytic if an incoming large DC voltage for some reason would last much longer than a period of the line level hum.
3) In the last example the Schottky diodes have been moved to the input side of the 47u electrolytic cap, protecting the caps a little bit better - but possibly providing less protection of the op-amp input, in case the electrolytic would become charged and a large voltage is then suddenly applied to its input. The op-amp input voltage is not guaranteed to always stay within the Schottky protection range because of the electrolytic and what happens on the input side of it.
So what would be the best practice of protecting typical op-amps like OPA1662, OPA1652, OPA1642, OPA1678, NE5532, TL074, RC4580 etc from input overvoltage in typical audio input scenarios? Op-amps can often have different types of internal diode protection between inputs and/or to the rails - do these op-amps have it, and if so, does it protect them from overvoltage in the third example?
