Mic Preamp output protection

[ddietz]

Hey folks,

I finishing up a dual chanel preamp for a friend, based on the INA217/THAT1510 chips, and have a question about protecting the output from phantom power. I'm concerned that since phantom supplies are usually easy to accidentally turn on (most are small push button switches right next to the mixer/interface inputs), it would be possible to damage the preamp. I cannot imagine that sending phantom into the output of the chip would be good for it.

What should I do to prevent phantom power from accidentally being sent into the output of the preamp? I'm using an unbalanced output and basically following the INA217/THAT1510 datasheets. A large electrolytic with a high reverse breakdown voltage? A pair of diodes like the 1N4148 to power rails used for input protection? What would have the least impact on sound quality? Transformers are not an option

Thanks!

[ddietz]

I am starting to realize that having an unbalance output may make this a non-issue. I am not experienced with many different mixers and intrefaces, but is is possible/common for them to allow phantom send over TRS inputs? I just checked my Focusrite Saffire 6 and there is no phantom over a TS nor TRS plug (device uses combo XLR/TRS) but there is using the XLR.

Is that common practice?

[Osvaldo de Banfield]

Yes, I believe a nonpol cap plus a diode reverse biased to each rails are sufficient, although I suggest to add some low value resistor as say, 10 ohm to limit the possible inrush current, and in extreme case it will blow protecting the output pin of the amplifier. Also, a couple of back to back zeners with a 10% VCC margin between out and gnd is capable of protect your device.