Soundcard Input Isolater/Saver

[cuibono]

I've damaged three out of four inputs on my soundcard - time to do something about it! Looking around, this isn't new territory. I thought I might develop it some more, hopefully.

As it turns out, the line input of my soundcard is DC coupled. It looks like there may be diodes protecting the input opamp (a 5532), but I'm not sure what they are doing, because something has been damaged - the noise floor is up 40dB...

So, I want to incorporate a few things:

-a DC block/HP filter, around 10Hz
-a LP filter for ultrasonic stuff, around 100kHz
-zener diodes, to protect from over-voltages
-an differential instrumentation amp, to prevent ground loops (has been handy in the past)
-a possible buffer on the output, to keep the filters well behaved

Here is the simplest option:

It has a voltage divider that attenuates the same amount as the gain of my amplifier. The LPF is about 10Hz, the HPF is about 100kHz, and the zeners should be 2.2V versions.

Here it is with opamps:

Still a work in progress, but this is about it. There is no input attenuator anywhere. The third opamp has 100V/V gain, which is something I use with my scope to look at small signals.

I'm thinking battery power. I'll draw up boards and BOMs, and post them.

Any feedback?