12voperational-amplifierpowerpower supply
I plan on building this buffer circuit to use my sound card as an oscilloscope, but I’m not sure on the best/acceptable way to get the -12V.
If it is only necessary to drive the MOSFET for a brief moment, you can use a concept like this:
simulate this circuit – Schematic created using CircuitLab
While the output of the comparator is high, the capacitor gets charged. Then when the output of the comparator switches LOW, the capacitor is still charged and the right plate will be pushed below 0V. The capacitor cannot discharge through the diode so it will stay charged for a while with only leakage current flowing.
Usability depends on exact specification of your components, but you can play with capacity, an extra pull up resistor and the cathode voltage on the diode.
The power rails probably don't need accurate regulation but they will need to be clean of noise. I'd consider using a flyback regulator with two output windings that can generate both dc rails then use a bit more LC filtering to smooth ithem more making it suitable for audio.
There are plenty of chips from linear technology that fit the bill.
Best Answer
Crazy answer. Don't.
TL082 will easily run off 9V. You'll be using the sound card's line input which is approximated 10k, which is easily driven by this op amp. So use two 9V batteries to create a +/- 9V supply. The power will be as smooth as possible without major shielding. It will certainly be quieter than the analogue and digital chaos inside your PC. Just use a double throw switch to control both sides of the supply simultaneously. Don't forget that you only need about 3Vp-p to max. out your line input, so 12V isn't strictly necessary to use it as an oscilloscope.
I don't know your use case, but I suspect that the number of hours you'll be using this will easily fall into the useful operational life of a good branded alkaline PP3. You'll only be consuming ~30 mA. And it's less hassle than building an active power supply. I've built a very similar buffer for my sound card and it works very well. However, I'm sure that I don't need to remind you of the very serious bandwidth limitations you'll face. You'll achieve 20kHz nominally, but might be able to push that to 96kHz if your sound card / software supports it.
Most importantly though I would share a personal philosophy. I never build test equipment. I buy the best I can and if I can't, I save up till I can. The reason is that if you build your own (and you're an amateur -like me), you'll never have full confidence in it's readings. You'll always be wondering if that 50Hz signal dominating all your results is from the device under test, or mains feeding through your circuit's voltage regulators. At least eliminating your PSU goes some way to meeting this philosophy.