Question: A couple of questions:

1) It looks to me that the three-prong ground connection in the bottom right of this schematic is the only ground connection. I used the GNDPWR symbol in my KiCad schematic to represent this ground connection. Can (or should?) this GNDPWR be connected to the "normal" GND which I am using on the jack sockets which connect to the inputs/outputs?

2) Why is the -12v connected directly to this GND? Is this just saying that the GND reference is -12v? But because I am using op-amps and a dual-rail powersupply, don't I also need a 0V GND? Won't it cause a short-circuit if I connected the -12v GND to the 0v GND? I do not understand.

3) Can I use any op-amp for U1-U4?

4) For the op-amps should I use a +/-15V power rails? Or should I use +/-12V?

5) What does the test point 4 mean (near the +12v)? Should I just ignore that?

[![enter image description here](https://i.stack.imgur.com/6CLXU.jpg)](https://i.stack.imgur.com/6CLXU.jpg)

Answer: 
1 and 2) I read that as chassis ground symbol, so negative supply pin is just connected to a metal chassis. This and the fact that the op amps do not use ground as reference but it is biased to half of the input supply pins would suggest that this is a single supply device so it needs single 12V supply and 0V ground, not dual +/- 12V supplies.

3) In general, no. You would have to know what this filter is for and what parameters of the op-amp are important for the circuit operation. If this is from the ARRL handbook, then this is for audio I suppose and it uses LM324 which is pretty generic. And pin numbering must match, but in general quad op-amps have matching pinouts these days.

4) It says 12V on the circuit, not 15V. And based on 1&2, this circuit needs +12V only, not -12v.

5) They are not test points, it just shows the supply input connections to op-amp supply pins 4 and 11, but here the label 11 is missing.
I breadboarded this up and after a couple false starts, I finally have some answers:

1) Connecting -12v and +12v causes it to start smoking. So the schematic is wrong labeling it as -12v. Don't do this.

2) Connecting it up with +12v and 0v (GND) works perfectly! They should remove the -12v label as it is simply incorrect, it is actually just 0V and GND, NOT -12v. This schematic is also missing a label #11 on the lower GND input, which is the negative supply for the op-amp. The op-amps are supplied by pin #4 and #11. I am assuming they incorrectly labeled it -12v because usually the op-amps are dual-rail, but in this case they are powered by just +12v/0v (single supply). The op-amp is the LM324 as indicated on the original schematic and text which can be found at <https://www.americanradiohistory.com/Archive-DX/Ham%20Radio/70s/Ham-Radio-197802.pdf> on page 70 (page 72 in PDF). Thanks to @SamGibson for finding the original schematic.

So for anyone else looking at this schematic, the -12v is wrong and should be marked 0v.

This is a great little filter.