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

  • \$\begingroup\$ This looks to me like a mistake in the schematic, honestly. But it could just be that the chassis ground is -12V relative to another ground in the system? \$\endgroup\$
    – Hearth
    Sep 19 '19 at 2:13
  • \$\begingroup\$ Note that this is a state-variable filter, so other schematics are available. \$\endgroup\$ Sep 19 '19 at 2:38
  • \$\begingroup\$ Nate Houk - I think I've found the origin of that circuit, which seems to answer some of your questions (e.g. that marking "4" is a non-obvious module pin number) & gives you more to research. I don't have time to write an answer, so here's the link for you (or anyone) to use: "discrete operational amplifier active filters" Ham Radio magazine Feb 1978 (pdf page 72, page number 70). See fig 4 That article references two articles in previous magazine issues, which you can find on the same archive site. \$\endgroup\$
    – SamGibson
    Sep 19 '19 at 2:43
  • \$\begingroup\$ @SamGibson Oh wow! How on earth did you find that!?! Did you find that by memory? Amazing! \$\endgroup\$
    – Nate Houk
    Sep 19 '19 at 16:23
  • \$\begingroup\$ Typo aside, I have worked with dual supply designs ,(single ended or dual ended) where the negative rail was considered system common and tied to chassis as well, it certainly leads to confusions on occasion, but internally it was always considered single ended supply , taking the full span of the supply for Vcc and Vee if it happened to be a double ended supply , and the supply midpoint ("0") used for optional features if installed. \$\endgroup\$
    – crasic
    Sep 19 '19 at 21:25

I suspect that this circuit is intended to use a single 12 volt power supply, with the negative terminal of the supply connected to the circuit ground. The power input labelling is misleading - it appears to imply a total 24 volt supply (+12 and -12) rather than a single 12 volt supply.

Test point 4 and the test point connected to Ground are just handy spots to connect your meter to measure the supply voltage - you can ignore them.

Does the place you found this circuit recommend any particular op-amp? If not, I expect any op-amp that will work from 12 volts will do.

When you want to ask questions about a circuit you found somewhere, you should include links to the source (and you should look around the source - it might answer your questions...)

  • \$\begingroup\$ You are correct, this is supposed to be a single supply 12v, and the -12v label is misleading or simply incorrect. The original schematic clarifies its the LM324 op-amp. The test point 4 means that you connect the +12v to pin 4 on the LM324. The other connection point from the GND should be to pin 11 on the LM324 which is labeled in the original schematic, but the pin 11 label is missing in the reproduced schematic which I posted above. Thank you for your help! \$\endgroup\$
    – Nate Houk
    Sep 19 '19 at 20:56

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.


The most important assumption not shown is what voltages are used by the OA V+,V-.

The certainty is that the midpoint between the external , DC V+,V- inputs shown will become the DC output of all Op Amps with a null input. The input and the notch out are AC coupled, the latter of which is dubious for the reason of blocking DC only on this output.

The missing assumptions for 0V and OA Supply and lack of specs , makes this schematic incomplete.


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.

  • \$\begingroup\$ Shouldn't smoke unless you connect the ground externally though. \$\endgroup\$
    – pipe
    Sep 19 '19 at 21:11

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