I'm trying to build a state variable filter, to this circuit diagram:

circuit diagram

All resistors are \$1k\Omega\$ and both capacitors \$1\mu F\$.

I've constructed it on a breadboard:


But the output I get, ignoring some distortion when the input is loud, is the same as the input, no filtering is done at all. I have tried powering the opamps with both a 9v battery and a +12v/-12v power supply, and still, the output is the same as the input. (I have of course done the calculations on the circuit and the output should definitely not be the same as the input). The opamps seem to be working (they're getting power and the + and - inputs are at the same voltage).

How can I diagnose this? I do not know where to start.

  • \$\begingroup\$ start with a frequency sweep measuring amplitudes at each output. \$\endgroup\$
    – Jasen
    Aug 9 '15 at 11:06
  • 1
    \$\begingroup\$ Do you have the ground point shown on the schematic at centre rail? \$\endgroup\$
    – Andy aka
    Aug 9 '15 at 11:48
  • \$\begingroup\$ @Andyaka as in connected to the centre (0v) of the -12/+12v power supply? Yes \$\endgroup\$
    – ACarter
    Aug 9 '15 at 17:13
  • \$\begingroup\$ Another good tool is LTSpice available for free. Consider simulating the circuit with that and ensuring it is not a design problem vs an implementation problem. \$\endgroup\$
    – cowboydan
    Aug 9 '15 at 21:45

At first, use split power supply only. Secondly, ground R1 and check if all opamp output nodes are app. at 0 volts DC. Then, connect a signal source (small source resistance) to R1 with a (small) level that does not cause any opamp to saturate. This level depends on the resistor values you have chosen. What are these values?

Example: At very low frequencies the lowpass gain (output at the most right opamp) is R5/R1. The maximum high pass output (first opamp) for large frequencies must be R4/R1.

  • \$\begingroup\$ Thanks. The op amp outputs are at -10, 0,4, and 10 volts respectively (somewhere around that, there's a bit of movement). That's very wrong, right? \$\endgroup\$
    – ACarter
    Aug 9 '15 at 12:34
  • \$\begingroup\$ Yes - something is wrong with the circuit. In the circuit diagram, all opamps have dc feedback. Hence, without any signal the DC output voltages for each unit should be app. 0V. Check the wiring again. \$\endgroup\$
    – LvW
    Aug 9 '15 at 14:55
  • \$\begingroup\$ Found it - after looking at why those voltages were there, I found a connection where there shouldn't be. Sorted. \$\endgroup\$
    – ACarter
    Aug 11 '15 at 12:48

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