I'm working on a project based on this publication. I've built the circuit but something is not working.
I've tried checking the voltage with a multimeter (I do not have an oscilloscope at hand) and there is voltage before the band-pass filter (1-1.1 V). As expected, there is no DC voltage after the band-pass filter (black arrow).
But after the last op-amp the voltage should be in the 0-5 V range, according to the publication, which also says that the last op-amp introduces a 2.5 V DC offset to the signal. I think I should be able to measure about 2.5 V at the red arrow, but I consistently measure -0.6 V. Why? Also, I am using a -12 V source with a 72Kohm resistor,but this shouldn't matter I suppose (-12/72 * 15 = -2.5)?