Hello everyone,
I'm designing a diode limiter with a 2nd order HPF before the limiting stage cutting at 12db/oct below 110hz.
I'm now struggling to find a low boost circuit that is the exactly the inverse of the 2nd order input HPF so that I can achieve relativately flat response at the output, so I've thought of using an identical 2nd order HPF with same cutoff freq and slope and adding it -out of phase 180- to the original signal a bit like the Dolby NR modules do.
To do this I sent the signal through a non inverting opamp, HPfiltered the output and fed it back to the inverting input. I'll attach the picture of the simulation schematic.
Now the mistery: tried an LTspice simulation first and found out that this configuration created a spike just before rolloff that I didn't really like -see picture.
I then inverted the feedback HPFiltered signal before adding it to the inverting input and got a really acceptable result on the simulation -see picture.
For some reason the first simulation gave a similar result on the breadboard but this latest configuration doesn't work in real life, as in the opamp becomes unstable and self oscillates.
I was hoping that someone would point out what's wrong with doing this, or why it could work in LTspice but not on breadboard?
Cheers!
Stay safe
I'm designing a diode limiter with a 2nd order HPF before the limiting stage cutting at 12db/oct below 110hz.
I'm now struggling to find a low boost circuit that is the exactly the inverse of the 2nd order input HPF so that I can achieve relativately flat response at the output, so I've thought of using an identical 2nd order HPF with same cutoff freq and slope and adding it -out of phase 180- to the original signal a bit like the Dolby NR modules do.
To do this I sent the signal through a non inverting opamp, HPfiltered the output and fed it back to the inverting input. I'll attach the picture of the simulation schematic.
Now the mistery: tried an LTspice simulation first and found out that this configuration created a spike just before rolloff that I didn't really like -see picture.
I then inverted the feedback HPFiltered signal before adding it to the inverting input and got a really acceptable result on the simulation -see picture.
For some reason the first simulation gave a similar result on the breadboard but this latest configuration doesn't work in real life, as in the opamp becomes unstable and self oscillates.
I was hoping that someone would point out what's wrong with doing this, or why it could work in LTspice but not on breadboard?
Cheers!
Stay safe
Attachments
I'm now trying as an alternative to sum a low pass filtered version of the original signal to regain the low end, only thing is that I get a notch where the two slopes meet due to phase clash. Once again inverting the low passed signal 180 degrees does fix it moderately well, though is feels wrong to have an output where the low end of the signal out of phase with the rest! Is that a reasonable concern?
