I am working on an audio filter, and my goal is to filter out noise below 90Hz and 20kHz (the range of my speakers). I will have some nearby circuits running at 25 to 30kHz, so I have decided to go with a second order high pass filter to avoid the noise from these circuits. So far, so good. However, when looking at schematics for active, low pass second order filters, I can only find this circuit (and similar variations):
This would be great, except I have read in several places (such as here) that my audio signal would be better if this were inverting. Unfortunately, after playing around with this for awhile and scouring the web, I could not come up with an inverting variation.
So here is my question: Is there a single op amp inverting second order filter, or do I need to choose between this one and cascading two first order filters (and if so, does anyone have opinions on which of the two may be better for audio)?
Best Answer
Answering my own question:
It would seem that multiple feedback filters do exactly what I was looking for, somehow missed them in my search (curses to the abundance of Sallen Keys). I will also be pursuing a higher order filter because, as pointed out, the noise reduction isn't all that great due to the relative proximity of my noise source and desired pass band. Again, thanks @TonyStewartolderthandirt for the helpful guidance.
As it may help others, I also came across this tool, which shows the response of multiple filter types and orders very nicely: http://www.analog.com/designtools/en/filterwizard/