0
\$\begingroup\$

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):

second order low-pass filter

(Source of schematic diagram)

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)?

\$\endgroup\$
  • \$\begingroup\$ Yes there are, but 1st define the band stop loss at 25k and 30kHz with band pass gain but 2nd order only gives you 12dB per octave, you need to eliminate the noise at source... and you only allow a half octave "Lowpass, Multiple Feedback, Bessel 4th order" \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 30 '18 at 23:19
  • \$\begingroup\$ @TonyStewartolderthandirt Thanks, I am looking into multiple feedback filters now. \$\endgroup\$ – Hufflet Jun 30 '18 at 23:35
  • \$\begingroup\$ 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. \$\endgroup\$ – Hufflet Jun 30 '18 at 23:56
0
\$\begingroup\$

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/

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ Do you know that there are also 2nd-order Sallen-Key type filters which are inverting? Surprisingly, they are less known - however, they have good properties, in same respects even better that positive-gain stages. \$\endgroup\$ – LvW Jul 1 '18 at 9:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.