Selecting resistors & capacitors for bandpass filters

The bandpass filters IC probably used switched-capacitor Butterworth Sallen-Key circuits.
Some other manufacturers make switched-capacitor lowpass filter ICs like Maxim.

audioguru said:

The bandpass filters IC probably used switched-capacitor Butterworth Sallen-Key circuits.
Some other manufacturers make switched-capacitor lowpass filter ICs like Maxim.

Click to expand...

Thanks but I think I'll pass. Something to be said about doing it myself. There's only so many functional brain cells remaining in my head, and I've already retasked a number of them to learning what I needed for this project. As it is, I've lost quite a bit of trivia already and forgotten a lot of the lyrics for a good majority of the Beatles, John Denver & Bob Dylan catalogs just to learn op-amp filters & the math required for high order bandpass filters. Formulas, algorithms, sine waves and trigonometry! My dreams at night are haunted by alphabet math.

Super-Dave said:

Thanks but I think I'll pass. Something to be said about doing it myself. There's only so many functional brain cells remaining in my head, and I've already retasked a number of them to learning what I needed for this project. As it is, I've lost quite a bit of trivia already and forgotten a lot of the lyrics for a good majority of the Beatles, John Denver & Bob Dylan catalogs just to learn op-amp filters & the math required for high order bandpass filters. Formulas, algorithms, sine waves and trigonometry! My dreams at night are haunted by alphabet math.

Click to expand...

Why strain your brain?, there are numerous electronic calculators available on-line, including ones for all kinds of different filters. Such as this one:

Why not do it with FFT and get rid of all the filters -



Video, code and schematic here for a ESP32 based array -





Regards, Dana.

danadak said:

Why not do it with FFT and get rid of all the filters -

GitHub - s-marley/ESP32_FFT_VU: A spectrum analyzer VU meter for the ESP32 and a FastLED matrix

A spectrum analyzer VU meter for the ESP32 and a FastLED matrix - s-marley/ESP32_FFT_VU

github.com

Tiny Arduino Music Visualizer

This easy project combines an Arduino with our microphone amplifier and bicolor LED matrix to create a music visualizer that responds to ambient sound.

learn.adafruit.com

Video, code and schematic here for a ESP32 based array -

DIY simple FFT Spectrum Analyzer

The device is made on 16x16 LED matrix with WS2812 chip on diodes.

hackaday.io

Regards, Dana.

Click to expand...

First off, it's not in stereo. 2nd, being Arduino based, there's software involved, I want analog, not digital. This is going into a '73 cruiser. I'm trying to keep it a classic, that means nothing that screams post millennial. If I could find a decent functional 8-track, I would. 3rd, it sources from a microphone. I want to be able to see only the music as it comes from the radio, not conversations from the back seats or the rumble of a big block or road noise. Finally, it has to fit into a 2"x7" single DIN chassis window. Or at least the display. Going digital somehow seems like cheating.

Thanks but no thanks.

Se page 46 of this magazine file:

A nice simple five band audio filter setup, specifically for visual display!

Nigel Goodwin said:

Why strain your brain?, there are numerous electronic calculators available on-line, including ones for all kinds of different filters. Such as this one:

Bandpass Filter Designer

Bandpass Filter Designer. This (Javascript) Tool lets you decide, which approach should be used. For small bandwidth's a bandpass is suiteable, whereas for larger a bandwidth's a highpass/lowpass combination is more promising.

www.changpuak.ch

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Thanks, but I found a site that will spit out actual realistic resistor & capacitor values that can be purchased, it even gives a bode plot and will tell you if it oscillates or not.

http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

Now he doesn't list 4th order low pass or high pass, but nonetheless, it works out the same, right? If I cascade 2 low pass filters back to back, then cascade 2 high pass filters back to back, I end up with an 8th order bandpass with 4th order roll-offs, right?

Here is a good filter design tool -



Regards, Dana.

I woke up this morning thinking about incorporating a rectifying peak detector into the design of my spectrum analyzer & I realized I was overthinking, it would be redundant. My transistorized diode ladder VU Meter already has all of that built in. That's why the low pass subwoofer level meter worked (150Hz 2nd order Sallen-Key). The very nature of the 1st input diode on the diode ladder was what led me here to begin with, seeking help with diminished sensitivity (which led to introducing an input bias to correct it). Now I can just concentrate on is the bandpass filters.

Thanks go out to all for your suggestions and input, even those of you whose advice I declined to pursue - you meant well. You guys were a bigger help than those around me IRL who know little to nothing about electronics. That includes my father-in-law who was an air force electronics specialist in radar & weapons targeting. Audio systems are outside his forte. I am grateful for this forum that allows me to pick the brains of those who know more than I do about this stuff.

I'm going to try breadboarding & cascading second, 3rd & 4th order low pass & high pass filters, see what works best for me. I'll write down my diagrams, post my results and likely be back with more questions. (Like, "Why doesn't it work? What did I do wrong?") Again, thank you.

Super-Dave

If you do a wide variety of audio stuff here is some switch cap filters that may
be of interest at a later time -


The beauty of most of them a simple clock determines center frequency, in
case of BP filters, or cutoff in case of LP, BP.

I used OpAmps to build variable filters back in the 70's. Architecture was State Variable.
Used multiplying DACs to control freq, BW, eg. in place of R4, R5 here. Beauty of architec-
ture is simultaneous LP, BP, HP outputs. I used these in synthesizers.



You mentioned in a prior post love for restoring older technologies. I do a lot
of that activity with shortwave receivers, transceivers, vacuum tube and discrete
transistor era. In some cases I mix old and new technology to get a desired result.
Its all good.

Lots of fun.


Regards, Dana.

A 2nd-order Butterworth highpass or lowpass filter has a fairly sharp -3dB corner and a flat response at the top.
Two of these filters in series are not Butterworth because they make a droopy -6dB corner and a curved (not flat) top.

Instead, the resistor and capacitor values must be set properly for 3rd-order or 4th-order Butterworth filters.