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I need help selecting a bandpass filter

Super-Dave

Member
The project I'm working on is a 12v 5 band audio spectrum analyzer for a car. I'll be using TL074 op amps, but I also have LM324s & LM3900s. What I desire is a relatively flat response curve and a steep roll-off (to suppress input outside the selected frequencies).
I am overwhelmed by how many topologies there are: Multi-Feed Back, Bessel, Butterworth, Chebyshev, Sallen Key, the list goes on and on...
I'm thinking I will probably want at the least second order. But I'm having a hard time choosing.
Most would be wide band, 20-150 Hz, 200-800 Hz, 1-3 Khz, 4-8 Khz, & 8-16 Khz.
Please help. Thank you in advance for your response.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Have a look at the MSGEQ7 IC?
(The title is misleading, it is for a display and is not itself an equaliser).

That does the multi-band filtering and allows you to use whatever display type you like, either basic logic with a counter and analog switches to feed bar graphs, or connect to an MCU analog in and logic pins.
 

Super-Dave

Member
Have a look at the MSGEQ7 IC
Not enough room in my display housing for 7 rows of lights. Each row of lights is laid on edge of a perf board with 3/8" spacers separating each board. If I use smaller spacers the circuits would come dangerously close to touching the next board, which could lead to a shorted circuit. Plus there's heat generated by the resistors and the LEDs themselves.
Gotta work within the space limitations available. It has to fit inside the window of an automotive DIN radio chassis. Maybe if it were all surface mounted components, but my soldering skills are limited to through-hole perf boards. Maybe I COULD leave off a couple rows and use the MSGEQ7 chipset to drive only 5 rows, but then there would be audible music coming from the speakers that wouldn't be represented by the display.
 
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Super-Dave

Member
This is what I have to work with:
IMG_20210516_153212700.jpg


No more room for additional rows of lights. I have to make due with 5 bands.
 

audioguru

Well-Known Member
Most Helpful Member
The old MSGQ7 has multi-feedback or gyrator filters with a narrow band of frequencies at the peak and a wide band of frequencies at low levels. It misses many frequencies and even more will be missed if you leave off some of the bands.
I have always used Butterworth Sallen-Key filters for audio.
EDIT: 170 LEDs to blind everybody and quickly kill the battery?
 

Super-Dave

Member
170 LEDs to blind everybody and quickly kill the battery?
Lol! The sound system for Project Turd is equipped with a 2nd trunk mounted deep cycle battery, a 1/2 farad cap, 5 amplifiers & 12 speakers (+8 tactile transducers mounted in the seats to augment the subs) pushing 1800 watts, with a battery isolator to charge it when it's running. All the resistors on the LEDs are 510 ohm (instead of 430s) to prolong their lifespan and reduce brightness, plus there's a tinted lens covering them.

In case you're wondering why I call my car Project Turd, it's because it's doggy doo brown and it gets crappy mileage. Big honking 4 door land yachts powered by muscle car big blocks tend to have thirsty appetites.
IMG_20210829_182555017.jpg

With a heavily cammed 400 bellowing thru headers, glass packs and side mounted 4" tips, I need a little power behind the speakers to hear my tunes over the music of the motor. And I want some visualization to accompany the audio.

What kind of cutoff slope will a 2nd order Butterworth filter give me? I don't want to see the thump of the kick drums or the crash of cymbals represented in the middle vocal bands.
 

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