66. Low Pass Active Filters

Low pass active filters are covered in this presentation. For those people who have never worked with filters, a short overview of terminology relating to filter characteristics and terminology precedes the actual discussion. The Sallen-Key topology and Butterworth low pass filter are discussed, operating parameters are calculated and then verified via experimentation.
A Multisim file of the 2nd order filter and the images used in the video are available at:
drive.google.com/open?id=1Z6M...
This is a re-upload (?) of the video correcting some editing errors.

[@ramzi-sah]

one hour video to understand active filters, no pain *no gain*.

[@TimothyBratcher]

I cannot thank you enough for taking the time to share this knowledge with the world. Such a noble thing to pass on such deep amd profound knowledge, even to those of us that exist only in the ether. Thank you so, so, so much.

[@dougfarrell5091]

Thank you for putting this excellent video together. I am a physics major with quite a few electronics courses in my background and I thought your lecture was clearer and more organized than the ones I took in college on filters. Doug

[@merwynvincent]

1 hr is not enough to cover the entire topic. Really thanks for your time and effort.

[@amazagx]

Thank you very much. Don't worry about the length. You give details and show them in the breadboard/scope, so it takes time. Good stuff!!

[@pawelczarnecki7780]

Thank you 🙏 for this great video, by far the best lecture I’ve seen since college in 1998, you sir have a gift of passing on knowledge. It was a pleasure to listen to you. I will share this video in my groups.

[@linzobeanz]

This has been incredibly helpful, thank you so much! I have been stuck on this topic for days now and this is the first time I feel I have a solid understanding.

[@37leonardo]

Thank you. This the best lecture that I experience since my graduation in 1973.

[@TheOffsetVolt]

Thank you very much/

[@UpcycleElectronics]

As a hobbyist and someone that has created a few videos (none monotized/just for fun), I have to say, you put a LOT of effort into this. Thanks. You must have spent ages editing this to cut the final production with so much detail section by section and point by point. I believe someone needs to try creating something like this to really understand how much effort it takes. Thanks again. I like your method of explaining the details. I watched (but mostly listened to) the first 35 minutes last night while goofing around on the bench. The math was a bit much for 1/2 of my attention span but that's my fault. In my (limited hobbyist) experience I have tried to use working examples of circuits to help me understand circuits like audio op amps filters. I quickly discovered how it's relatively easy to learn individual filter configurations of first and second order filters. I found it challenging to understand a complete circuit like a preamplifier in a stereo system. For example I was curious what exactly makes a "subwoofer" unique as bass frequency driver in typical consumer home stereo systems. I specifically wanted to compare the powered subwoofer units to the typical entry level guitar amplifier. I have a couple guitar amps, so I found a few powered subs and tried to figure out their circuits. As I suspected, swapping the driver from a sub with one from a basic Fender Frontman shows no difference that I can hear even after recording and replaying them back to back. This lead me to one conclusion, an entry level, home stereo "subwoofer" is just a preamp filter circuit with a more powerful amplification stage. My problem (and the point I am trying to convey) is that it is really difficult for me to figure out the circuit theory in practice. I haven't gone through the standard EE network theory classes, and at 33yrs old, my memory of algebra is getting rusty. It might be helpful to many people like myself if you were to use some real world examples to show the various filters and configurations in practice. In particular I would appreciate examples that utilize the 4558 op amp for audio and the LM358/324 in general practice. I know the 741 is the original 'jellybean' op amp and has not been replaced as such is most of academia. However, the 741 is not used in practice in consumer electronics. If someone is interested in goofing around with hobby electronics, they can take apart almost any electronic device and find either a 4558 if it's audio, or a 358/324 for general purpose use. It's just a simple suggestion, but these are the most practical chips for anyone to play with as they can be found everywhere. A couple examples of circuits I started trying to understand are a Sony powered subwoofer model SA-WMSP-76, and a Soundtech 4150 multi channel PA Amplifier and equalizer. Both have PDF schematics available. I'm still trying to wrap my head around the entire PA system EQ but that's a big circuit to start with. The Sony sub or anything similar is the main kind of thing I think people can play with and potentially understand in practice. I tried to figure it out on paper several times but ultimately failed and decided to figure it out empirically by hacking switches between each of the different op amps and bypassing them to hear the results. I wish I had spent longer doing this but I ended up bypassing 2 op amps and reconfiguring the last 2 as a unity buffer and gain stage with a passive tone filter circuit in between, and a single N-channel JFET amplifier stage set up as the input. I would have loved to understand the circuit better, but I had trouble understanding the circuit in the schematic despite watching and reading several things online and making the mistake of trying to use The Art of Electronics to understand something so specific without going through a ton of background information. It is super easy to get a circuit like this from any second hand store for less than $20, and less than $10 or even free if you know where to look. I find similar circuits in almost every home theater receiver. I think a powered sub (the kind that has a mains AC cord attached) is a better device for understanding op amp filters though. It is guaranteed to have a op amp based filter circuit on board, and one that is very specific in it's design. It would make an excellent fundamentals demonstration. I don't dare to try such a thing myself as I would certainly mislead and misrepresent the details with my current understanding of such things. Sorry if the length of this offends anyone. I hope that my experience helps others that are traveling a similar path in life. Thanks again for all your efforts to help others understand electronics, -Jake

[@TheOffsetVolt]

Wow! Glad you liked the video. I will look into ripping some circuits out of devices and doing an analysis on them, it would be fun for me and and everyone might learn something. Starting with a PA and EQ circuit might be a little much but a sub woofer circuit is definitely doable. The channel, despite it's 2 plus year age, is still a work in progress. The goal was, and still is, to give everyone the material that would be learned in a 2 year program. The ideas you have would fit into the 2nd year side of that plan. One day I will actually have to go backwards and cover the first year DC and AC theory. Cart before the horse and all that. Again, thanks for watching.

[@tommcqueary7831]

It would help us older readers with poorer eyesight if you used 5 or 6 paragraphs with a blank line between them to break up this long comment. Thanks.

[@edtracey]

Really appreciate the time you put into this. 👍

[@TheOffsetVolt]

No problem 👍, my pleasure.

[@vikkydon1057]

Thank you for this excellent video

[@jakewangax4164]

great video, this is no pain but gain to me

[@DavidMsg]

Happened to stumbled up on this video looking for building a 2nd order (or more) active filter to roll off from 15khz. What a find! Thanks for the excellent video and explanation of how it works, and calculations.

[@DavidMsg]

If you were to use a dual opamp, and used one side for the 2nd order low pass filter and the other as a variable gain stage, which would you put in front?

[@TheOffsetVolt]

Glad I could help!

[@ece-getsrmpl9010]

Truly amazing explanation on the topic. Worth the time. Hope you would create more amazing videos.

[@TheOffsetVolt]

Thank you, I will

[@lordjoshuarobindumbleton275]

This video helped me a lot with my electronic engineering A-Level Exam, thank you a lot ! Infact im off to the exam hall right now, wish me luck!

[@TheOffsetVolt]

Good luck on those A-Levels and thank you for watching.

[@TomLeg]

Love the video! Not too long, I'm looking for that detail (though I admit I took a meal break and other pauses). You need to white balance the camera, so the paper isn't blue .. and brighter would be nice. The easy way to do that is to bounce some bright white lights off the ceiling, which produces a soft, mostly shadowless light. Of course you'll need to choose the right angle to avoid glare reflections. Would love to see a tutorial comparing different topologies .. I believe you do some of that in #67.

[@AbhishekDubey-mp3ys]

The video is super awesome! Thanks for investing your time and effort into this ! Just a quick suggestion, can you also add high pass filters to the title of this video as you explained both of them and I was not aware of that and it can help others out for not jumping to any other video. Thanks !

[@ScireMi]

Love your channel. Keep up the good work.

[@TheOffsetVolt]

Thanks, will do!

[@tuna3226]

Thank you for this helpful video and appreciate your effort.

[@TheOffsetVolt]

I'm glad you liked it and thanks for your time watching it.

[@nippysmilk6267]

Awesome video, many thanks!

[@saarike]

Excellent information about filters. My humble thanks.

[@TheOffsetVolt]

Glad you liked it. Thanks for taking to time to watch my videos!

[@JS-uz5tl]

Thanks so much! Great info, very well presented. excellent

[@TheOffsetVolt]

Glad it was helpful!

[@RakhmanovRakhat]

Thank you. Perfect explanation !!!!

[@SureshKumar-nk2ok]

thank u very much sir and happy Christmas,can i use this as filter for my 9 volts 1 amps power supply application

[@bfgoalie99]

Thank you very much for a great explanation on the math side of this!

[@TheOffsetVolt]

I'm glad it was helpful! Simple maths are the only ones I do! :)

[@JurekPrzezdziecki]

Excellent and very useful video. Thanks.

[@TheOffsetVolt]

Glad it was helpful!

[@jaydee9968]

Sallen-Key filters should always be designed with the op-amp buffer gain = 1 in order to maximize the loop transfer function, thereby providing the best performance of the closed-loop op-amp buffer. Design is easy because the normalized resistors are one ohm and the normalized capacitor values are easily calculated based on normalized [w0 = 1 r/s] pole locations.

[@sebastianmeert6160]

What a great video!

[@Bianchi77]

Nice info, thanks for sharing it:)

[@phillipneal8194]

Great presentation. Thank you. Question: I want to build a Sallen-Key filter for wspr on 20 meters. Do I have to worry about the slew rate on a 14 Mhz type of signal ? If so, can you recommend an op-amp that would handle this frequency level ?

[@peterholm9619]

I just found this excellent video, thanks! I have one question regarding the last filter version, the "unity gain" without feedback resistors (using capacitors to set the desired damping factor instead). I guess this solution also can be used to get higher damping factor (skirt), but how do you calculate values for a second stage? /Regards Peter, Sweden.

[@TheOffsetVolt]

The filter with the single RC pair followed by the op amp is really a passive device, not an active filter. The op amp acts as a buffer for the RC filter! My staff, consisting of one person (me), erred. Hard to find good help!

[@jakep8484]

Just found your videos, some good stuff on your channel, why no recent videos? Was hoping for a part 2 to the am radio video.

[@stefano.a]

technically it is also an active filter because it doesn’t work without buffer

[@DIYerGuy]

Jeez... that'a lots of people you have working for you.... :)

[@37leonardo]

You are absolutely wrong, so as OP Amp removes shunt load for RC and that makes big difference. So it is still active 1 poll filter. Need to learn and not to argue, my advice.

[@adrianvonino2574]

Thanks for the video

[@Yorumcu63]

Amazing video.I was see new this channel .What a bad youtube algoritmys

[@tommytan8571]

well,i give a A+ for this little project.

[@bones1225]

Love the video. Watched 3 times re the math. Have no clue on the benefits, less creating a better audio output. I guess it should work with a square wave. Pity about the geeks, who are more interested in your presentation than content. Last time I looked, it was shared knowledge for free, courtesy of your hard work.

[@TheOffsetVolt]

Glad you enjoyed it and thank you for watching.

[@merwynvincent]

Have a doubt in the unity gain schematic in the last part. By saying we don’t use the Rf feedback resistor did u mean we simply connect the output to the inverting input of the op amp? (Coz that’s how the basic unity gain buffer stages work). Pls reply.

[@gloriannikolov7297]

Why in your Multisim file the cutoff frequency is not at -3dB?

[@deonhugo3442]

Hi Marvellous video and very educational. This is the best I could find on Sallen and Key uni-polar second order low pass Butterworth unity gain filters on the internet. How about you make a video on the Deboo uni-polar integrator? Uni-polar or single-supply op-amp circuits are actually high in demand due to their simplicity of design. Feel free to contact me. Regards, Deon.

[@tonycarboni2002]

excellent teacher--ty

[@pir869]

Hi, At 30':26" the sound craps out just long enough to miss the dB to Vout calculation procedure,try as i might,i can't get the same answer you come to. Could you reiterate it please,i can't tell what the 10 is written for as the sound just comes and goes after you write the Av = . Very frustrating to say the least. I'm brushing up on my op-amps theory,from 30yrs ago when i studied for electronics design . Great videos,wish the internet was around when i was in college,young 'uns don't know how lucky they are now. Best wishes from uk,scotland.

[@joparkcat]

DB/20 to the power of ten.

[@brdlygy]

Such a good video mate

[@TheOffsetVolt]

Glad you enjoyed it

[@theengineer9910]

1 hour video of pure gold, i just cannot get good at the transfer functions its impossible. The first math I've encountered in electronics that has totally killed me its annoying as hell. Do you think its important for me to know transfer functions since theres calculators online that can basically do it for us? Like no doubt I'll figure it out soon but is it that important to know by hand? Besides that the stuff you did in the video seems super easy all the gain formulas and stuff, but goddamn the transfer function is the only thing that confuses me when it comes to active filtering

[@stefanb.933]

hi. What final should I use so that the sine wave can be played on a 4 ohm speaker?

[@christianreyalison8082]

thanks

[@MasterMindmars]

Very good I've been simulated a Butterworth 8th order but without Rf nor R1, negative direct realimentation, and works fine. I take account DF can't be more than 2 to be Butterworth. DF = 2 - 0 = 2 So, the circuit is more simple. Thanks a lot for your work.

[@TheOffsetVolt]

Thanks for sharing

[@MasterMindmars]

@@TheOffsetVolt I've make a program to calculate all the capacitors of the n - order at any fc. The program is tested and works fine. I've simulated a 16th order filter fc=1kHz. Furthermore, I can calculate the circuit for any frequency. Only 371KB in rar.

[@darrenrahnemoon2684]

If you put a Chebyshev filter through another negative feedback op amp can you remove that gain fluctuation?

[@fatemahalkandari5165]

which components were used in the circuit?

[@JurekPrzezdziecki]

@The Offset Volt 36:00 When you click and hold your cursors should stick both to the waveform :) You don't need to set them separately.

[@TheOffsetVolt]

Hi Jurek, That is very true. Naturally I have another scope now and need to learn a new set pf operations particular to it. Thanks for watching.

[@DIYerGuy]

Really excellent video... even in a college course this quality of explanation - which is necessary - is often not covered. One issue with the video is that at some points your voice drops out for a second of so - I don't think it's on my end. To the main question... everything is clear up to the point of where you derive a formular to get the voltage at the -3db point.. which in the video starts around 30:23... Is that formula Av=10 to the exponent of db/20 ??. Thanks for this video, I just Subscribed and look forward to more of your videos.

[@chasw36]

Yes, the formula is 10 to the (db/20)ith power. This formula is often used when you know the attenuation and want to get the voltage out.

[@nobody853]

i'm trying to do this circuit in proteus, but the plus and minus ends of opamp(ua741) are exposed(4,7). how many volts should I connect to those ends.

[@enriquegonzalez2802]

What if I build a 4 pole "butterworth" filter with identical resistor ratios of 0.603 in the first an in the second stage. Av1=1.603, Av2=1.603; Av=2.57 ???

[@chasw36]

I am also not seeing where the 2.57V is coming from for the AV in the 4-pole configuration. Unless it is being calculated directly from the total AdB which i calculate to be 8.22dB, then I get it.

[@overbuiltautomotive1299]

very in depth video thank you .i need to subsonic filter board as i am modifying a boom box and it uses a 3 inch ported sub-woofer i am using 2 now it tuned sounds as good as it can but a tune with all very low frequencies gone would be great

[@TheOffsetVolt]

Glad it helped

[@herisuseno2569]

Yes

[@hibakhk1404]

What happens if the components in the circuit are flipped and made LPF instead of HPF ?

[@massinissaautoscan]

dear sir please how to get quasi sine wave (inductive crankshaft signal for automobil) from square wave i.e crankshaft signal (hall effect) varing from 0 to +5 VOLT using for exemple a low pass filter note: the frequency range is : 800HZ TO 30KHZ THANKS SIR

[@hibakhk1404]

what happens if the components in the circuit are flipped and made lpf instead of hpf ?

[@TomLeg]

Picky detail ... @4:00, you say the gain is unity if Rf & Ri are the same ... that would actually produce a gain of 2 (1 + Rf / Ri). For unity, have a short for Rf or an infinite impedance for Ri, such as an open circuit. That provides 1 + Rf/inf or 1 + 0/Ri.

[@sreenandhan2544]

thanks a lott !!!!

[@TheOffsetVolt]

Welcome!

[@kcha986]

what is the formula for the phase lag?

[@user-uz6xn7ux8p]

v-good

[@user-hu4md7wy9n]

Wouldn't Q = 1/(2zeta)? where zeta is the damping factor

[@hamidmohammadi5744]

Hello dear I'm Hamid I would like to learn hpf and others filters

[@believethat6739]

Make video on chebyshev filter circuit

[@TheOffsetVolt]

I'll see what I can do - been working on my "real" job so the channel has been on the back burner.

[@astromania1257]

ok

[@santowalterscaramella6323]

aa

[@talentedboysciencematerial1972]

Nice but it is long time

[@incaroads001]

Audio dropouts? Really?