LM324N - Active Filter Audio Bandpass Filter - Need Help Changing B/W & Fc - Page 1
Forum section: electronics / projects

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 04:47:55 pm
content: All:I posted the following over on the RF and Microwave forum, because the attached circuit comes from an Ancient Heathkit HW-99 Novice Transceiver I am building. It was recommended I post here, thus the double post:"I am trying to modify an Active Audio Filter utilizing a LM324N. Attached is a copy of the schematic in question. According to the Circuit Description the following was given:- Fc = 700Hz- B/W = 450Hz (-3dB)- 6dB BW = 500Hz- Bandpass- Purpose = Active CW FilterIt is my goal to modify this circuit in a way to shift up the Fc and increase the B/W so that I can utilize it for Single Sideband Reception (SSB) and Possibly AM. The approach I have taken (Right or Wrong) was to attempt using some of the modeling software for Active RC Filters to simulate this change. To date I have tried TI FilterPro, and Ti Workbench to simulate. When I enter the specifications listed above, I am not getting R/C values anywhere close to what is seen in this schematic. As a matter of fact, all the simulations seem to spit out asymmetrical designs, where this appears to be symmetrical. It appears most of the modeling software I have used seems to favor Dual RAIL Devices, where this schematic is single ended, and uses pseudo ground.Wondering if anybody here can point me in the right direction as to why I am seeing such a discrepancy, and possibly recommend another approach to modeling this active filter so I can begin the process of changing its design parameters?Many Thanks,Rich"

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 07, 2025, 05:20:32 pm
content: Hi,don't double-post.(https://www.eevblog.com/forum/rf-microwave/lm324n-active-filter-audio-bandpass-filter-need-help-changing-bw-fc/)...you should know. ;-)

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 05:34:58 pm
content: I am trying to delete the old post after being directed to change forums. Its just taking me a minute, I apologize.Rich

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 07, 2025, 06:53:09 pm
content: I don't know the exact answer because I suck at math, but there are usually several ways of calculating filter blocks, sometimes keeping certain capacitor values the same or using identical multiple sections. They do it to make the math easier or simplify the component selection. Look up FSF and ISF for filters. Those are frequency scaling factor and impedance scaling factor, which allow you to easily change frequencies and overall component values.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 08:38:34 pm
content: Hi Conrad -I will look those up. From what I have been reading on the topic (interesting topic BTW), your assertion about keeping certain values fixed does come up in the research I have been able to do thus far. Other than a very brief introduction to Active Filters in University over 30 years ago, I have never attempted to design or redesign an active filter. I do have experience in RF Filter design, so I find the topic interesting, and would love to learn more.Rich

author: Benta
author_rank: Super Contributor
timestamp: August 07, 2025, 08:39:35 pm
content: What you have are two identical 2nd order MFB BP filters in series.There may be reasons for this, although it's unusual.The circuit will not give a "usual" 4th order response, but rather "2nd order squared", which in the end is also a 4th order response with suboptimal cutoff performance.For synthesis and simulation, I'd use a single stage with the parameters you listed, but use -1.5 dB and -3 dB for the cutoff specs instead of -3 and -6.TBH, I doubt you'll be able to get that response, but give it a try.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 08:45:19 pm
content: Hi Benta:I will give that a try shortly, thanks for the suggestion.ALSO - I have tried to remove the double post, and for some reason it just keeps coming back. I will see what else I can do to delete the original post placed in the RF, Microwave, Ham Radio Forum. I apologize for any inconvenience, as I have never been asked to move a post to a second forum before, thus I am learning.Rich

author: Benta
author_rank: Super Contributor
timestamp: August 07, 2025, 08:58:38 pm
content: AJ3G, since you're into RF, I'll give you a comparison:A 2nd order active filter has exactly the same response as a passive LCR filter, no matter whether LP, HP, BP, BS or AP. It's just built differently.Your schematic is equivalent to two identical resonant LCR BP filters in series, with a buffer in between for matching.Your specs indicate that you need a flat passband with very good cutoff at the ends.The right way of doing that is a LP section followed by a HP section.

author: bson
author_rank: Unknown
timestamp: August 07, 2025, 09:11:31 pm
content: CW, as in hearing an IF key tone? Or for music? Very different problem domains, more specifically phase linearity (group delay), gain compression, modulation artifacts, etc won't matter for a single tone.

author: Benta
author_rank: Super Contributor
timestamp: August 07, 2025, 09:15:07 pm
content: If you read the OP, the CW filtering was the original purpose of the circuit.The poster is asking about modifying it for SSB.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 09:20:03 pm
content: Benta -That makes much more sense to me. I think I now understand and will attempt the synthesis as you suggest in an attempt to get a result that matches the circuit I presented. I think the results I am getting from the software tools are probably correct as they appear to have a LPF and HPF thus the asymmetrical simulation output. This is what was throwing me off. This is very helpful, thanks!

author: AJ3G
author_rank: Regular Contributor
timestamp: August 07, 2025, 10:46:24 pm
content: Benta:That pointer you gave got me very close to what I am seeing in the schematic. Enclosed are the results from FilterPro. I do note R1 is about 2X in value, but I think I understand why. All the rest of the values seem to be very close when I set R2 to 680K Ohms. This is definitely progress. I think I have enough here to simulate some other filter parameters, and can prototype a circuit on breadboard.Unfortunately, FilterPro will not let me change cutoff frequency attenuation to anything less than 3dB.

author: Benta
author_rank: Super Contributor
timestamp: August 07, 2025, 10:57:42 pm
content: Yep.And if you double the dB amplitude readings, you'll get what the original Heathkit filter does. (Two identical filters cascaded).As you also see, it's a resonant filter and not really what you need for SSB.I take the above sentence back.This is really interesting.I think you may have a smart idea here: two resonant BP filters with staggered peaks. Adding the amplitudes is no problem, there's no phase dependancies in this configuration. Choosing the right Q for each filter could give you a "semi flat top" response.But you won't find any free filter programs for this, it's going to be maths and spreadsheet, I think.

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2025, 12:03:03 am
content: A word of caution wrt to comparing typical RF Filters (LC types) and Active types (Op-Amps). As mentioned by Benta if you cascade two typical op-amp based identical active filters the response is the single response squared, however this is not true with LC types filters which require proper termination.Because of the termination the cascaded LC types will have a distorted response since they require the proper source and load impedances to absorb and reflect energy to achieve the desired response, whereas an active type filter based around op-amps have a ideal isolation between sections provided by the op-amp (ideally a zero source impedance). Since the LC types require termination impedances the 1st LC filter doesn't "see" a proper load termination and the 2nd section doesn't "see" a proper source termination impedance and the response suffers!! If one must cascade these type LC filters it's best to include and isolation matched buffer amplifier or matched attenuator between stages.Best

author: AJ3G
author_rank: Regular Contributor
timestamp: August 08, 2025, 12:08:17 am
content: I am going to build the circuit just as it is in the original circuit and use an Audio Scalar Network Analyzer (Sound-card with Software) to get a feel of what it’s really doing as designed. The high level description in the manual does not say much other than what I have pointed out in my opening post.The IF Strip has a 3KHz Filter, and I plan on adding other XTAL filters on a separate PCB (i.e. 250Hz, 2.1KHz, 2.7KHz, etc) which will be switched in and out. Those should provide most of the heavy lifting. I may try an asymmetrical design (LPF/HPF), but I could look at trying to pick two Staggered Fc’s to see what the result is, but you’re are right, this would take some time to develop.I have a dedicated external Active Filter which I will be using anyhow, so the goal is to at least allow 2.7 - 3.0KHz to pass from the product detector, which might also be accomplished with a simple Active LowPass. One other Idea I had was to simply snip pins 9 & 2 on the IC, and place a jumper across those pins on the IC Socket, which effectively bypasses the Active Filter (U203C/U203D) but leaves the Buffer Amp U203A intact to overcome the drop caused by Q218.I am simply grateful I have a good place to start, which as of yesterday I was really scratching my head as to what was going on with the synthesis tools I was using.

author: Someone
author_rank: Super Contributor
timestamp: August 08, 2025, 02:34:16 am
content: Quote from: Benta on August 07, 2025, 10:57:42 pmSuch responses can be found with existing filter tools/programs even the TI online tool. Just ask for an Elliptic, Chebyshev, etc and specify the pass band ripple. Those polynomials fit an "ideal"/optimal response based on their constraints.

author: Someone
author_rank: Super Contributor
timestamp: August 08, 2025, 02:36:21 am
content: Quote from: AJ3G on August 08, 2025, 12:08:17 amWatch out as the original circuit does not bias the DC point for the intermediate stages of the filter (pulls it to ground!) so there can be lots of distortion etc.

author: Benta
author_rank: Super Contributor
timestamp: August 08, 2025, 08:50:46 am
content: Quote from: Someone on August 08, 2025, 02:36:21 amSo what's R2019 and R2021 doing?

author: Someone
author_rank: Super Contributor
timestamp: August 08, 2025, 09:31:25 am
content: Quote from: Benta on August 08, 2025, 08:50:46 amSetting the opamps around the middle of their rails. Which is not where the other intermediate stages are biased to. So gets a little confusing for a beginner to walk through.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 08, 2025, 12:16:53 pm
content: Did you look into filter design tools, most analog IC makers have one. AD, TI, Microchip etc.Most simulator apps have a tool for this too.Do your googlering before starting a thread.The filter is a multiple feedback bandpass (MFB) type. See attched on how to work out the values.Also seehttps://webench.ti.com/filter-design-tool/filter-typeThat'll even draw the schematic for you. Easy-peasy.You could suffer the network analysis if you're a masochist, but you don't need any advanced maths and basic arithmetic will do.Get out your four-banger and get to it. If you're very lazy use this app:https://sound-au.com/mfb-filter.exeWindows might bitch but it is safe to run.https://sound-au.com/project63.htm

author: AJ3G
author_rank: Regular Contributor
timestamp: August 08, 2025, 12:25:05 pm
content: Quote from: Terry Bites on August 08, 2025, 12:16:53 pmI did do research prior to posting, and I am using the tools you describe. If you look at my earlier post, the results I was getting from the tools you mention left me even more confused. Benta was able to straighten me out, so I now understand what I am dealing with. The Network Analysis is being used to compare the Texas Instruments simulation results to the circuit in question, so I have a good foundation to start from.

author: Benta
author_rank: Super Contributor
timestamp: August 08, 2025, 12:29:33 pm
content: Quote from: Someone on August 08, 2025, 02:34:16 amNot really. There's nothing Cauer, Chebyshev etc. about the response discussed here.We're talking twin-peak BP filters. I haven't found any of the filter programs being able to do that.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 08, 2025, 12:52:57 pm
content: Quote from: Benta on August 08, 2025, 12:29:33 pmThat is good to know too.. Just a few moments before you posted I was thinking this topology reminded me of the Cauer L/C Filter I used to design.

author: Someone
author_rank: Super Contributor
timestamp: August 08, 2025, 12:56:15 pm
content: Quote from: Benta on August 08, 2025, 12:29:33 pmTI webench (and other various tools) do exactly that: using a pair of resonant 2nd order bandpass sections to flatten the passband of a 4/5th order design.

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2025, 01:00:36 pm
content: Quote from: Benta on August 08, 2025, 12:29:33 pmReminds of Zverev Handbook of Filter Synthesis with Helical Resonators where the coupling between the shunt helical resonators is critical is setting up the slight coupling between resonator stages to achieve a given flat bandpass response.Best

author: RFDx
author_rank: Regular Contributor
timestamp: August 08, 2025, 01:02:40 pm
content: Quote from: AJ3G on August 07, 2025, 04:47:55 pmIt's certainly possible to broaden the BP-filter as needed for SSB/AM operation but the selectivity would be terrible. The resulting filter would have strong high-pass but almost no low-pass characteristic.Consider replacing the band-pass section with a 4th-order MFB low-pass filter. All that needs to be done is to adjust some of the passive component values, swap some resistors with capacitors and vice versa. An example that uses standard component values is attached. No layout changes needed. The bandwidth is 100Hz-3kHz which is a good compromise for both SSB and AM. Don't mind the current source at the input, it is just a placeholder for the actual product detector.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 08, 2025, 01:03:29 pm
content: Someone:This is the simulation I had planned on running this afternoon. Did you utilize the same center frequency of the resonators in your synthesis?

author: AJ3G
author_rank: Regular Contributor
timestamp: August 08, 2025, 01:19:34 pm
content: Quote from: RFDx on August 08, 2025, 01:02:40 pmA LPF design was one I was going to simulate today as well. I am ok with the compromise, as external to the circuit in question, I would have a Five-Pole LPF and Five-Pole HPF which is adjustable. I had mentioned earlier a LPF may be the correct choice for this application, especially considering external active filter.

author: Someone
author_rank: Super Contributor
timestamp: August 09, 2025, 01:32:10 am
content: Quote from: AJ3G on August 08, 2025, 01:03:29 pmThose center frequencies are what the Chebyshev constraints produced for some arbitrary filter response, chosen to highlight how multiple bandpass sections can be tuned to "flatten" the pass band. TI webench doesnt have a way to limit the order? so it might not be the easiest way to arrive at a solution where that is a hard constraint, but with some back and forth between putting in the limits and seeing what it produces you should be able to find some options.Personally I'd consider some asymmetric options with all low pass in the opamps and high pass in the coupling (as above by RFDx).

author: AJ3G
author_rank: Regular Contributor
timestamp: August 09, 2025, 02:18:13 am
content: Quote from: Someone on August 09, 2025, 01:32:10 amTI WorkBench and FilterPro are very limited in flexibility. I have found some ways to work around its limitations, but it’s not as straight forward as I am used to in other Filter simulators used for RF. I did run a few simulations, and I am really leaning towards RFDx’s solution. With an outboard Five Pole LPF & Five HPF (Variable), It looks like the response of the LPF will be the best. I am limited to two op-amps using the stock board, and even if I went to a better op amp the BPF frequency response is not stellar in the slightest over a 2.5 - 3KHz B/W. To get anything reasonable enough to rely solely on the Active Filter inside the receiver for a BPF it appears as though I really need four devices. Fortunately, I will be using tight XTAL filters in the IF Stage for the mode of operation I intend on modifying this receiver for, so that will definitely help, even if I ultimately choose the LPF as the way to go.I am having fun experimenting now that I not as confused as I was yesterday. I think early next week it will be time to breadboard a few examples including RFDx’s proposed design and seeing what those results yield.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 09, 2025, 03:39:10 am
content: You can certainly simulate with LTSpice, QSpice or any other. Still, I like using formulas if not too complicated. There are a couple books I've found useful, published long ago by Burr Brown. Try the active filter sections of:http://www.bitsavers.org/components/burrBrown/Burr_Brown_Function_Circuits_1976.pdfandhttp://www.bitsavers.org/components/burrBrown/Burr_Brown_Operational_Amplifiers_Design_and_Applications_1971.pdfShould also gethttp://www.bitsavers.org/components/burrBrown/Burr_Brown_Applications_of_Operational_Amplifiers_3rd_Generation_Techiques_1973.pdfjust to have it.

author: AJ3G
author_rank: Regular Contributor
timestamp: August 09, 2025, 06:45:03 pm
content: Quote from: Conrad Hoffman on August 09, 2025, 03:39:10 amHi Conrad:Thanks for the references. Now that I have been playing around with using these as filters, I have become more intrigued. As I said, the last time I as introduced to Active Filters was in University over 30 years ago, and it was only addressed in brevity. Since taking this journey, I have looked for some reference material and Burr Brown did pop up. I was also looking for something a bit more focused on active filters, and there are some text, but they were very expensive. Anxiously awaiting some parts to come in, so I can try a few of these designs I have modeled, and those given to me here on this forum, and see how they really behave.

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 06:57:05 pm
content: FYI - I was using these LP filters in my ham radio constructions, with SSB etc. (well, perhaps 20y back)https://www.analog.com/en/products/max7404.htmlThere are tons of variants:https://www.analog.com/en/parametricsearch/11484#/You set their corner frequency by a single capacitor, or by external clock..

author: mawyatt
author_rank: Super Contributor
timestamp: August 09, 2025, 07:05:58 pm
content: A healthy dose of mathematics and network analysis can serve one well when it comes to electronics filters. Active filters bring additional considerations/parameters/restrictions/limits into the filter arena that further complicates things. Filter design programs are great IF one takes the time and effort to understand the above, otherwise the final results in physical hardware are likely to disappoint!!Best

author: mawyatt
author_rank: Super Contributor
timestamp: August 10, 2025, 12:52:49 pm
content: Here's an example of an active 3rd Order LPF and shows how the SPICE op-amp model is inadequate in showing the actual physical response in a simple VCVS (Sallen-Key) active LPF. Check the entire thread as it also shows some interesting means to show effects of the output impedance of linear regulators and op-amps.https://www.eevblog.com/forum/testgear/sds800x-hd-actual-use-cases/msg5510947/#msg5510947Best

author: AJ3G
author_rank: Regular Contributor
timestamp: August 10, 2025, 06:03:20 pm
content: Mawyatt:Thanks, yes those are interesting frequency response plots. I am tying to configure my Tektronix MDO 3XXX so I can make some frequency response plots similar to yours, but I am going to have to do this measurement the hard way, I think.The designs I have been contemplating do have the output terminated into Hi-Z, but this is why I want to breadboard and try a few modeled examples before deciding on how to proceed.

author: mawyatt
author_rank: Super Contributor
timestamp: August 10, 2025, 07:05:22 pm
content: Quote from: AJ3G on August 10, 2025, 06:03:20 pmSorry we don't know the modern Tek DSOs (or Keysight), just the Siglent's we have. They come with a built-in Bode Feature and directly control an AWG (LAN or USB) to produce the input stimulus. Even the entry level SDS800X HD supports this!!Buried in that mentioned thread has to do with the op-amp output impedance becoming inductive (rising with frequency). This is a condition which can cause resonance with parts of the circuit and create some of the unintended consequences shown in the stop bands.Also some of the general purpose op-amps like the LM358 and LM324 have a unique but troublesome output amplifier design which adds additional grief with active filter use. As mentioned in the thread if you load the output of these type GP op amps with a shunt to the negative rail (or ground if single supply operated) this will bias the output amplifier npn bipolar transistor to supply this bias current and thus becomes an emitter follower. With enough bias current this can significantly improve actual results at the expense of additional power consumption.If you can spend some time studying this mentioned thread and the related links, there's some good information contained within!!Best