Help needed with low-pass Sallen-Key filter - Page 1
Forum section: electronics / projects

author: modul
author_rank: Newbie
timestamp: July 08, 2025, 10:28:30 am
content: Hello, everyone. New here, so be patient.I am trying to make a low-pass filter with a cutoff frequency of 400Hz. My schematic is in the attached file (Screenshot.png). The PCBs are manufactured and assembled, so changes in the schematics are a problem. The best way would be BOM change.I was using Analog Devices Analog Filter Wizard for reference, and it promised me something like (Example.png).But when I measured a few points to get a crude frequency response, I got what is in (Fresponse.png)It kind of works, but at the same time it doesn't. It is cutting way too much in the low frequencies.About the schematics, I am using a resistor summer (R15 and R20) to combine left-right channels and get a single output because it is for a subwoofer.The filter itself is connected to a D-class amp made of TPA3128D2DAPR that works fine on its own. The problem is the low-pass filter.I am dividing the voltage for the op-amp inputs with R23 and R25 so that I can get the best swing in both directions. The output is decoupled with C32.I know that R27 is too high, with 8 mA supply current of the op-amp. With 20R it works fine.Sorry for the long introduction. Now, can someone tell me why my filter does not work? And preferably, is there a way to fix it with a component change?If I left anything out, let me know.Thanks in advance

author: joel16
author_rank: Contributor
timestamp: July 08, 2025, 12:58:16 pm
content: The inputs are AC coupled but the op-amps are grounded. I think there is some other funkiness going on as well.

author: Circlotron
author_rank: Super Contributor
timestamp: July 08, 2025, 01:27:42 pm
content: C37 is a bit sus. It is connected to what is a virtual ground node at the opamp input. My guess is it is doing something unexpected to the negative feedback at that point.

author: modul
author_rank: Newbie
timestamp: July 08, 2025, 02:03:00 pm
content: @joel16The op-amps are grounded because I have only single 24V supply. The resistor divider (R23, R25) is moving the inputs at half the supply thus making vitual dual supply.I think the input should be AC couled to remove the DC bias. I may be wrong.@CirclotronR15, R20 and C37 make the first low-pass, the second one is R16, R21 and C36. If I leave only one the cut-off will not be as sharp.Forgot to mention that R24 is not mounted. The output from the first op-amp (U1A) is at 12V, so U1B does not need biasing.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 08, 2025, 02:16:18 pm
content: On your Fresponse.png, what are the units on the y axis?

author: modul
author_rank: Newbie
timestamp: July 08, 2025, 02:44:24 pm
content: Volt. I am using a sine wave gen with 100mVpp at the input and testing it with different frequencies.2,5V might be a bit much but I do not hear any distortion/clipping when music is played.If needed I can lower the gain of the first op-amp with R14.... R30 is also not mounted.

author: themadhippy
author_rank: Super Contributor
timestamp: July 08, 2025, 03:27:14 pm
content: 400hz cut off seems rather high for a sub,thats into low mid territory.100Hz or maybe less if the bass drivers can cope, is more were i likes to be.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 08, 2025, 03:51:30 pm
content: Yes, way too high for a sub. But we should leave that for later! 😁Back to the topic... I'm sorry I can't contribute anything useful at the moment.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 08, 2025, 04:52:48 pm
content: C37 should not go to ground directly. U1a has unity gain at DC so it will have an output set at Vcc/2. Pin 5 is already biased to Vcc/2 via R16, R21R24 should be connected via a capacitor to ground not to R23, R25 etc.A series resistor is required and the pot needs to be moved to control gain.(I'd forget about gain in this circuit and control it at the power amplifier input)The value of R27 is way too high. It's starving your opamps of operating current.10R might be nearer the value needed. I think I'd omit it and use a linear regulator to take the 24V down to ~15V.You don't need a 20V swing capability to drive the lin amp! A couple of volts is more than enough.A regulator will help keep the psu crap out of your opamps eg LM317. 15.31V , "R1 = 240, R2 = 2700"C34 needs to be higher, say 47u. Put 100n in parallel with it.R17, 10k, why? If you want a bit of line matching/ output protection a 47R would be fine. Not really needed though.If the lin amp has an ac coupled input (highly likely) then C32, R17 are not necessaryIm guessing the inputs are at "line level" so about 2.8vpk-pk. If you want line level out then just make U1a unity gain.Set your high pass corner in the suming amp and your low pass in the filter. 16Hz and 200HzSwap the positions of R15, C30 so that the cap is sat the input rather than at the opamp.Do the same for R20, C35.C37 and the series resistor that needs to be added must set a highpass corner 10x lower than your frequency of interest. So that 1/2pi()Rnew.C37= 1.6HzUse passive addition to convert from stereo to mono. That will free up U1a letting you make a higher order filter.Free simulator tools let you verify your circuit.eg Qspice. Use generic opamps if you cant find the NE5532 to see the DC conditions in your circuit.

author: Circlotron
author_rank: Super Contributor
timestamp: July 08, 2025, 11:50:53 pm
content: Quote from: modul on July 08, 2025, 02:03:00 pmMy point was that any HF roll off achieved by C37 on the incoming signal will be countered by C37 also rolling off any HF in the feedback signal, therebyincreasinggain at HF. What the left hand gives, the right hand is taking away. Much better to put a cap across the feedback resistor as shown by Terry Bites in post #8

author: JoeyG
author_rank: Regular Contributor
timestamp: July 09, 2025, 01:07:01 am
content: Spend a full day learning LTspice or QSpice or MINDI or NGSpice.Then model the circuit in one of these spice simulators.There's 47K members in LTSpice that can help youANDVideoshttps://www.youtube.com/results?search_query=LTSpiceIt will be worth the effort for future analog audio designs also.

author: modul
author_rank: Newbie
timestamp: July 09, 2025, 12:46:35 pm
content: @themadhippy400Hz is a bit high, but it can be tweaked.@Terry Bitesoh boySo C37 should be connected to half the supply?R24 is not mounted I mentioned it in the original post. I can try with a cap to ground.The pot is there because TPA3128D2 has only for steps of volume control. I can add a series resistor.R27 is 20R I mentioned it in previous post.I know that 24V is a lot, but it is used in the power amp. R27/C34 is poor's man LM317.I can make C34 higher and 100n is always a good idea.R17 is 10k because at 36dB the input impedance of TPA is 9k. Maybe it is not needed.TPA3128D2's datasheet recommends decoupling caps for the inputs.I did not think that swapping R15/C30 would make a difference but I can try.I need some time to think and experiment.@JoeyGI used LTspice prior to going to Altium, and it was looking good.

author: bson
author_rank: Unknown
timestamp: July 09, 2025, 10:17:56 pm
content: Looks like a biasing problem. The AC-coupled inputs are intended to be biased between C37 and the pot, but for this the pot should be connected to +24V, not to the opamp output.