Schematic Review Request: High Impedance Input Buffer and Filter  - Page 1
Forum section: electronics / beginners

author: R-802
author_rank: Contributor
timestamp: June 22, 2025, 03:47:47 pm
content: Hello, looking for opinions, critiques, and optimization suggestions for this analog circuit design. It's an input buffer for an effects processing unit, made a post here last week about it and have since made a few improvements.Overall, the circuit consists of light bandpass RC filtering directly after the jack, into the buffer which is AC coupled and single supply biased with 12V/2=6V and then into a third order butterworth filter designed with TIs filter design tool.It's mainly the power supply I'm concerned about, I know noise can make its way into the bias supply which would propagate through the system. This is something I will try to avoid through filtering, just unsure if what I have setup now is adequate. The 12V 3A supply is necessary for other digital circuitry, so decreasing the main supply is not possible.USB-C PD supply will also be designed, this filtering circuitry will clean both the wall adapter or usb-c pd supply.I made the opamp supplies 12V because that apparently improves dynamic range and performance of the opamps, however I don't know how much say a 9V supply would change things. If anyone has experience or opinions about this that would be very helpful because if it doesn't change much I was considering stepping the 12V down to 9V with an LDO which would potentially produce a cleaner supply?I have attached LTspice results of AC and Transient analysis simulations and schematic.

author: RFDx
author_rank: Regular Contributor
timestamp: June 22, 2025, 06:09:41 pm
content: A few remarks (PDF schematic):1. There is a cap missing cap in the 2nd order S&K LP-Filter stage. In LTspice it's fine though.2. C5 (22 pF) is (way) too small.3. What is C15 (220 pF) for? Op amps do not like capacitive loading.4. Absorb the 1st order filter stage into the second stage.5. There is already a 6 V bias voltage applied at the input, no need to do it again in front of the LP-filter stage(s).

author: Benta
author_rank: Super Contributor
timestamp: June 22, 2025, 06:18:31 pm
content: Could you remove the little boxes and repost, please? They donotimprove readability.

author: R-802
author_rank: Contributor
timestamp: June 23, 2025, 06:42:03 am
content: Done, first time I've been told that the boxes don't improve readablity, I've done that on a few schematics in the past. Will take that into account in the future.

author: R-802
author_rank: Contributor
timestamp: June 23, 2025, 07:59:56 am
content: 1. Thanks, did not notice that.2. Thanks again, C5 should have been 22uF.3. C15 is my attempt at shunting high-frequency noise before it gets into the opamp’s input. If it is that harmful to performance I will remove it.4. By absorb do you mean something like this (https://habr.com/en/articles/542496/)?I suppose the leftover opamp could be used in the output buffer.5. Yes thanks for pointing that out, it's there because I'm planning to send the high impedance buffer output to a relay to switch between XLR and TRS inputs both of which will be filtered by the same 20kHz butterworth low pass filter before the ADC. Just trying to get the single ended TRS input working for now.

author: tooki
author_rank: Super Contributor
timestamp: June 23, 2025, 08:28:38 am
content: Quote from: R-802 on June 23, 2025, 06:42:03 amIt’s really quite a matter of opinion, not an objective fact. (At least, not until someone does empirical testing of schematics formatting!) I personally think that grouping with boxes can make circuits easier to understand quickly. One can, of course, overdo it, and there are myriad ways to actually format the grouping (e.g. thinner lines, colors, etc.).I do recommend usingdashedboxes/areas for optional/variant sections only, like Sony schematics do. (For example, a dashed box in the main schematic will show the configuration for one geographic region or model, and next to it, an alternate version of that section is shown for a different region or model.)What I do consider very bad for understanding a circuit is the style that you see a lot now (perhaps inadvertently encouraged by KiCad’s somewhat weak schematic editor, because moving already-connected components is so clumsy) of disjointed schematic blocks with only net labels connecting things, with no wires anywhere. It’s one thing to separate the power supply section, for example. But the signal flow through the “meat” of the circuit needs to be clear, and that should fundamentally be with wires. (As you have done.)

author: MariuszD
author_rank: Regular Contributor
timestamp: June 23, 2025, 09:08:43 am
content: Quote from: R-802 on June 23, 2025, 07:59:56 amIt makes the op-amp unstable, so it will produce high-frequency signal.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: June 23, 2025, 09:17:16 am
content: There probably is a mistake:On OPA U2A likely the inputs need to be swapped.

author: R-802
author_rank: Contributor
timestamp: June 23, 2025, 09:49:35 am
content: Oh yes that is a VERY big mistake, can't believe I didn't see that. Need to be more careful in the future, thanks for that.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 23, 2025, 12:11:53 pm
content: Quote from: R-802 on June 23, 2025, 07:59:56 amС14 and С16 are real examples to filter high-frequency noise. It is done by putting enough resistance before a filtering capacitor (LowFreq. RC-filter). But C15 is an example of just unhealthy capacitive load at opamp output.

author: RFDx
author_rank: Regular Contributor
timestamp: June 28, 2025, 09:57:28 am
content: Quote from: R-802 on June 23, 2025, 07:59:56 amYes, a 3rd order filter with just one op amp, no need to split it up in 1st and 2nd order. An active rail splitter (U1A) is also not strictly necessary, you can perfectly get by with just a passive version.