# Simple analog Low Pass Filter self oscillating with no resonance **URL:** \ **Category:** DIY STUFF\ **Created:** [August 27, 2021, 8:00am UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941 "2021-08-27T08:00:16Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Spocky007](https://avatars.discourse-cdn.com/v4/letter/s/e79b87/32.png) [@Spocky007](https://lookmumnocomputer.discourse.group/u/Spocky007)\ **Post date:** [August 27, 2021, 8:00am UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/1 "2021-08-27T08:00:16Z")
My filter was working but now its self oscillating when i turn the cuttoff knob even though the resonance is turned down. No audio input plugged in. The resonance LEDS don’t light up unless i turn the res pot up. I’ve triple checked everything. Rgds. Joe ---
**Author:** ![konthaar043](https://avatars.discourse-cdn.com/v4/letter/k/a88e57/32.png) [@konthaar043](https://lookmumnocomputer.discourse.group/u/konthaar043)\ **Post date:** [August 27, 2021, 8:21am UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/2 "2021-08-27T08:21:34Z")
What filter did you build? Do you have a schematic or name by any chance? If the filter has an output level control, does it still oscillate when you turn it down? Have you tried switching ic’s and did you add a bypass capacitor between the power pins of the chip? Oscillation is a product of positive feedback in your circuit, but can also occur when high gain circuits have unstable power supplies. ---
**Author:** ![Spocky007](https://avatars.discourse-cdn.com/v4/letter/s/e79b87/32.png) [@Spocky007](https://lookmumnocomputer.discourse.group/u/Spocky007)\ **Post date:** [August 27, 2021, 9:25am UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/3 "2021-08-27T09:25:25Z")
It’s the LMNC ms20 simple low pass filter. > **[STUFF - LOOK MUM NO COMPUTER](https://www.lookmumnocomputer.com/simple-filter)** ---
**Author:** ![konthaar043](https://avatars.discourse-cdn.com/v4/letter/k/a88e57/32.png) [@konthaar043](https://lookmumnocomputer.discourse.group/u/konthaar043)\ **Post date:** [August 27, 2021, 12:58pm UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/4 "2021-08-27T12:58:58Z")
> **[KS-20 Filter](https://kassu2000.blogspot.com/2019/07/ks-20-filter.html?m=1)** > > My interest in the MS-20 style filter was sparked by videos from Look Mum No Computer, who built a DIY low pass version. This filter makes g... I think this should be the same design overall, but if you look in the comments, there can be some issues when using this circuit with a jfet input opamp, like the tl074. Jfet opamps can swing their output to the wrong voltage rail, if the input is too close to the negative rail. If you look up “opamp polarity reversal” on google you will find a pdf if you want to learn more. Probably not all jfet opamps will do this, so it might be worth swapping it out and see if the problem persists. On the kassutronics page, there is quite a solid reason stated to use this circuit with bipolar opamps. If you find a quad with the same pinout, you can probably just swap it out as well. I don’t have experience with the circuit, so that’s about all i know, apart from my previous post. Hope this helps, maybe someone else can chip in? 🙂 ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [August 27, 2021, 1:45pm UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/5 "2021-08-27T13:45:32Z")
I think there would still be a problem with the build somewhere, as I have built this filter with Sam’s stripboard several times without this issue. maybe check again your circuit, components values, short … have you some pic of your build ? maybe that can help 🙂 ---
**Author:** ![konthaar043](https://avatars.discourse-cdn.com/v4/letter/k/a88e57/32.png) [@konthaar043](https://lookmumnocomputer.discourse.group/u/konthaar043)\ **Post date:** [August 27, 2021, 2:11pm UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/6 "2021-08-27T14:11:16Z")
Yes maybe there still is, i must have checked my tube amplifier more than 10 times, and it still had a very simple error that i didn’t notice. ---
**Author:** ![Spocky007](https://avatars.discourse-cdn.com/v4/letter/s/e79b87/32.png) [@Spocky007](https://lookmumnocomputer.discourse.group/u/Spocky007)\ **Post date:** [August 27, 2021, 11:00pm UTC](https://lookmumnocomputer.discourse.group/t/simple-analog-low-pass-filter-self-oscillating-with-no-resonance/3941/7 "2021-08-27T23:00:32Z")
Fixed! Originally the faceplate was bare metal. i have since painted it which meant the resonance pot wasn’t getting its ground off the CV Input jack via the faceplate. 🤦‍♂️ This filter sounds brilliant. so good!! Thank you very much for your help. Rgds. Jo