# MS20 LPF Problem? **URL:** \ **Category:** DIY STUFF\ **Created:** [March 12, 2021, 12:02pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143 "2021-03-12T12:02:37Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![lu.caas](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/lu.caas/32/12420_2.png) [@lu.caas](https://lookmumnocomputer.discourse.group/u/lu.caas)\ **Post date:** [March 12, 2021, 12:02pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/1 "2021-03-12T12:02:37Z")
Well I built a MS20 style LPF, but I have some problems with it 🤔 When I set the cutoff pot up to the maximum frequency setting, the filter stops and there are no sound and no LEDs anymore. I don’t know if it’s because of the input signal (±12V) or the components value (Schmitzbits) So if anyone of you know what I’m talking about well I’d like to have your explanation 😅 I will send a video to show the Problem! [![](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/3/38f97b683664b0c8a216cfaf05159a61d01a939a.jpeg "MS LPF Problem") ](https://www.youtube.com/watch?v=k35XHkkVFeI) ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [March 12, 2021, 12:06pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/2 "2021-03-12T12:06:00Z")
The TL07x can lock up if the signal gets too close to the negative rail. There are some earlier threads about it, including suggestions for different input resistors. Using a different opamp is also an option. ---
**Author:** ![lu.caas](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/lu.caas/32/12420_2.png) [@lu.caas](https://lookmumnocomputer.discourse.group/u/lu.caas)\ **Post date:** [March 12, 2021, 12:11pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/3 "2021-03-12T12:11:05Z")
I’ll look up for that! Thank you ^^ Well I will try to replace the 10k input res with a 100k ---
**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:** [March 12, 2021, 12:20pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/4 "2021-03-12T12:20:12Z")
For me it only happened with a square signal (stronger than the others), I added an attenuator pot on the Input of all my MS20, which allows to fix this problem, and also with the other signals to lower the entry to have more resonance 🙂 ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [March 12, 2021, 12:32pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/5 "2021-03-12T12:32:21Z")
This post links to an article that explains when and why this happens: > [@LMNC MS20 Filter Low Output](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/43): > > Ah, phase reversal, our old friend… Is it only when you crank up the resonance, or does it happen also at low resonance levels? What signal source are you using? ---
**Author:** ![lu.caas](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/lu.caas/32/12420_2.png) [@lu.caas](https://lookmumnocomputer.discourse.group/u/lu.caas)\ **Post date:** [March 12, 2021, 1:45pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/6 "2021-03-12T13:45:14Z")
Yeah! Exactly the same thing happens to me: square wave the worse, sawtooth it’s medium and triangle it’s okay ---
**Author:** ![SYNTHGUY57](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/synthguy57/32/14422_2.png) [@SYNTHGUY57](https://lookmumnocomputer.discourse.group/u/SYNTHGUY57)\ **Post date:** [March 13, 2021, 1:27am UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/7 "2021-03-13T01:27:49Z")
possibly need to add a resistor to that pot to extend the full cutoff cycle and should stop that , hopefully So 100k pot and 10k then from the last pin on pot extend that to resistor … ---
**Author:** ![Circulartonic](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/circulartonic/32/10399_2.png) [@Circulartonic](https://lookmumnocomputer.discourse.group/u/Circulartonic)\ **Post date:** [March 13, 2021, 7:02pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/8 "2021-03-13T19:02:27Z")
Maybe Try a different Tl072 op amp and see if the results change and tweak the values of the resistors. It is called Opamp Destructive latch up some op-amps are more prone to it then others. Could also try a NE5532 they are better at handling this problem and is featured in kassutronics KS-20 Filter for this reason. [KS-20 Filter](https://kassu2000.blogspot.com/2019/07/ks-20-filter.html) [Good Examples for Causes of it](https://electronics.stackexchange.com/questions/304798/op-amp-symptoms-of-latchup) ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [March 13, 2021, 7:07pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/9 "2021-03-13T19:07:34Z")
As mentioned earlier in the thread, this is phase reversal, it’s inherent to the TL07x series (and similar opamps) when you go outside the input range, and it’s (luckily) not really destructive. ---
**Author:** ![Circulartonic](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/circulartonic/32/10399_2.png) [@Circulartonic](https://lookmumnocomputer.discourse.group/u/Circulartonic)\ **Post date:** [March 13, 2021, 7:20pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/10 "2021-03-13T19:20:59Z")
Thanks for that write up on that other post. That explains a why my Tl072s started to act weird when I would try and set my CV Joystick controller beyond -8V Range. ---
**Author:** ![BobbyZincone](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/bobbyzincone/32/30171_2.png) [@BobbyZincone](https://lookmumnocomputer.discourse.group/u/BobbyZincone)\ **Post date:** [March 23, 2021, 11:38am UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/11 "2021-03-23T11:38:54Z")
Hello! I think I also have the same problem. With a square wave, the filter is killed about halfway the cutoff level. I’m not entirely sure what would be the best solution. Should I swap the 10k input resistor to 100k? Or should I add a 10k to one of the pins of the cutoff pot? Which one? Thanks for the information on phase reversal, I was not aware of this! ---
**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:** [March 23, 2021, 11:49am UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/12 "2021-03-23T11:49:12Z")
> [@MS20 LPF Problem?](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/4): > > For me it only happened with a square signal (stronger than the others), I added an attenuator pot on the Input of all my MS20, which allows to fix this problem, and also with the other signals to lower the entry to have more resonance slight_smile some explainations here > [@LMNC MS20 Filter Low Output](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/56): > > People keep using the filter in various configurations, so not everyone is hitting the same issues; it’s perfectly possible to use it out of the box without any issues, especially if you’re used to attenuate your way to the right signal levels (which is something you usually need to do anyway in a full rig). There are three level issues that people keep running into: The MS-20 filter has [different response curves](https://lookmumnocomputer.discourse.group/t/1113-performance-filter/14/22) in low pass and high pass mode; in low pass, it’s two −6 dB/octave filters in a … ---
**Author:** ![BobbyZincone](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/bobbyzincone/32/30171_2.png) [@BobbyZincone](https://lookmumnocomputer.discourse.group/u/BobbyZincone)\ **Post date:** [March 23, 2021, 12:12pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/13 "2021-03-23T12:12:49Z")
Thanks Dud! Yeah, this makes sense. I would not like to use a built-in attenuator (mainly because I’ve already drilled the panels and I’m stubborn), but a simple attenuator module is something I’m missing and would be extremely useful - so this might be a solution. ---
**Author:** ![BobbyZincone](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/bobbyzincone/32/30171_2.png) [@BobbyZincone](https://lookmumnocomputer.discourse.group/u/BobbyZincone)\ **Post date:** [March 26, 2021, 7:27pm UTC](https://lookmumnocomputer.discourse.group/t/ms20-lpf-problem/3143/14 "2021-03-26T19:27:02Z")
I finally got to building an attenuator. Solves the cutoff problem as stated, thanks a lot!