# LMNC MS20 Filter Low Output **URL:** \ **Category:** DIY STUFF\ **Created:** [April 18, 2020, 7:26pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660 "2020-04-18T19:26:11Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![ChristianBloch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/christianbloch/32/56_2.png) [@ChristianBloch](https://lookmumnocomputer.discourse.group/u/ChristianBloch)\ **Post date:** [April 29, 2020, 10:01pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/41 "2020-04-29T22:01:32Z")
Yes, I’ll try with some that are a little higher. It does indeed stop working at the top. ---
**Author:** ![Caustic](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/caustic/32/15588_2.png) [@Caustic](https://lookmumnocomputer.discourse.group/u/Caustic)\ **Post date:** [April 29, 2020, 10:35pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/42 "2020-04-29T22:35:25Z")
That was me. It was also affected by the resonance. I sorted that out, though its hard to tell if it was my redesign or just building it again and not making the same mistake. @antoine.pasde2 recently asked me about that redesign and i will have to look at what i was thinking at the time. The past few days have bee rough so i havent had a chance to really look at it. ---
**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:** [April 30, 2020, 12:46am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/43 "2020-04-30T00:46:18Z")
Ah, phase reversal, our old friend… > **[What's the deal with phase reversal?](https://northcoastsynthesis.com/news/whats-the-deal-with-phase-reversal/)** > > Intuitive overview of what causes the notorious "phase reversal" issue with popular JFET-input op amps. 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:** ![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:** [April 30, 2020, 8:26am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/44 "2020-04-30T08:26:54Z")
I added on my filters an ATT potentiometer on the input signal, to be able to attenuate a little some signal (square for example) ---
**Author:** ![sebastian](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sebastian/32/39_2.png) [@sebastian](https://lookmumnocomputer.discourse.group/u/sebastian)\ **Post date:** [April 30, 2020, 1:10pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/45 "2020-04-30T13:10:53Z")
So, what are good options? Use a different opamp? Limit the input with diodes? Keep using the 100k and add an amplifier at the output? ---
**Author:** ![fluxpavilion](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fluxpavilion/32/2475_2.png) [@fluxpavilion](https://lookmumnocomputer.discourse.group/u/fluxpavilion)\ **Post date:** [May 26, 2020, 8:56am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/46 "2020-05-26T08:56:19Z")
working on the performance filter and just came accross this issue swapped out the 100k resistor for 10k resistor and it fixed the issue, but on LPF mode the input gain makes the filter shut down thinking maybe trying out a 20k resistor instead to see if it can duck just below the panic level? anyone found a better way of doing it? ---
**Author:** ![fluxpavilion](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fluxpavilion/32/2475_2.png) [@fluxpavilion](https://lookmumnocomputer.discourse.group/u/fluxpavilion)\ **Post date:** [May 26, 2020, 9:10am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/47 "2020-05-26T09:10:44Z")
can confirm, just put 18k in place and seems to be fine no audible signal drop between modes and stays working all the way to the top ---
**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:** [May 26, 2020, 10:18am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/48 "2020-05-26T10:18:55Z")
for this problem i put a attenuator pot in my Input in all my filters, but i made them with stripboard and diy panel, if yours is the Perfomance one more diffucult with pcb and panel ready made (but allways possible i think) ---
**Author:** ![TayBae](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/taybae/32/4180_2.png) [@TayBae](https://lookmumnocomputer.discourse.group/u/TayBae)\ **Post date:** [June 7, 2020, 8:08pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/49 "2020-06-07T20:08:53Z")
Hey is it normal for this filter to self oscillate? I used an oscope at the output of the filter, with no signal going into the input, and got something resembling a triangle wave at the output. ![IMG_1698](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/d/d996291aad4c49b709d86c524dea5f5e24372b85.jpeg) ![IMG_1699](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/d/dce3d6530a9f1421d89dc138ef9fbf6b366a98b0.jpeg) ---
**Author:** ![ChristianBloch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/christianbloch/32/56_2.png) [@ChristianBloch](https://lookmumnocomputer.discourse.group/u/ChristianBloch)\ **Post date:** [June 7, 2020, 8:34pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/50 "2020-06-07T20:34:29Z")
Oh, definitely. The resonance is quite severe. ---
**Author:** ![Caustic](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/caustic/32/15588_2.png) [@Caustic](https://lookmumnocomputer.discourse.group/u/Caustic)\ **Post date:** [June 7, 2020, 8:55pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/51 "2020-06-07T20:55:54Z")
yup yup. totally normal! 🙂 ---
**Author:** ![TayBae](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/taybae/32/4180_2.png) [@TayBae](https://lookmumnocomputer.discourse.group/u/TayBae)\ **Post date:** [June 7, 2020, 9:44pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/52 "2020-06-07T21:44:40Z")
Awesome! and kinda unexpected 😂 ---
**Author:** ![TayBae](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/taybae/32/4180_2.png) [@TayBae](https://lookmumnocomputer.discourse.group/u/TayBae)\ **Post date:** [June 19, 2020, 3:30am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/53 "2020-06-19T03:30:34Z")
I’m confused, does this only apply for the performance filter? I’ve been running into the same issue with the basic MS20 LPF. If the source is a square wave, the signal will drop out when the resonance is high. ---
**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:** [June 19, 2020, 7:56am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/54 "2020-06-19T07:56:49Z")
a simple solution for this problem > [@Dud](#): > > for this problem i put a attenuator pot in my Input in all my filters, ---
**Author:** ![fluxpavilion](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fluxpavilion/32/2475_2.png) [@fluxpavilion](https://lookmumnocomputer.discourse.group/u/fluxpavilion)\ **Post date:** [June 19, 2020, 8:30am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/55 "2020-06-19T08:30:13Z")
couldn’t say for sure but i think that the performance filter IS the MS20 filter in a funkier configuration, so im not surprised of the same issue, seems like a weird one to keep repeating though ---
**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:** [June 19, 2020, 8:52am UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/56 "2020-06-19T08:52:52Z")
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 row so −12 dB/octave in total, while in highpass it’s one −6 dB/octave filter and the other stage is only used to inject resonance feedback. - The MS-20 itself has one low-pass and one high-pass filter in sequence, so it’s not really an issue there, but here you have only half the filter, and if you flip the switch the levels will be off. - The performance build has a [much larger input resistor](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/34) than the stripboard and other builds, which affects the signal level. You can swap this for lower values, but go too low and you run into the next issue sooner: - The TL07x series isn’t a rail-to-rail opamp, and the input range (“common mode input voltage range” in the datasheet) is limited; if an input voltage goes below −8 V or so, the opamp misbehaves (this is called [phase reversal](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/43)). This is not a problem with inverting topologies, but the TL07x amplifiers in the filter chain are non-inverting voltage buffers, so the inputs will see the full signal voltage. Too much amplitude in or via resonance, and the filter locks up. To avoid this, you have to either manage the levels, or switch to a different opamp. So as long as you have the right levels in and the right gain after it, the stock builds work just fine. If not, some tweaking might be needed (e.g. replacing the input 100k with 20k seems to bring up the levels to nicer but still reasonably safe levels) but you may still run into issues that are inherent in the MS-20 design and the component choices. ---
**Author:** ![ChristianBloch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/christianbloch/32/56_2.png) [@ChristianBloch](https://lookmumnocomputer.discourse.group/u/ChristianBloch)\ **Post date:** [June 19, 2020, 4:02pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/57 "2020-06-19T16:02:48Z")
> [@fredrik](#): > > To avoid this, you have to either manage the levels, or switch to a different opamp. Any suggestions for opamps to switch to? ---
**Author:** ![TayBae](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/taybae/32/4180_2.png) [@TayBae](https://lookmumnocomputer.discourse.group/u/TayBae)\ **Post date:** [June 19, 2020, 5:36pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/58 "2020-06-19T17:36:57Z")
Thanks, I’ll give this a try and see if that solves the issue ---
**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:** [June 19, 2020, 5:56pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/59 "2020-06-19T17:56:15Z")
I’ve used two TL072s for my builds, giving me the option to swap the one in the filter path for something else, but haven’t tested anything. [Kassutronics](https://kassu2000.blogspot.com/2019/07/ks-20-filter.html) uses an NE5532 but if you have quad opamp that won’t help you. I don’t think this circuit demands anything from the opamps, though, so something really basic like an LM324 might work. (I’m a little bit tempted to experiment with using the built-in darlington stage, like in the original MS-20 design; all the filter buffers do is to convert a current to a voltage, while isolating the filter stages from each other, and if the darlingtons were good enough for Korg it might be good enough for the rest of us too 🙂) ---
**Author:** ![ChristianBloch](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/christianbloch/32/56_2.png) [@ChristianBloch](https://lookmumnocomputer.discourse.group/u/ChristianBloch)\ **Post date:** [June 19, 2020, 6:02pm UTC](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/60 "2020-06-19T18:02:41Z")
> [@fredrik](#): > > I’m a little bit tempted to experiment with using the built-in darlington stage, like in the original MS-20 design Well, don’t let me stop you 😜 [Previous page](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660.md?page=2) [Next page](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660.md?page=4)