LMNC MS20 Filter Low Output

Hi all, I just built the LMNC MS20 stripboard filter last night. The output is VERY low, like almost inaudible…I can hear the cutoff and resonance working and the LEDs are lighting.

What is the normal operation of the LEDs? BOTH of mine are lighting up when there seems to be a heavy load. Are these supposed to operate independently?

Someone also mentioned on a different thread to try passing audio through the filter without any power. I tried that and did not get any sound out of the filter.

Anybody experience any thing similar? Any tips?

Thank you!

I should mention that I daisy-chained all of the grounds between the sockets and the pots. The stripboard layout makes it look like there are separate grounds between CV-CV IN-RESONANCE and the OUTPUT jack. Would having them all on the same ground make a difference?

The LEDs are in the feedback loop, so they only light up when the resonance setting is high enough that the feedback is clipping. They’re hooked up to the same signal, just in two different directions, so will light up at the same time.


EDIT: Sorry, missed that you said stripboard:


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Strictly speaking the LEDs will never light up at the same time. They light up in an alternating fashion. But if the frequency is high enough mere mortals can not see that. Aliens probably can :wink:

So, let’s assume I hooked up the LEDs wrong since mine are both lighting up on high loads. Would that be enough to affect the output volume, or is that a separate problem?

That is hard to say, because you could have made any mistake. Did you add the 10K resistor from pin1 to pin 2? And I’d suggest you meticulously compare the your soldering to the bread board and to the schematic.

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They’re fed the same AC signal, so they are both lighting up at the same time, unless you’re some weird robot that can see several hundred or thousand pulses per second as individual pulses :robot: (in which case you probably have bigger problems with modern electronics then these LEDs…)


Daisy chaining the grounds is fine.
In fact all the grounds should be connected together, including the input jack sleeve which is not shown on the stripboard diagram.

Did you cut the traces under the ICs and near the transistors?
Did you install the 470nF output coupling capacitor?

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Yup…10k resistor from pun 1 to pin 2 is there.

I will go over the board again though…

It’s just weird that it’s actually working, but at a really low level.

Yup, input jack sleeve is to ground. All the strip board rows are broken under the ICs…and 470nF capacitor is there.

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Has anyone gotten this design to work properly?

yes i did it several times without any problem

but i don’t know what the level out is low for him

check that the welds are good, maybe a faulty component ?
can you post a pic Casiokid ?


Mine works properly.

there’s already one topic for the same thing here :

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Hey Dud, that’s actually my post! I put that there when I noticed that someone else had a similar issue, but didn’t seem to post a solution, so I started a new thread.

It’s possible that it’s a faulty component…I can’t imagine a cold solder or a faulty weld would allow it to work at all…that’s what is throwing me. The fact that’s it’s working like it should, but at a really low volume.

I’m posting pics of my strip board in case anyone has a better eye than me!!!

Any help is appreciated!!!

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maybe not a defective weld but a small burr between the lines for example, are you shure that you put the correct value for resistor and pot (maybe check an check again)
a error of resistor (multimeter), or eventualy a defective components
i don’t know …

All good points…I’ll keep checking the resistor values on the strip board. The cutoff pot is a 4.7k and the others are 100k.

Does anyone know of a good test point to check values? Maybe that would help me narrow down the error.

now that the resistors are soldered, it’s hot to check them but maybe compare the values ​​with the colors

Yeah, I’m going to go through every resistor and compare it to an unsoldered version.

Noob question, but where is the gain stage of this circuit? Maybe that would help me focus my efforts?

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the gain , i don’t know
the output is pin 8 of TL074
check also capacitors (nf /uf/ pf , maybe you make a mistake of value?)…good luke