Ducking module (Juanito Moore) problem

I am stuck on my last module which works strangely.

here is the schem

my stripboard version

i keep out the second led change nothing
i checked again and again but i don’t see an error on my stripboard

Symptoms :
it seems that the Amount pot is not working.
But stranger when I touch the pot knob, the led lights up, it looks like an gnd problem.

I feel like I’ve tried everything and can’t figure out where the problem is coming from, so I come to ask for help.

Maybe a different eye, or someone who had a similar problem, an idea …?

thanks in advance :slight_smile:

Edit : i also keep out the vactrol and use only a led for testing

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I continue to search

it seems on the instruction (build on the chip)

one diode go to gnd (pin 5)

a mistake on his schem with the diodes ?

EDIT : sorry it’s not that, pin 5 is not connected to gnd, i use the different opamp

here’s the pin numbers i use in my stripboard

EDIT : I also add 2x 10uf on rail power, not in the original schem but …?


Your Stripboard looks good to me! I don’t understand what the opamp with all the diodes is doing, probably a kind of precision rectifier, but with more diodes and no resistors…
So the circuit works but the amount pot does nothing? Can you send in not a kick, but a constant voltage and see what happens with that?
This is what I Imagine what it should look like:

Edit: I mean the waveforms, not the circuit. The wiggles of the kick should be made only positive and then control the led.

Edit2: maybe you should add a resistor as in my picture (R2), because in Juanito’s version the opamp operates in open loop mode when the diode is not conducting, so differences between opamps might be important ( how fast they are etc.)

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thanks a lot @sebastian !

i’ve just re do all my wire connections, changed the power wires, change the TL074, and re do all my gnd solder on my board

and it’s better ! surely a bad gnd connection (even though I have the impression that I have already done it several times :crazy_face:)

the Amount pot works now !
and the led seems to be cool
i will add the vactrol and test all the module after eating :slight_smile:

maybe try to add your resistor too


The strip board layout looks good to me.

My first suggestion, change the pot, but im sure you have tried that.

oh I see it’s fixed while I was replying, nice :slight_smile:

@sebastian it is a strange inverting rectifier, I was intreged and decided to simulate it. This is a plot for a 1kHz sinewave input, and a link to the simulation.


thank you for watching, yes i thinks it’ good too (+ @sebastian ) so we are allready 3 :slight_smile:

and yes i’ve change the pot too, even it was not it the problem.

btw for the rectifier, i don’t really understand what do you mean


too high for the led ?

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I was commenting on the original design, as I have not seen that configuration before. I would expect it to work fine given the author, Juanito Moore.

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there was some interogations like you about the diodes part, when i posted it the first time

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Hm, I did a simulation too, just up through the rightmost diode.

Except I assumed the input signal was synth level so used a 10 Vpp sine, in which case the first op amp output is almost always ±12 V, and what comes off the pot wiper is a near square wave. (Note the pot is configured like a voltage divider, but there’s current through the wiper when the voltage is above ~700 mV.)

I think what’s happening is:
When the pot wiper voltage is negative the leftmost diode has only a small leakage current. That current has to come through the top left diode, so the second op amp output must be a diode drop above -(the wiper voltage), and the output is a diode drop below that.

When the pot wiper voltage is positive so is the inverting input on the second op amp; it has no way to change that, so the op amp output is -12 V. Then the output is three diode drops below the wiper voltage.

I don’t really see the point of the rightmost diode other than to reduce the output by a diode drop; it’s always in conduction (for ±12 V out of the first op amp).

Output for different pot settings:

Why they’re going through all that just to drive a vactrol I don’t know, it’ll presumably just smooth this out into an envelope, so it’s kind of an envelope follower but seems an oddly obfuscated one. On the other hand just this part of the circuit, without the vactrol and with a coupling capacitor to remove the DC, might make an interesting distortion.


Well, after eating, i come back to my build, and the Amount pot works like this morning test, but when i touch (or near) the pot : the led bright up !?

With my finger on the pot to have the problem led light up
and with a wire connected to the gnd i looked for where to connect the other end so that the led goes out

i found it but it’s still strange

it was my standoff near the pot

very strange because the standoff dont touch the led, the knob, the pot …


on the other side there’s 2 lines without short between the rest of the circuit


i put a wire on the standoff line on the board, connected to gnd

and all is ok now :slight_smile: (ouf!)

the standoff surrounded by the led and the gnd can become an antenna ?
always weird stuff for me


@analogoutput Thanks for the explanation, however, I still had to think this through for myself, it took a while to realize the second opamp is effectively bypassed when the wiper is positive, and an inverting buffer when negative. This is a nice low component active full-wave rectifier, if like in this module, the inconsistant diode drops and non-lineararities are not of importance, I might try this for driving signal led’s

Nice to know im, not the only one using Circuit labs, I only found it a couple of weeks ago. I’m still only a few chapters into the tutorial, but find it really useful.


Conclusion of the day, I have the impression of having isolated something, as if I was isolating my wooden panel !!!
I have a little trouble thinking about that :crazy_face:


I wonder if you have just enough bias across the diodes that you are creating an AM radio of sorts, with your hand as the antenna?

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it’s a guess, I don’t know what’s really happened in this circuit :slight_smile:

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