Introduction: Freestyle High Fidelity Ducking Circuit

About: I love dancing, building circuits, and making electronic music. My wife and kids are pretty cool too.


Okay so first, what is a ducking circuit!?? So glad you asked!

Ducking is also called sidechain compression. This effect is most commonly found in electronic music, where when the kick drum hits, the rest of the music is reduced in volume. My favorite and most outrageous example is the silly French electro track Satisfaction by Benny Benassi. Look it up, maybe watch the video if you're not offended by over-the-top exploitation.

Anyway, this is one of my favorite audio effects, and this simple cheap little circuit will get you there! In high-fidelity! Because most analog VCAs use chips which introduce distortion and noise, and this circuit uses a low-noise audio op amp and a photocell as a variable shunt resistor, which is very low-distortion and noise.


  • 1 TL074 quad op amp
  • 1 100nF ceramic disc capacitor
  • 1 1uF electrolytic capacitor
  • 2 220R resistors
  • 2 1K resistors
  • 1 10K resistor
  • 1 33K resistor
  • 2 47K resistors
  • 2 100K resistors
  • 1 100K potentiometer
  • 2 10K potentiometers (100K is okay too)
  • 2 LEDs (any color besides red or ultraviolet)
  • 1 light-dependent resistor/photocell/photoresistor
  • 4 diodes, 1N4148 or basically any diode
  • wires and stuff
  • E6000 or Goop or basically any super-sticky clear glue
  • Something to make it dark inside the LED/LDR, tape, heatshrink, poster putty, black paint...
  • Faceplate, jacks, bipolar power supply, stuff like that

Step 1: Jumping Spiders Do This

Jumping spiders are incredible hunters. They'll eat anything that they can catch and overwhelm. The dudes are smaller than the ladies, so when they want to mate, they have to find a female and dance for her. If they don't dance just right, fitting into the female's ruthlessly biologically determined expectations of vision and vibration, she'll pounce and have herself a nice little spider meal.

If you ever see a jumping spider and have a little mirror handy, try to show the spider its reflection. If it's a dude, it will probably raise its front legs like this and quickly lose interest. It's pretty cute.

Anyway, this is the only chip we'll need for this project! It's a TL074, and we'll be referring to their pins by their numbers in this project so we can be sure to get the right ones!

Microchips all have a notch or a circle indentation to indicate which pin is number 1. If you look at your microchip with the notch or indentation pointing north, pin number one will be the top pin on the left. The pins get counted counter-clockwise from that pin all the way to the opposing pin, which is pin 14 for this chip.

The reason pins are counted like this goes back to when electronics were all in glass tubes. Technicians worked with the bottom, or pin-end of the tubes, counting the pins clockwise. Nowadays we look at the top of our electronic devices, meaning we count the OTHER way around!

Oh my word, why did I just write all that?

So for this project, we need to bend pin 1 up, with some of the skinny part pointing out. Pin 14 gets the same treatment. Pin 2 and 13 just get a bit of the skinny part bent out. Pin 3 and its opposite, pin 12, get bent right under the chip, as does pin 10. All these pins will connect to ground later on. Pin 4 and its opposite, pin 11, get the skinny parts bent straight out. Those two pins are the power pins. Pins 5, 6, and 7, and pins 8 and 9 get the skinny part cut right off. This last step isn't actually necessary, I just prefer working with shorter pins that aren't as poky to my fingers.

Step 2: Hey Crazy Little Jumping Spider, Jump Upside Down!

Here's a quick view of the underside of our TL074. Make the one on your desk look like this one!

Step 3: Our First Two Resistors!

Here's the first resistors we're adding to our project! These resistors set the gain of our two amps that will be processing audio.

<geek>There's a good reason not to use resistors rated this high for audio circuits, since there's a thing called "Brownian Noise", which is caused by electrons going through resistance, but this particular op amp has incredibly high input impedance, so there won't be appreciable current going through these 100K resistors, so yeah, don't worry about it. If you're using the other very popular low-noise audio op amp, the NE5532 for some other project, try not to use resistances of higher than 20K.</geek>

Step 4: Bypass Capacitor!!!

Here's a capacitor the shape and color of a lentil. It's there to reduce noise from moving from once circuit to the next through power lines, and to keep this op amp from self-oscillating. There's lots of capacitors more expensive than this type, but this type is actually perfect for this application!

The two pictures are of the same thing, in the second one I've soldered the leads.

Step 5: A One Kilohm Resistor!!!

I got a couple thousand of these antique 1K resistors with really hefty thick leads, which I really like, from a really cool electronics/robotics/hacker/maker space in my city that was forced to shut down amidst rumors of tax evasion, fraud and sexual misbehavior. None of which I put ANY stock in, but wow, did I get some cool stuff from their close out sale.

Anyway... your 1K resistors will probably not look like this, but still, this is what we're gonna do with them, no matter how they look.

Take the short end of the 1K resistor and solder it to pin 5. Then ruthlessly bend it under the chip, bend it up, and solder it to pin 10. Pin 10 is one of the three pins on this chip which needs to be connected to ground. The other two pins will be connected to ground in the next step!

Oh hey, look carefully at these two pictures. Those are not perfect solder joints. The parts didn't get hot enough to really get the solder to flow properly. In the next few steps, I go back and fix that problem, which you'll see if you look carefully.

Step 6: Hey You Pins, You're GROUNDED!

Take that lead and bend it over to connect to pin 12. Pin 12 should already be soldered to pin 3, so now all three of our ground points are connected together! They're all grounded. For life. Sorry not sorry.

Step 7: Diodes!

Here's a couple diodes with the extremely catchy part number 1N4148.

Twist those suckers together like that! Please note that one end of each diode has a stripe. We're going to twist together one stripe-end with one non-stripe-end.

Electricity will only flow through these things one way. Looking at the schematic of this circuit back on the introductory step, you'll see that all the diodes in this circuit sort of point the same way.

So how come we're connecting them heel-to-toe? Because electricity is going one way through the pair of them!

Step 8: And Hook Them Up There

The twisty ends of the pair of resistors goes right there. Pin 9.

To make our projects match each other, put the diode with the stripe "up" towards the "bottom" of the "chip." That should "be" awesome, "let's" move "on."

Step 9: Whoah, Another Diode?

Grab another diode, and solder the non-striped-end to pin 8! Hopefully your solder joint will look better than this. I don't remember if I went back to fix this joint up.

In the next step, we're going to add the final diode to this project! Well, the last non-light-emitting diode, at least.

Step 10: Yet Another 1N4148

Take the last 1N4148 diode you've set aside for this project, and connect the striped side to pin 5. Then, three of the diodes that are sticking up in the air like the quills of a startled porcupine will be connected together.

The two diodes right next to each other connected to pins 8 and 9 that have the black stripe away from the pins connect together, and arch across the chip to connect to the one diode that we just soldered to pin 5. There isn't really a super clean way to get those three leads connected, so just kind of bend them so they're all touching and flood the connection with solder. At this point, with all the diodes held in place, we could theoretically go back and reflow all those cold joints that some of us made earlier in the project.

The last picture shows how we'll bend over the last sticky-up diode. That is where an audio signal will enter this part of the circuit. If you're interested, all these diodes force the audio coming in to this area to be going the "same way," so all the audio signal will be in the positive-voltage realm.

Step 11: Smoothing the Waves

All those diodes forced the signal to be rectified to be positive-voltage-only. This here capacitor will smooth out those ripples and peaks, and depending on the setting of a potentiometer we're going to add later on, will let the current leave more gradually. This will make it so we can make the audio be "turned down" for a longer amount of time.

This is an electrolytic capacitor, which means if too much voltage gets into them the wrong way, the voltage will blow the dielectric anodization off the aluminum foil and outgas energetically, causing it to pop! Not in a good way. in a bad way.

Right, so make the stripe side of the capacitor connect to pin 3, which is one of the grounded pins, and the non-striped-side of the capacitor connect to pin 5.

Step 12: A Sanity Resistor

Argh, I accidentally labeled this as 33K. Don't worry, it's a 220R resistor. I might fix the picture if I find the original.

Here's a cute little 220R resistor that will make it so that at the minimum-decay-setting of the potentiometer (zero ohms) we're going to eventually connect here won't overwhelm the output of the op amp that's feeding the 1uF capacitor.

Don't worry about it, just hook that bad little boy to pin 5, where the non-striped-side (the + side) of the capacitor is connected. Then bend the other lead of the resistor over like that so you don't accidentally lance your fingertip.

Step 13: Oh Em Gee What Is This?

Thanks for asking. This is an LED. When you connect LEDs in the feedback loop of an op amp, the op amp automatically adjusts so that the LED will light up in a more exact way. See, LEDs light up when there's enough voltage to sort of "push through" the quantum weirdness that's going on deep inside them. That'll be between about 2.5V for red LEDs, and up to 4V for blue or ultra-violet LEDs.

But when we put an LED in a circuit like this, the op amp will put enough voltage into the output to make the voltage seen by the inverting input pin equal to the voltage seen at the non-inverting input pin. Our rectified-and-smoothed kick drum signal will go in to pin 5 (non-inverting input) and, let's say it's 1V. That's not enough to light up any LED, but the op amp wants the voltage at that pin to be equal to the voltage at its other input, so it'll output enough positive voltage to overcome the LED's forward voltage drop, and light the LED up just a little bit.

This precision LED circuit is important to how well this circuit performs!

Right, anyway, current can only go through an LED one way, so we need to connect the positive side of the LED (look inside the plastic, the positive side tapers down to a small flat bit) to pin 7. The negative side of the LED (the negative side forms a little bowl or anvil shape) we'll connect to pin 6, which already has the 1K resistor connected to it.

Oh, and we're going to make sure we leave plenty of LED lead hanging out there. Trust me.

Step 14: It's Resistors This Time.

Here's a pair of 47K resistors. The full audio that this project will be attenuating (turning down) goes through these two resistors, with a variable resistor (the light-dependant resistor we'll attach in a step very soon) shunting some (most!) of that signal to ground.

Twist them together!

Hook one of them up to pin 2!

Step 15: Weird Twisting

Okay, so this is what we need to do to that poor LED. It's got to twist and bend so it's pointing, like, pointing like that.

It'll make sense real soon.

Step 16: THIS Is an LDR!!!

I love LDRs. They just look so cool.

And they're usually made of cadmium sulfide. I don't even know what that is, but it sounds totally kickass, and I just learned it's severely restricted in the EU! So cool!

Right, so one end of the LDR goes to ground (pin 3), and the other end goes to where the two 47K resistors are twisted together. The LDR needs to face the LED as directly as you can make it.

Step 17: A Pot and What to Do With It.

Here's a 10K pot. It's going to take some, all or none of the incoming kick signal and feed it to the full-wave rectifier and smoother. That's called an envelope follower.

Another cool thing I got at that weird place that got shut down was rainbow ribbon cable. It's so cool! I love ribbon cable for freestyle circuitry anyway, but rainbow ribbon makes it so easy to keep track of which wire is which! Get some if that's your thing!

I think of potentiometers as having a "high" side and a "low" side. When you turn the potenitometer as though you're turning the volume up, the wiper that follows the knob will go to the "high" side of the potentiometer. In this example, that's the orange wire. The "low" side is the green wire, and of course the wiper is the yellow wire. Okay. The "high" side (orange wire) connects to pin 1, the "low" side (green wire) connects to ground, which is just that hoop of resistor lead. The wiper (yellow wire) goes to the diode that enters the envelope follower, which is that diode that we bent over in step 10.

Step 18: Another Potentiometer and Another Thing to Do With It!

This potentiometer actually has to be 100K. Also, we're going to connect the "high" side of it to the wiper, turning it into a variable resistor instead of a voltage divider.

Notice the bit of resistor lead connecting those two legs together.

When you've got that done, hook up wires to the "low" side and either the "high" or the wiper, doesn't matter since they're connected.

Step 19: Hook Up That Pot!

Since this potentiometer is a variable resistor, it doesn't even matter which wire goes to which connection! Liberty!!!

So hook one of the wires to ground (pin 3 in this project, same place the LDR connects) and the other one hooks to that 220R sanity resistor which we curled over back in step 10.

Step 20: Aaaahhh!!! Three Steps in One! Buckle Up!

We want to be able to have the kick drum mix in to the rest of our audio. That 33K resistor connected to pin 2 is where we will do that in an upcoming step. So right now we're just going to connect a 33K resistor to pin 2.

The other thing we have to do now, because somehow I left the glue until too late (???) is cover the LED and LDR in ultra-sticky clear glue. If you want to, you can use hot glue, but it's very messy. E6000 or Goop (etc.) is much stronger and reliable, and if you use a tiny screwdriver to kind of push a blob of it around where it needs to go, it's not super messy.

Much later when the glue cures, in a step I didn't take a picture of, we're going to make the inside of that thing dark using black paint (might theoretically be electrically conductive) or electrical tape (hoo boy, good luck) heatshrink (maybe too late for that) or my FAVORITE, blue poster-putty.

The third step which we also have to do now, is a 10K resistor connected to pin 13, in the background of the third picture. Not even labeled. What a mess. Go ahead and connect the 10K resistor to pin 13, cut the other end off and curl it over maybe, although I didn't. Remember this resistor, we're going to use it in the next step.

Step 21: Our Last Potentiometer!

This will be the potentiometer that mixes the kick drum into the rest of the audio. It'll work most like you expect if it's a 10K resistor, but anything less than 1M should be totally fine.

Again, I'm wiring the "high" side of the potentiometer to orange, the wiper to yellow, and the "low" side to green.

The "low" wire goes to ground (that resistor lead hoop).

The wiper wire goes to the 33K resistor that connects to pin 13.

The "high" wire goes to....... why don't I have a picture of this? It goes to the 10K resistor from step 3 of step 20 LOL. You can see the 10K resistor I'm talking about in the third picture, sort of out of focus coming into the foreground. That resistor is where the kick drum signal will be coming in to the circuit.

Step 22: The Electronics Are Basically Done!

Here's a faceplate I scavenged from an old module in my system. You're probably going to use something slightly less tin and slightly less round. Maybe?

This faceplate has holes for the three potentiometers and three jacks, and an LED (that also has a 1K resistor to ground). I chose to label this hideous faceplate with a Sharpie like in the third picture.

Step 23: Connections to Jacks

The first picture shows a red wire that we connect to the "Kick In" jack. It's connected to the 10K resistor that the "high" side of the mix potentiometer is connected to. That resistor goes to pin 2 of the TL074.

The second picture shows a white wire which we connect to the "Audio In" jack. It's connected to the 47K resistor, the first of the pair that have the LDR in the middle.

The third picture shows a blue wire connected straight to pin 1, which will go to the "Out" jack. I forgot to include it in my build, but it's not a bad idea to include a 220R resistor between pin 1 and the output jack.

Step 24: A Second LED!

It's fun to have an LED to show you how much your circuit is working! The positive leg of the second LED gets hooked up to pin 8, the positive leg of the LED that's already included in our circuit. There's a 1K resistor on the negative leg of the LED already in the faceplate that connects to ground.

The second picture sort of shows what's going on.

Step 25: By the Power of Greyskull, I Have the Power!

I use wires pulled from Cat5 network cable. Works super great.

Get yourself some, decide to follow my color convention, which is...

Orange = +12V, Brown (or white) = 0V/ground, Green = -12V

...or make up your own, but make sure you're very happy with it and don't forget.

The +12V wire goes to pin 4 of the TL074. The -12V wire goes to pin 11 of the TL074. Make sure sure sure that you don't hook the power wires up backwards. In my build here, the chip is sort of upside down, so it would be easy to mix up the power wires. These chips burn out instantly when you try to power them up backwards. A situation to avoid!

The ground wire goes to any convenient ground. In this build, it's going to pin 12, where the LDR connects, but you can connect it anywhere convenient.

A final thing to remember (a thing I have forgotten many times) is to ground your front panel.

Step 26: Good Job!!! Oh Wait...

And with that, we're done! Oh wait... you still have to make it dark inside your LED/LDR device. The glue is probably dry by now, so get some blue (or otherwise opaque) poster putty and make a little darkbox for your home built Vactrol!

Enjoy the goofy ducking effect! It's worth it!