Crossfader Circuit Point-to-Point

This is a crossfader circuit. It accepts two inputs and fades between them, with the output being a mix of the two inputs (or just one of the inputs). It's a simple circuit, very useful, and easy to build! It does invert the signal going through it, so you won't be able to use it for control voltages.

Here's what you'll need:

• 1 potentiometer, 20K will work best, but you can get away with anything from 5K to 100K
• 4 10K resistors
• 1 20K resistor
• 1 100nF ceramic disc capacitor
• 1 TL074 quad op amp
• Various wires for power and stuff
• Pliers to bend stuff
• Clippers to cut wires
• A soldering iron and solder
• A desire to become a soldering ni

Behold. It's a good chip. Reliable, sturdy, easy to understand, and cheap!

Notice the semi-circular notch in the near end of the chip. That's the "top" end of the chip, and the pins of the chip are numbered, from 1 to 14, starting with the pin to the left of the "top" of the chip, going counter-clockwise around the chip.

Microchips pins are numbered this way from back in the day when there were no microchips; all electronics were tubes, which are round. The important bits of the tube would be in the round glass envelope, and the technicians working with the business end of the tube numbered the pins clockwise from the notch. Looking at the bottom of a microchip, the pins are numbered the same way!

Point-to-point electronics aren't kind to chip pins. I'm really glad chips don't have, like, ACLs and stuff.

Bend the pins on the left side of the chip like this. We'll bend pins 1 and 2 together, bend pin 4 with the skinny bit pointing out, and bend pins 6 and 7 together so they touch.

Here's what the other side of the chip will look like.

Bend pins 8 and 9 together, bend pins 10 and 12 so they're lying flat against the bottom of the chip, and bend pin 11 so the skinny part points out.

Hope our soldering iron has warmed up, because it's time to melt lead!

Here's the first part I add to every build, a bypass capacitor. Every chip should have a bypass capacitor close to the power pins. Bypass capacitors help keep noise from getting into the circuit from the power wires, and also keep noise from the chips from getting into the rest of whatever circuitry is close to the circuit. Some circuits won't end up injecting noise into the power rails (this one won't) but some are not so kind, so bypass capacitors are a great idea. Use them!

Okay, wrap the capacitor legs around pins 4 and 11, and hit those spots with solder. Also, solder the pins which we bent to touch each other.

Here's a potentiometer!

This circuit works by carrying "GROUND" to one of the incoming signals, causing it to fade out and then disappear, while carrying "GROUND" away from the other incoming signal. The wiper of the potentiometer is the part that'll be carrying that "GROUND", so we'll take some solid wire and bend it around the middle leg of the potentiometer.

I like bending all the legs of my potentiometers like this. Be gentle and they won't break.

There's two pins of the TL074 chip which also get grounded. It's the two pins we bent down to lie flat against the bottom of the chip. We'll solder the ends of the V of solid wire to those two pins.

If we feel like it, we can stick the chip to the potentiometer. Double-sided tape works great, superglue works, my favorite glue (Goop or E6000) works, but takes a while to dry, and that glue would be overkill for this project LOL

Let's make four 10K resistors look like this!

Look! It's like a little Richard Nixon doing the twin-victory-fingers thing!

We're going to take the short twisty bits of the resistors and solder them to the two side legs of the potentiometer.

The white layer should be more transparent. Thanks a lot, The Gimp, for having 20% opacity looking different in different versions.

Anyway, let's bend two of the 10K resistors over the ends of the chip and connect them to the pins that are bent together. Be careful not to be too rough with the thicker bits of the resistors, since that bit is actually a metal cup covered with a layer of paint. It's possible to scrape through the paint and make stuff short out! I try to not let resistors touch other metal parts.

Okay, you know the one pair of chip pins on the corners of the chip that we did not bend together? The pin shown (pin 13, if you're keeping track) is where both of the other two resistors are going to connect. Only do the closest one right now, since there's another resistor we're going to attach first.

This resistor can be 20K, higher, or lower! Resist to taste!

A larger value resistor here will make the output louder. 47K, 100K, 220K, these value resistors will make the output of this circuit much louder, up to the point the op amp won't be able to output the voltages it wants to, and it'll clip. You do you, but op amp clipping is a harsh sound.

If you are happy with the voltages of the incoming signals, you can have the gain of the circuit be one (okay, technically negative one, since this circuit inverts the signal, but for audio 1 and -1 sound the same), which means a resistor value of 20K or 22K should be perfect.

If you want this circuit to make the signal quieter for some reason, use a lower-value resistor. 10K, 4.7K?

Remember the resistor we left hanging all lonely? That resistor is going to stretch way over the two pins that were grounded and the middle pin that the capacitor is connected to, and connect to the same spot (pin 13!) as the gain resistor and the other resistor too.

And it's time for power!

Let's bring real electricity into the picture!

I use network cable wires for power. Brown or white is always ground, green is always negative power and orange is always positive power. Use my system or make up your own, but choose one and stick with it! You don't want to be confused later on and blow up a bunch of projects!

The ground wire should connect to the middle leg of the potentiometer.

The negative voltage needs to go to pin 11 of the chip, the bent-sideways pin that's nearest to the leg-side of the potentiometer.

Just copy the picture!

Here's a good view of where the positive power rail needs to attach. We're going to attach it to pin 4 of the TL074.

My projects all use +12 volts and -12 volts, and of course ground. This is called a bipolar power supply, and is very common in synthesizer or audio circuitry. I would love to show how to build a power supply, but working with household current is dangerous and you might electrocute yourself, so I'll just give you a hint: daisy-chain regular power supplies, and use the middle of the chain as your ground point. Also, use regulated, switching power supplies so they'll behave gracefully when there's a short circuit.

Look at this! We're basically done!

Your two signals will enter the circuit right here.

Now, this project right here is already installed in a module, where it's fading between two oscillators. The inputs are hard-wired into the two signal sources. But if the inputs will ever be disconnected, like in a modular synth or in a guitar pedal, you will need to add resistors from the inputs to ground. Any value from 10K to 100K (or more!) will be fine.

How come? you are asking.

Well, the inputs on the TL074 are very high impedance, super high impedance. What that means is that it's very very easy to change the voltage of that spot of the circuit, so any stray voltage floating around in the air will change the voltage of the pin. The TL074 also has a bit of input bias, meaning with no signal coming into the inputs, the output will flip to as high voltage as it can manage and just sit there. Not useful.

Oh, whoops, I forgot to label the output of this circuit. Okay, so see that crazy resistor leg that's bent, sticking up? That is the output.

If you want to change this to a circuit that doesn't invert the signal, like for control voltages, you can change the input section of the circuit. First, you'd need to not have the pins connected to each other. Then, ground the other two + input pins, pins 3 and 5, use pairs of 10K resistors wired together just like step 7, connect the twisted ends to the - input pins, pins 2 and 6. One of the 10K resistors in each pair would bend over and attach to the output pins of those two op amps, pins 1 and 7, and finally, the input would be through the unconnected resistor. In this configuration, it's not necessary to tie the inputs to ground through resistors.

Hope this project has been useful to you!