Pixie Dance Floor Example: Mixer

Here is a simple mixer circuit that can be built on your Pixie Dance Floor. Start by checking that the DPDT power switch is off and clearing the breadboard. Get all of your components ready. You will need a TL074, 2x 100nF capacitors, 1x 1k resistor, 6x 100k resistors, and 16 dupont cables. 

Place the TL074 in the center of the breadboard. If this is a brand new IC chip you will have to bend the pins slightly inward to match up with the holes on the breadboard.

Remember that the side of the IC with the U-shaped indent is the "top" of the chip. Now the pin on the top left corner is pin 1, and counting up counter clock-wise from there. So pin 2 is below pin 1, pin 3 below that and so on until pin 7 at the bottom left corner. Pin 8 is on the bottom right corner, pin 9 above that and continuing up to pin 14 at the top right corner of the IC.

Next we'll place the 100nF decoupling capacitors. One side of each capacitor is connected to the power pins of the TL074, and the other side is connected to the ground rail on the breadboard - on the left side we will use the negative rail and on the right side the positive rail.

Decoupling capacitors help keep the supply of current at the power pins of ICs steady and smooth. Without them there is the possibility that the voltage at the power pins can be pulled down, causing issues with the circuit.

Next we will place the feedback resistors for the two op-amps we are using, and also the input resistor for the second op-amp in the circuit.

The first op-amp is the one that does the actual mixing, however it results in the output being the inverted sum of the inputs. So we use the second op-amp to re-invert the signal back to the correct polarity.

Next place in the 100k input resistors and the 1k output resistor. The input resistors all connect together at pin 2 and for now simply land the other side of each resistor on its own row somewhere below the TL074. Same for the output resistor - place one end on pin 14 and the other end on its own row somewhere below the TL074. We will connect them to the I/O jacks with dupont cables later.

Now connect the power rails of the breadboard to the power I/Os on the PCB headers. +12 connects to the positive rail on the left of the breadboard. -12 connects to the negative rail on the right side of the breadboard. Connect one GND to the negative rail on the left, and the other GND to the positive rail on the right. 

This would be a good time to double check that your decoupling capacitors are connected correctly - one side on the power pin of the IC, the other side on the ground rail.

Next connect the non-inverting inputs of the TL074 - that's pins 3 and 12 - to the ground rails. Also connect the free ends of the input and output resistors to the 6 pin header at the bottom of the breadboard - this will connect the 3.5mm jacks to your circuit.

Now we can terminate the unused op-amps in the TL074. Doing this prevents unwanted noise and helps keep the circuit stable. Pins 5 and 10 connect to the ground rails. Pin 6 connects to pin 7, and pin 9 connects to pin 8.

Next, connect pin 4 to the positive rail and pin 11 to the negative rail. Check that none of the resistor legs are shorting to each other. Before connecting your other modules to the inputs of your new mixer, power the Pixie Dance Floor on. Sending voltage to the inputs of the circuit while the main power switch is off could damage the TL074.

Finally, you can connect your other modules to the mixer. For a simple test of the circuit connect two different LFOs to the inputs and connect the output to an VCO's control voltage input. You should hear the oscillator change frequency.

You can also incorporate the Pixie Dance Floor's on board potentiometers as input attenuators very easily. Disconnect two of the dupont cables joining the input resistors to the I/O header. Reconnect these cables from the input resistors to the wiper connections of the pot header. Connect the counter-clockwise connections of the pot header to the ground rail, and finally connect the I/O header to the clockwise connections of the pot header. You've now got volume pots on your mixer!

For another simple but useful mod to the circuit, just connect pin 1 to a free spot on the I/O header via another 1k resistor. You've now got an inverted output in addition to the non-inverted output you already had.