# How to Design a Linear Mixer Circuit With an Op Amp

Written by j.t. barett
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For electronics hobbyists, designing a linear mixing circuit using an operational amplifier, or op amp, is a good introduction to this important component. The op amp is an essential building block for active linear or analogue circuits. Used in a linear mixer circuit, it takes on the role of an analogue adder, mixing all the inputs together to a single audio channel. This simple design involves no math and will take you an hour or two to complete.

Skill level:
Moderate

### Things you need

• Chart of electronic schematic symbols

## Instructions

1. 1

Review the chart of electronic schematic symbols, paying special attention to the symbols for wiring connections, fixed and variable resistors, and op amps.

2. 2

Draw the right-pointing triangle symbol for an op amp in the centre of a sheet of paper. Label its top input with a minus sign and bottom input with a plus sign. Draw a horizontal line leading to the right from the op amp's output. Label the line "Output." Sketch a ground symbol near the positive input and connect it to the input with a short line.

3. 3

Sketch a short zigzag symbol just above the op amp indicating a resistor. Label the resistor with a value of 10K ohms. Draw a straight line from the right side of the resistor to the op amp's output. Connect the left side of the resistor to the op amp's negative input with another straight line.

4. 4

Sketch a short horizontal line that ends at the right with the op amp's negative input. At the left end of this line, draw a vertical line several inches tall. For every input you want for your mixer, sketch a resistor that connects at its right side to this vertical line. The resistors will resemble the rungs of a ladder. Label each of these resistors "10K ohms."

5. 5

Draw a vertical variable resistor symbol for every one of the fixed resistors from Step 4, connecting the variable resistor's centre tap to the fixed resistor's left side. Sketch a ground symbol on the bottom end of every variable resistor. Extend a short horizontal line going to the left from the top end of every variable resistor.

6. 6

Terminate the left end of every short horizontal line with the symbol for an input jack. The jack's hot terminal should connect to the horizontal line. Draw a ground symbol that connects to each jack's ground terminal.

#### Tips and warnings

• As this design uses the op amp's inverting, or negative input, the output will be negative when the sum of the inputs is positive, and vice versa. This is relatively unimportant if you're mixing audio signals. If you want to mix direct current (DC) voltages, you'll need a slightly more complex noninverting amplifier design.

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