- 1 op-amp
- 1 resistor
- 1 potentiometer, resistor, and non-polar capacitor per channel
- signal source(s), this could be any music player with a line out
- lab supplies: breadboard, wires, DC power supply.
- another potentiometer, resistor, and op-amp, to build a master volume
- 1 switch per channel
At the heart of a mixing circuit is the basic summing circuit. One of the cool things about this circuit is that there is no limit to the number of channels we can add. But just because you can add 50 channels for you and your closest 48 friends to sing karaoke doesn't mean you should. If you were to freeze an AC signal in time, it would look like a DC value. If we summed 50 DC values together, there is a strong probability of saturating our amplifier. With an AC signal, sometimes one channel will be positive and another negative so it may not clip and distort your signal, but it's better to play it safe than sorry.
Let's start designing our circuit for the mixer. For your party, you want to have 4 singers and 1 song mixed together. This is a total of 5 channels, so you will need 1 op-amp and 5 potentiometers. An op-amp is not well suited to directly driving a speaker, so a power amp is normally included to drive the speaker. Also it is common to add a pre-amp to normalize all signal inputs. I have shown this in the schematic but will not be including the designs of how to build a power amp or pre-amp.
A word on microphone pre-amps, and power amps. Many microphones may require "phantom power" depending on type, and some microphones may be damaged by providing phantom power. Other microphones may use battery power. This design does not include a microphone pre-amp that provides phantom power. If you choose to add microphones look into the type of power required by your microphone, common specs on phantom power are 12, 24, or 48 volts; digital microphones often use 10 volts for phantom power.
Many pre-built sound systems include a built in power amp, such as stereos and computer speakers. If you choose to build your own power amp pay careful attention to the speakers power requirements and the power your amp is capable of delivering. Also for large power amps take proper precautions to ensure sufficient cooling is provided.
You will notice that I have the same setup in the first four channels: a microphone followed by the optional pre-amp followed by the potentiometer, capacitor, resistor, and then a switch. The capacitor is used to block any DC signal that may be coming from one of your sources. This is not necessary, but if all of your sources have a DC bias of 1 volt you may quickly be approaching your positive rail, causing significant distortion. Because of this it is very strongly recommended to include the capacitor. The series resistor in each channel is to keep the op-amp from functioning like a differentiator. For the fifth channel--the microphone input--is replaced by the music sources, which could be a computer, tablet, phone, or mp3 player. If a master volume is desired, you can build a second a second inverting stage for a master volume using the same design as an individual channel.
An important note on potentiometers is that they come in linear and logarithmic varieties. Although either variety will work, the logarithmic taper is preferable. This is because human hearing is logarithmic in nature, that is why use the decibel system for measuring volume. The logarithmic taper is desirable because if you want to make one channel twice as loud you would want to turn the potentiometer twice as high. If you use a linear potentiometer it will not be twice the volume, but rather twice the voltage. A logarithmic or audio taper potentiometer would be twice the volume.
Designing The Circuit
I measured the max output of my microphone at 2 Vpp (on battery power) and my tablet's max output was measured at 3 Vpp. This required a larger gain on my microphone than on my music input channel. The full output of the mixer circuit is Vout = -Rf(V1/R1 + V2/R2 + V3/R3 . . . + Vn/Rn). Thus, by making RF larger the gain can be increased. The other option is to make the resistor in the channel(s) smaller, but you are already using a potentiometer that can become very small, thus increasing the size of Rf is a better choice.
For my system, I used an LF347 op-amp (just about any op-amp will work for the mixer), a 47k audio taper potentiometer, 100k linear taper potentiometer, a 470k resistor, two 10nF capacitors, and +-12 VDC for my op-amp power. The small capacitors give a poor frequency response, with a lower -3dB point at 4.1kHz, but will work for my proof of concept circuit. I also only had 2 channels, so mine was a little simpler. In the simulation I disconnected the second channel to simplify the simulation. The different colors are different positions of the potentiometer.
I also designed a better mixer, but didn't build it. The simulation only has one channel for simplicity reasons. I used the normal 10k potentiometers in the design, but for the feedback resistor I used a 5k resistor. This is a little unusual: normally you would also use a 10k resistor, but this only gives you the ability to reduce the gain to unity. I liked the idea of being able to attenuate a channel instead of just increasing the gain. I also used a second inverting stage to build a volume control; the design is the same as the input channel.
If you desire more gain, I would increase Rf on the second stage: it could be increased on only the second amplifier. If you want a lower -3dB point, increase the size of the capacitor.
The simulation is shown below, as the circuit reaches the limits of the potentiometer the -3dB point shifts from 42Hz at unity to 69Hz.
For the input of your system, it may be desirable to use a standard 3.5mm stereo audio jack. If you choose to use this, it is helpful to understand which wires correspond to the jack's connections. The tip is the left channel, the middle section is the right, and the bottom section is the ground. Inside the cable, you should have loose wires that are the ground connection, then you should have a white, and most often a yellow or red wire. The white should be your right channel, and your other color the left channel. As there are many variations of the 3.5mm jack, it would be best to test your cable. If your source uses a mono audio jack the left and right channels will be joined. Some mono devices will use a stereo jack.
This is my audio mixer working (it's difficult to hear the microphone in the video). My microphone runs on battery power, and then I am driving a set of computer speakers that have a built-in power amp.
There you have it! Go out and get your party started!