I am going to share with you all an important type of gadget that any musician or any electronic enthusiast would like to have. It is an active sound mixer, and remember i am talking about active sound mixers, not passive one. I searched instructables and found out that there is not much of the topic discussed. I could only find projects on passive mixers and i couldn't any active mixer projects discussed here in instructable.  So therefore i thought of taking up this project and document it step by step from beginning on.

So lets start by explaining what actually Active Mixers and Passive Mixers are? Passive mixers are those mixers which helps to mix the signals with the help of only some resistant. They main component is the mix resistor and it works but the sound mix would not contain quality. It does not contain any active elements to mix the audio signal from the inputs. It just acts as a divider circuit and there is not much of quality mix.

Therefore we would want to have an active mixer, An active mixer has various electronic components in the circuit and it would help to distribute the audio signal in a balanced manner. The output from the mixer would be matched to feed it to any power amplifier. Therefore you would like to have an active mixer than to have a passive mixer for any quality audio.

So now i will be explaining how to build you own Three channel active mixer with a very few electronic components that i will be showing you here in my project.

Step 1: Parts You Will Be Needing

Here i have chosen a simple active mixer circuit and the parts are readily available at your nearest electronic shop. I am underlying the needed parts.

  • 2N3904

  • 100K Variable Resistor (4 Nos)
  • 1K5 (1 Nos)
  • 100K (1 Nos)
  • 5K6 (1 Nos)
  • 10uF/ 10V
  • 0.1uF Ceramic Capacitor  (3 Nos)
  • 10uF/16V Electrolyte Capacitor (1Nos)
  • 3.5mm Mono Sockets
  • SPST Switch (3 Nos)
  • PVC box

Step 2: Circuit Overview

The main component used here in the circuit is the transistor, It will intelligently divide the sound signal to a source. It has three variable resistors for controlling the input sound signal. The signal is then fed to the ceramic capacitors that would block any DC voltage and only allow the signal to pass through the line. The ceramic capacitor is very useful as because there might be DC voltage either present in the signal or to the mixer itself, so this will stop the voltage, thereby protecting the source signal, as the source signal might be coming from an expensive smart phone or an expensive laptop or anything very valuable of cost.

The signal is then fed to the mix resistors that would take the signal to the transistor that would mix the sound signal with high gain and quality. Finally at the end there is a filtering capacitor that would filter out any unwanted signal arising from the mix of the transistor  and the DC voltage, so the output is received from the filtering capacitors and this signal can be directly fed to any power amplifier circuit.

Step 3: Compiling the Parts in the PCB

Lets start by using a general PCB. Start by fist soldering the ceramic capacitors as while doing so you would also get the feel how the signal would pass in the mixer. Try to make a mental picture while soldering, taking view of the Circuit diagram. After the ceramic capacitors are soldered. Then solder the main part the transistor, Be careful not to over heat the transistor while soldering. After the transistors is soldered, put on the rest two of the biasing resistors. Finally solder the filter capacitor and more or less you mixer board is ready for further transformation.

Now as you have finished the soldering part, now comes the work of wires. It may get really messy with the wires, but try to be neat and bunch up the wires evenly. I would recommend to use audio wires as there would be less noise if you use audio wires. Audio wires are wires that have a shield covering the outer part as to make the audio signal free from interference. A simple example of  It is a good practice and i generally use such type of wires. Remember to ground the shielded wires.
Finally take out the two leads for Positive and negative. Here the positive is the blue wire and white wire is the negative. The red wires coming out from the board are shielded wires as because i want to look like more professional and eliminate the extra sound as less as possible.

Step 4: Putting It Into a Neat and Clean Enclouser

I Started by choosing a PVC box, as because it is very handy and holes can be made to this box very easily. I started by drilling small holes for the 3.5mm socket, three holes at a constant interval. I then added the socket and screwed it firmly to the base (as shown in the picture 1). 

Again i drilled holes for the Variable resistors (100K) with the given intervals so that look professional. I just drilled the three holes directly above the 3.5mm socket so that one can find out which channel they will be controlling.(Picture shown 2).

There was more drilling now for the switches that would be inserted to select the inputs as needed. It was again drilled just above the holes drilled for the variable resistors. (Picture 4) and in the middle of the switches, a single hole was drilled for the main volume control.

A finished upper part of the unfinished looks like in the picture 5. The shot is taken from the bottom angle.

Now it was simple process of fitting the variable resistors and switches in their respective places and bolting them up, please follow the pictures (6,7,8,9). It is very simple just screw in the bolts of the variable resistors with a nose or a monkey pliers firmly it the front end of the PVC box. 

In the final picture 10 you will be able to see all the variable resistors at their places and now only we need to add in the switches.

In Picture 11 you will be able to see the back of the PVC box where all the variable resistors are fixed to the box and now they are ready to be soldered to the respective connections.

And finally in picture 12 you will be able to see the switches placed to the respective holes and now you can see the top of the three channel audio board. On top of the board is the master volume control. It will be responsible for the overall output to the power amplifier. Just below the master volume control are the three channel selector switches responsible for selection the audio channels. and below the selector switch are the individual channel volume control which will be engaged for varying the input audio for the mixer. It is partly finished and looking great for further alterations.

Step 5: Wiring the Components

After every thing is screwed to the PVC box, now we will complete the instructable with wiring the variable resistor and the 3.5mm socket to the main mother board that we have prepared.

We will start by soldering the stereo socket and then gradually finish up with wiring the variable. In doing so, you will have a rough idea on how the audio feed is getting in and will be distributed over the circuit for mixing purposes. 
Soldering is pretty simple just go with the schematics and you will find you way. 

At the end the finished product will be looking as shown in the figure and you will only need a power supply for the mixer to power it on. You can do this by simply taking a 5v DC power from any power source and you are ready to mix. Here in my mixer my power supply is from the Blue and White wire coming out from the mixer as shown in the figure. You can add a maximum of three (3) inputs and test you brand new active mixer. 

The quality you get with this mixer is pretty good and it works really fine. I generally use this mixer when i am monitoring various frequency in the air, while i am listening to various audio programs. This gadget is a must have for every electronic enthusiast. 
<p>Hey wesome project, I am looking for a solution that will make me able to connect my amplifier output and a track source, such as PC audio, as Inputs. And mixing them so that the output would be my headphones. My question is, will this work or should I settle with a op amp mixer?</p>
<p>If i add more inputs, do i still need to change the circuit values/parts?</p>
<p>Hi! I just made this mixer and it doesnt work. As I can see, all the signal is going through the two parallel resistors and then directly on the electrolic capacitor. The transistor is not being used. Unless there is a connection between the colector of the transistor and the electrolitic capacitor, but in this case, the sound goes out with a lot of distortion. Can you please help me?</p><p>Thank you very much! </p>
<p>thanks for the tutorial, can another transistor be used instead of the one in the schematic diagram. pls give me the name, because i couldnt that transistor here, answer plsssss</p>
<p>This is awesome! Thanks! I would like to ask if in order to use slider potenciometers on the inputs, would I have to change any of the resistors or capacitors? Also, how complex can it get to integrate 2 filters for each input to have low and high frequency pots in action? </p>
<p>Hi, this is awesome tutorial. Thank you.</p><p>Is it possible you write a bit of guidence about what would change to make it stereo, and with more input channels ?</p><p>Either way, thank you very much.</p>
<p>Hi! Maybe you can build this instructable two times and then just wire the second channel of the stereo input to them. That would be even greater 'cos you would've control over each line in the stereo input. But maybe there's another more efficient way to do it.</p>
<p>you need linear pots (two in one basically) and stereo jacks, that and maybe more power, just be careful not to mis solder l or r to ground :)</p>
<p>can it be used for surround or do you have to change it then?</p>
<p>Thank you for the tutorial. :)</p><p>I'm thinking to build this mixer but I'd like to add some more input channels. Should I make some changes in the resistor values? Or is as simple as join the new input channels in parallel with the three you created?</p>
The 2N3904 is to amplify? or rectify?, if its to amplify, can i assemble one without amplification?
No, you can use this Circuit for only pre-Amp purpose, so therefore you need to have an amplifier connected with it <br>
<p>What's the expected signal level of the mixer? I want to feed the output to a video digitizer card and I suspect I can do that directly from the mixer output...</p>
<p>How can I add a headphone monitor output? Can I stick a headphone on the transistor collector terminal with some resistor to impedance match? Circuits looks very simple and elegant but for my application (translator's booth), I would want to add a headphone port. Thanks for the great project!</p>
Will this work for recording on a PC, connecting the output to PC&acute;s microphone input? <br>Thank you very much!!!! <br>Answer please!!!
Yes, it can work, but you need to have an amplifier between the mixer and the computer

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