Monitor Mixer with DI Box

One of the problem for low budget band online streaming is the mixing part. The biggest challenge is getting the mix right. Why is it hard? Guitar and other instruments have their own amplifier so when you are doing the mix, you would likely decrease the level of these instruments to the main mix. If you are feeding the main mix into your online live feed, chances are these instruments are too silent on the main mix.

This circuit can be use as a headphone amp mixer with built in DI box so you can directly feed the output into the mixer. There are 2 volume knobs, one knob is used for your instrument volume and the other knob can be used as a monitor level feed from the mixer.

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4Ch Mini Stereo Headphone Mixer

4 channel stereo headphone mini mixer 3d render

Here is a simple stereo headphone mixer that can be used for monitoring and mixing 2 sources. A very helpful device for online streaming. How? For example you are playing a keyboard on a band in an online stream and using a headphone as monitor. You can use this to mix the headphone output of you keyboard and headphone output of the streaming mixer so you can hear more of your keyboard.

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Simple USB powered Audio Mic Mixer

Here’s a simple Mic mixer that can be Powered with 5V. There is currently 3 channels on the circuit but you can add more if needed.

About the Circuit

The circuit is made up with BJT common emitter configuration. OPAMPS that can operate at low voltage are hard to find at a local electronic stores. The very common TL072 opamps can still operate at 5V at a very limited output voltage swing.

The circuit provides ~29dB gain, you can reduce that gain by increasing the value of the emitter resistors(R8,R9, and R11).

R3 and C1 is a filter for the power supply. C1 can be increased to reduce the noise if your power supply is noisy. R3 can also be increase up to around 100 ohms for better filtering.

You can connect this circuit to the USB audio interface circuit in this post so you can have this connected to you PC for recording.


On the simulation below I used waveform generator with sinewave output at 100mV at frequencies 100Hz, 200Hz and 500Hz respectively. The volume setting is set at 30%, 10% and 10% respectively.

Simple 9V 4 channel Mic mixer

Here’s an improved version of my simple mic mixer featured on this page. It is powered by 9V battery so you can make it portable.

About the circuit

The circuit is made of transistors only to make it simple. I didn’t use any op-amps on this project. You can basically build this circuit using universal PCB’s

It has 5 potentiometers. 1 for each channels and 1 for master volume. It can be used for guitars and mics.

Circuit Simulation

For simulation I used signal generator set with sinewave output at 50mV and each are set at different frequencies.  This is to be able to see each frequency on the output waveform. As you can see on the simulation of the output waveform, it has different frequencies.


PCB design

Here’s a simple PCB design that I made. As you can see, the design is very simple and only has few components. The layout is not perfect but at you can have something to start to. You can still improve this layout. 🙂

3D render by Kicad

You can download the Kicad project file here:

Download page <- find the download here


Digital Audio Mixer Project with ADAU1701

It has been my long desire to design a digital audio mixer but there a too little resources regarding digital signal processing. While researching I stumble upon this little chip from analog which don’t require DSP knowledge. It is program using a software called sigma studio. There is no programming language, you just need to drag and drop modules. I am still on learning process, and this is just a test of what can I able to pack on a cheap ADAU1701 DSP. Considering the price of this DSP and the size of its memory, these modules that I was able to put was amazing.

What I was able to get with the limitation of ADAU1701:

  • 8 input channels
    • all have 3 parametric equalizer
    • all have compressor
    • all have volume control
    • all have 1 pre aux
    • all have 1 post aux
  • 4 output channels
    • 2 as main output with 6 band equalizer
    • 1 as pre Aux output
    • 1 as post Aux output

That’s about it. Adding more modules will prompt an error that you are out of memory. Considering what most 8 input analog mixer are offering on the market, this features are better. This project can be further improved so I have uploaded the project file on my download page so you can edit it yourself.

Sigma Studio Project

Main Schematic

Channel strip

These controls can be accessed through a microcontroller connected to it via I2C. The microcontroller can then be connected to user interface such as buttons and displays like LCD.

10 Channel variant

I have tried to use the maximum possible inout of the ADAU1701 which is 10 inputs but simplifying the schematic and removing the compressor on channel 7 and 8. The 2 additional channel can be used for bluetooth audio. I was able to achieve the following:

  • 10 input channels
    • Channels 1-6 has HPF, 3 band Parametric EQ, Compressor, 2 aux sends
    • Channels 7 and 8 has 3 band Parametric EQ and 2 aux sends
    • Channel 9 and 10 has 3 band EQ
  • 4 output channels
    • 2 main out with 10 band graphic EQ
    • 1 Pre Aux out
    • 1 Post Aux out

I think that is the best I can have for ADAU1701 DSP.

Multi-track Recording

This is the biggest challenge for this project, for now I don’t have any idea on how can it be done and how it can work with the ADAU1701. What’s clear for now is that the ADAU1701 DSP can’t be used for this purpose. I know it can be done using the XMOS chip but the development board is too expensive for me.


The ADAU1701 is a powerful DSP for audio application. It can surely be used for digital audio mixer application with limited input channels. 8 channels maybe the most channels you can have for this application. You can have as much as 10 total inputs but you need to limit some functions like compressors to be able to use all the 10 inputs or else you will ran out of memory and the project will not compile at all.


  • ADAU1701 only have 2 ADC channels build in, you need an external ADC to be able to add 6 more inputs.

I have yet to build the actual hardware but I am not sure when I can do it. When I got the fund and time for sure but I am not sure when.


Download page