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.

Continue reading “Monitor Mixer with DI Box”

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.

Continue reading “4Ch Mini Stereo Headphone Mixer”

Mic Preamp With bass treble Tone Control for videoke

If you are looking for a mic preamp with bass and treble control, you may consider this one. I designed it for my future mixer build. I am planning to build one someday when I am not too busy.


The circuit is only on design stage, not yet done any prototype but it is working on my simulation. This is an improved version of the preamp on my audio mixer design I posted.

The rolloff on the 10k-20kHz range is caused by the capacitor C8. This is done to make sure the system will not allow any radio frequency to pass especially AM radio signals. If you want that roll-off to be more aggressive, you can increase it. Treble frequency is set by C3 and C4 while C2 is for setting the bass frequency. Increasing those capacitor values will decrease the frequency. For instance you want the bass to have more boost on the lower end frequency, just increase the value of C2.

Gain is set by R1, if you want more gain, increase R1 value. Potentiometer used are 100k, using 50k or lower will decrease the boost and cut range by around 3dB.

Simulation Results

All range: bass and treble at 0%, 50% and 100%


Bass control (0%-100% at 10% increment)


Treble control (0%-100% at 10% increment)


DIY DI Box v3

I just noticed that many of my visitors are searching for DIY DI Box so I decided to make another design project for DI Box. This will be powered by a 9-15V AC/DC adapter or a 9V battery.

About the Circuit

The circuit is basically made up of 2 opamps, 1 is configured as a buffer and the other one is configured as an inverting opamp.

R2 and R3 is used as a voltage divider to provide 1/2VCC for the opamp. This is needed for single supply operation of opamp. At single supply, the output of the opamp cannot swing at negative voltage so the output of the opamp is offset to 1/2VCC so the output can swing below 1/2VCC for negative voltage and above 1/2VCC for positive voltage swing. at the output stage, a capacitor is used to remove the DC offset and output only the AC voltage. C5 is used to filter the 1/2VCC from the voltage divider R2 and R3. R1 and C1 is used to filter the supply. This is used to filter out the 60Hz/120Hz line noise on the supply. If you are using a battery, you can just remove R1 instead. Potentiometer value can be 100k to 1M. I used RCA connector simply because I can’t find any Audio phone jack on my library and I don’t have time to search for it.haha

C6 and C7 voltage rating should be at least 50V, this is to protect the cap when phantom power is accidentally turned on. Other capacitors can be rated 16V and above.

Phantom Powered DI Box

If you want it to be powered via Phantom power of mixer, you can check this version of my DI Box here:
You just need a 1/2W 6.8k resistor and a 15-30V zener.

PCB Design

PCB design is made with KiCad, a free open source software. Designed with 1 layer PCB for easy DIY at home.

3D Render

Simple Direct Injection Box

Here’s a simple Di Box that you can build. In case you don’t know the purpose of Di Box, it is used to connect unbalanced output to balanced inputs. For example, a guitar has unbalanced output and a mixer have balanced input. Balanced lines are used for long cables audio because it is able to cancel noise on long cables.

About the Circuit

The circuit use an NPN general purpose transistor. You can use any general purpose transistor here similar to 2N2222 transistor.



The positive output(emitter side) has slightly higher gain because emitter current is slightly higher than collector current. The difference is minimal and may not be an issue.

Vintage Guitar Preamp re-design

I have seen this circuit many years ago and I did planned to build this circuit but I never got a chance. This time though let’s try to re-design it to work with lower voltage and use an smaller amplifier for practicing. I am planning to use it for 12V or 9V amp so we need to modify it to work below 9V instead of the 60V supply which is the original design.

About the Ciruit

This circuit is all over the internet now and you can find it in many website. It is quite simple although the biggest problem in building this circuit is the operating voltage which is 60V. I will try to modify it to work with 9V supply. This task should be easy by using simulation software such as simetrix. I’m too lazy to compute so I just modify it by trial and error on the simulation by intelligent guessing of course! The main focus here are the high side resistors such as R3, R6 and R17. The goal is to decrease its value until you will come up with same bias level with the original supply voltage.

There is a small error I spot on this design – C11 should be connected to ground and not on the emitter of Q3. I wonder how they made this error.

Simulation Circuit

Again, I am lazy to match the names of the device on my simulation model so let’s keep it this way. 🙂

I did not included the “Harmonic Modifier” circuit on the simulation circuit because I want to keep it as is.

Bias Levels

I only have noted those points of interest here:

The level at Q3-emitter is around 26V, since our goal is to operate it at 12V, it wont be able to reach 26V when VDD is only 12V. We just need the voltage at this point to be half the supply. Same is true with Q2-emitter voltage.

Frequency Response

Here’s the frequency response of the original circuit.


Here’s the 12V modified version:

There are minimal changes, the base bias of Q1 and Q3 have been modified to make it simple. Added R20 and C7 to make up the gain at low voltage operation.

Simulation result of the modified circuit

The circuit has slightly higher gain. You can try to experiment on the values of the  resistors and capacitors. You can download the simulation file below. Simetrix is used for this simulation. It is a free software.


PCB Design

The PCB design is made using the kicad software, it is also free so anybody can download and use it. The design is very simple because I have no plans to build it for now. I just wanted to share the design for those who want to build this kind of project. You are free to edit it for your own requirements.

The design of the PCB is pretty basic. You can download the PCB project file below so you can edit it based on your requirement.

The best feature of Kicad is that you can view your design in 3D! This is a feature you wont expect on a free software! 🙂



Download Page


Guitar Preamp Design

If you are a electronic hobbyist and at the same time a guitarist, this project is for you. I have seen many circuits of guitar preamp online but none of them gives you the PCB design and circuit simulations. I already made a version of this project years ago and it works. I also have included simulation files together with the eagle PCB files for you to download if you want to build this project. Take note that the capacitor sizes are not yet taken into account. You will need to edit some few capacitors like the C10 which will be bigger than it is on the board.

Continue reading “Guitar Preamp Design”