Audio Mixer





Introduction: Audio Mixer

I like to have long talks with my friends by Skype. I love also to listen music. During the conversation is not convenient to use loudspeakers because the music sometimes disturb my friends and when the connection is slow the continuous sound can overload the data rate and the communication can be interrupted.
After thinking, I found the best solution – it is an audio mixer, which can combine the two signals: the coming from the Skype audio and the signal generated from other sound device : MP3 player, radio, second PC audio card…etc., and both signals mixed, can be applied to the headphone or the PC headset. The levels of both signals should be regulated separately to make the device extremely flexible.

I decided to make the audio mixer with two stages: first - the mixer stage, second – a headphone amplifier. The audio mixer has two audio inputs (their number can be increased simply by adding inputs in parallel). For my case two inputs are enough. The device has one audio output (the jack in the middle), which can be used as headphone driver or as Line output and connected to some power amplifier.

Step 1:

The schematic and the PCB of the device are presented in the PDF files.
There we can recognize the two stages.
The mixer stage is realized by the Texas Instruments (NSC) high quality micropower dual opamp LMV797B, connected in inverting summing amplifier configuration. The left/right audio signals from both inputs are summed separately (left with left, right with right). With the stereo trimmer potentiometers both signals (left and right) from each input are tuned simultaneously. The opamp performs also some band pass filtering.
The second stage is based on the Texas Instruments (NSC) high quality headphone amplifier LM4880. It requires only few external parts and sounds pretty well. I used it also to create the virtual ground node for the mixer opamp. The audio mixer is supplied from the PC USB voltage output. Of course some AC/DC adapter with 3-5V stabilized voltage output can be used

The design database files in "Eagle" format are available for download.

Step 2:

The list of the used parts:
1x  LM4880M    (SO8 package)
1x LMV797      (8-MSOP package – very small pin pitch!) – there can be used another audio opamp, with bigger package – must be dual, single supply, low noise. I used this because I had one available and I saw in its datasheet that it is suitable for this purpose.
7x10uF 10 V capacitors – I used Tantalum Elcos from Kemet
2x2,2uF 10V capacitors – from Kemet
2x100uF 10V Electrolyte capacitors from Panasonic
10x20KOhm SMD resistors  (muRata)
1 x510 Ohm SMD resistor (muRata)
2x100pF SMD capacitor (muRata)
1x Blue bright light LED
3x Audio stereo jacks like these :
2x Stereo potentiometers 10KOhm :
1xPower supply jack :
1x Power supply plug :
4x 2 pin connectors (male - female pairs)
     Additionally I bought a plastic case and two plastic knobs

Step 3:

For the designing of the project PCB I have used the cadsoft PCB design tool "Eagle".
It can be downloaded from this site: There is also free version available.
The PCB’s I ordered at - 10 pcs, cost 25 USD. The quality is not bad.

Step 4:

After collecting all the parts (I have to wait for some them for a weeks) I soldered them on the PCB.

Step 5:

Using "Dremel" like tool after carefully measuring and marking I did a holes at the front side of the plastic case for the audio jacks, potentiometr axles, the LED and at the back side for the power jack.

Step 6:

To supply the device I have mastered a special cable. I took an USB cable from defect web camera, cut it and mounted the power supply plug. The internal pin is the positive node. This cable allows me to supply the mixer directly from the computer. Having power jack is useful when the device must be used standalone (without PC connection).Then a DC adapter plug can be inserted in the power jack.

Step 7:

The audio mixer supplied by the computer:

Step 8:

The device assembled and supplied:

Step 9:

The picture above shows the testing of the device, with one channel connected to the external USB audio card (also my project).
The sound quality is impressive!
As conclusion: The mixer performance is awesome. Now I can enjoy both activities simultaneously - chatting with friends and listening of music, without disturbing the talk.

The device can be used also as standard phone amplifier - if only one signal source is connected. Additionally it can be used for:
1) sound mixing effects when editing films  (for example for "Youtube");
2) DJ mixing at parties
3)  sound recording activities
4)  gaming effects...etc.

A populated PCB can be ordered for 35 USD (Free shipping) at e-mail address Bare PCB costs 10 USD with the delivery.




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    The input resistance depends on the potentiometers used.

    The input resistance of the mixer amplifier is 20k (for audio). As it is, it can work also with line level signals. If the gain is not enough - you can change it for both stages (increasing the feedback resistors), but you have to check what is the recommended gain for the output amplifier. If you want to increase drastically the input resistance, you can change the configuration of the input stage to non-inverting. The output impedance of the mixer is low. Not more than 100 Ohm for 20 Hz signal. It can drive both - headphones and line.

    What's the input and output impedances of the mixer? Also if I wanted to modify this mixer to work with line level inputs instead of headphone level inputs would doubling the gain of the left and right channel op-amps and increasing the input impedance be sufficient? How do I increase the input impedance?

    Hi Milen, since i don't want to use the mixer with headphones, is it okay to simply remove the headphone amp?

    Also what's the purpose of C9 and C10 ?

    Hi MartinI34,

    You can remove the headphone amp.
    You have to create additional reference voltage for the first amplifier - voltage divider - supply/2 and to keep the filtering cap (C11).
    C9 and C10 prevent flowing of the DC current through the load (the headphones) - they pass only the AC (the signal). If you omit the headphone amplifier, you have to use also such AC coupling capacitors between the mixer and your device - you can take the signal after the capacitors C7,C8.

    "You have to create additional reference voltage for the first amplifier - voltage divider - supply/2 and to keep the filtering cap (C11)."

    By this you mean to provide +2.5 on pin 8 and -2.5v on pin 4 on the opamp since i would not have any virtual ground (the one that was provided by the headphone amp) ? What would happen if I don't provide +-2.5v and only provide 5v as in your schematic? Also can I use mcp6022 in schematic?

    Sorry for this kind of questions, im bit of a newbie :)

    That is true. You have to create virtual ground. In this case we use single supply of 5 V. The best voltage for the virtual ground should be 2.5V.

    That means - you connect two identical 5 to 10 K resistors in series between the 5V and the ground. At the middle point you put capacitor (1 uF) and connect this node to the pins 3 and 5 of the opamp as virtual ground. That will force the AC signal to dance around this voltage :-).

    Seems that mcp6022 is a nice alternative for this purpose.

    Hi Milen, thanks for your reply. I've created some schematics, can you check if i messed up something?


    For me seems to be OK.

    Hey Milen, Im wondering why C1, 2,3,4 are reversed. Shouldn't the + side be facing the audio source?

    From left side these caps see the vdd/2 voltage.

    If no signal applied from right side they see the ground potential through the pot.

    Of course, the potential from left side cna become more positive, but normally the signal source is also AC coupled, which defines the DC potential at the capacitors left side always to 0.