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 : http://www.ebay.com/itm/260867290343
2x Stereo potentiometers 10KOhm : http://www.ebay.com/itm/250956199680
1xPower supply jack : http://www.ebay.com/itm/270758466145
1x Power supply plug : http://www.ebay.com/itm/270979725550
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: http://www.cadsoftusa.com/. There is also free version available.
The PCB’s I ordered at http://www.iteadstudio.com - 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 chicho.mecho@gmail.com. Bare PCB costs 10 USD with the delivery.

<p>The input resistance depends on the potentiometers used.</p><p>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. </p>
<p>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?</p>
<p>Hi <a href="https://www.instructables.com/member/Milen" rel="nofollow">Milen</a>, since i don't want to use the mixer with headphones, is it okay to simply remove the headphone amp? </p><p>Also what's the purpose of C9 and C10 ?</p>
Hi MartinI34,<br><br>You can remove the headphone amp.<br>You have to create additional reference voltage for the first amplifier - voltage divider - supply/2 and to keep the filtering cap (C11).<br>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.
<p>&quot;You have to create additional reference voltage for the first amplifier - voltage divider - supply/2 and to keep the filtering cap (C11).&quot;</p><p>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?<br><br>Sorry for this kind of questions, im bit of a newbie :)</p>
<p>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.</p><p>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 :-).</p><p>Seems that mcp6022 is a nice alternative for this purpose.</p>
<p>Hi Milen, thanks for your reply. I've created some schematics, can you check if i messed up something?</p>
<p>For me seems to be OK.</p>
Hey Milen, Im wondering why C1, 2,3,4 are reversed. Shouldn't the + side be facing the audio source?
<p>From left side these caps see the vdd/2 voltage.</p><p>If no signal applied from right side they see the ground potential through the pot.</p><p>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.</p>
Thank you very much Milen. <br>It works on protoboard, tomorrow im etching a pcb, ill post my results here once i have something to share.
<p>Hi guys, I can't find the LMV797 where I live it's possible to use the RC4558 instead?</p>
<p>Hi <a href="https://www.instructables.com/member/JXplicits">JXplicits,</a></p><p>RC4558 could be used, but it requires min. 10V supply. The LM4880 will not work at 10V. You could use additional regulators, but this will make the design more complicated. If you want to use the PCB without changing it, you have to keep also the package type of the active mixer op-amp. One possible solution could be the OPA2316 in MSOP package. I hope you can find it.</p><p>Regards</p><p>Milen</p><p><br></p><p><br></p>
<p>Thanks for reply,</p><p>Yeah, maybe I will stick with the RC4558 cuz I dont find anything else and with your permission of course, reroute the whole board with thru hole components, put aside the second stage and use 12v supply. </p><p>I just want to mix the PC audio output with a floating aux input where I can connect any device (tablet, phone, mp3, etc) and doesn't have to rely strictly on the PC for listen music or do whatever and the mixer output go straight to the existing amplifier that I use with the current setup.</p><p>Sounds ok this use or maybe I can go with a different approach simpler I guess?</p><p>English isn't my main language, sorry if there is any mistake. </p>
<p>Hi everyone! Will this work if I wanted to input a microphone and &quot;mix&quot; it with the audio output of my TV, which will then go to the same speaker system? We had this problem a couple of weeks ago (karaoke session) where we would play youtube karaoke videos, but we had no way of mixing the mic audio with the music.</p>
<p>Hi,</p><p>The microphone signal is too week to work fine with this mixer. If you put a microphone preamp b/n the mixer and the microphone - it will work.</p>
<p>can you do surround sound with this?</p>
Hi <br>This is stereomixer - two channels. For 5+1 you would need 3 of them.<br>The problem should be how to synchronize the potentiometers. But some solutions can be found - for example using digital ones.
<p>Hi Milen, thanks for help, I will do 12V preamp and mixer version (as the low voltage IC and op amps costs are quite cheap) and if that work I will change to low voltage OP amps) and I will donate 12V version to my guitarist friend.</p><p>Thanks again</p><p>Michal</p>
<p>Hi Michal,</p><p>I do not have experience with JFETs. I found a JFET circuit which Works at 5V:</p><p><a href="http://www.radartutorial.eu/21.semiconductors/hl21.en.html" rel="nofollow">http://www.radartutorial.eu/21.semiconductors/hl21...</a> . but seems that they are mostly used with voltages overt 9V.</p><p>Here are some links: </p><p><a href="http://circuit-diagram.hqew.net/General$2dPurpose-Jfet-Preamp-Circuit_14846.html" rel="nofollow">http://circuit-diagram.hqew.net/Generalhttp://circuit-diagram.hqew.net/General$2dPurpose-Jfet-Preamp-Circuit_14846.htmldPurpose-...</a></p><p><a href="http://circuit-diagram.hqew.net/list-guitar$20preamp_1.html" rel="nofollow">http://circuit-diagram.hqew.net/list-guitarhttp://circuit-diagram.hqew.net/list-guitar$20preamp_1.html0prea...</a></p><p><a href="http://www.planetz.com/simple-jfet-preamp-for-an-idevice-guitar-interface/" rel="nofollow">http://www.planetz.com/simple-jfet-preamp-for-an-i...</a></p><p><a href="http://www.rason.org/Projects/jfetamp/jfetamp.htm" rel="nofollow">http://www.rason.org/Projects/jfetamp/jfetamp.htm</a></p><p>If you need 5V preamp - instead JFET you could use also an opamp in a small package (vsop or similar) - it will occupy the same place as the JFET.</p>
<p>Hi Milen,</p><p>Thank you for your advice, I ll try what you suggest but first I will build this <a href="http://www.till.com/articles/PreampCable/index.html" rel="nofollow">http://www.till.com/articles/PreampCable/index.htm...</a> as it explains lack of high frequencies in the sound, I will have a nice preamp then and then I will go ahead and build your mixer. I plan to run it either to headphones or to power amp so I will build it with LM4880 or some other IC of headphone amplifier.</p><p>The only question that remains to me is whether I could get FET transistor that would work at USB voltage or I will need to move to higher voltage. (e.g. 2x9V battery). Do you know anything about such FET Transistor?</p>
<p>Hi Milen, nice circuit, I like it very much, I will be using it for my guitar play along exercises. Do you have any idea of schematics on how to add electric guitar pre amp (clean sound only - no distortion) as another line input to your device? USB powered ideally.</p><p>Michal</p>
Hi Michal - to add additional line input, you have to put once again the following parts group: potentiometer (connected to the output of the preamplifier), capacitor, resistor and to connect all this to the inputs of the first amplifier. I suppose that the signal will be mono - I would suggest to use dual potentiometer, which inputs are connected together, but the tap points go to the separate amplifiers (both opamps). How to make the preamplifier - you can find also in instructables : https://www.instructables.com/howto/quitar+amplifier/<br>I think that you do not need even a preamplifier - you can try to connect the guitar directly to the potentiometers inputs. If the gain of this channel is not enough, you can try to reduce the 20K resistance (only for this channel). If you want to use external power amplifier, you can omit the headphone amplifier (LM4880). You can find the best performance by yourself with the play...
<p>Do you still have populated PCB's or bare PCB?</p>
I have one populated and 2-3 bare PCB,s.
Also why do I need 7 10uF caps? I only see 5 on the sch. (C1,C2,C3,C4,C12) According to the schematic 2.2u's go on C7 and C8. Thanks for the help by the way. I'm looking forward to getting this project completed!
Hi,<br><br>C7, C8 can be either 2.2uF, either 10uF..In the original design I put 2.2uF, but after that I put also 10uF - I do not see any difference in the performance.<br><br>C12 and C13 form capacitor bank. Normally the electrolyte capacitors are not able to filter high frequencies, because their internal inductance is too high.For this reason parallel with them are used ceramic capacitors with lower inductance (and capacitance). In one word :-) C13 is 100nF ceramic cap. <br>Tha value of C11 is defined in the datasheet of the LM4880 chip and its value 1uF. Higher or lover value possible can cause instability of this reference voltage, so...I decided not to play with different values :-).<br>As I understand, you are close to complete the project. Please, provide me you feedback - how do find it performance?<br><br>Regards<br>Milen
Are C11 and C13 100uF? if not which capacitors are they? One is labeled &quot;.01&quot; and the other &quot;1u&quot;
I can't seem to find the LMV797 for purchase in MSOP06. Do you know where I can get one or how I could install the 8pin package on this pcb?
Sorry, there was an error in the text - the chip is LMV797, and the package is 8-MSOP. <br>The PCB is designed for 8-pin package - no problem... The only problem is that the pin pitch is very small, but the nice thing is that pin count is not high :-)
pcb's still available?
Hi, I think that I have few remaining.
I really like the design! What is the purpose of JP2 and JP3? Also, where did you acquire the case and knobs?
Hi Bramathon, <br> <br>I put jumpers JP2 and JP3 with the idea to connect there in the future VU meters. This is future project, which I want to make - to put analog audio signal indicators on the top of the box. I have bought two of these : http://www.ebay.com/itm/200693370727, but I do not have time to do it. The case and the knobs I bought from a local electric store, but I think you can find in the online shops suitable ones. For Europe you can check here: http://www.conrad.com/ce/en/category/SHOP_AREA_14740/Enclosures-Cabinets-Housings . <br> <br>
love the mixer, any idea on the cost of the build.
I can send you populated PCB against 35 USD...or bare PCB for 10 USD, You can check what the components cost simply clicking on the links at step 2. The chips you can try to find in Digikey, Farnell or other online shops. The case will cost not more than 3-5 USD. You can try also to make your board by the laser ink transfer technology...<br>
very nice! what will you do with the other 9 boards?
I made some presents for my friends. Some still remain unused. :-)

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