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.
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.
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
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 firstname.lastname@example.org. Bare PCB costs 10 USD with the delivery.
THANK YOU FOR THE ATTENTION! :-)