Welcome to my very first instructable!

Why?

I've always had windows computers with 5.1 sound cards. Therefore the setup I've put in my room is also a 5.1 speaker system. It has three 3.5mm audio jacks: front speakers, rear speakers and middle speaker + subwoofer. Five years ago I started using a Macbook pro, with only one audio jack output. This rendered four of the available six speaker channels useless. (There is some crossover to get sound from multiple speakers, but it's nowhere near true 5.1 sound)

After five years of under-using my speaker setup, I decided to do something about it. I didn't want to buy an expensive sound card, and neither my itunes library or my speakers are top quality, so I settled for a cheap and medium-quality solution.

Goal: Combining three cheap 2.0 USB sound cards to act as a single 5.1 sound card

Skills needed: Soldering

Step 1: Materials

The materials you need in order to complete this project are:

A Mac computer
USB hub ($1.50)
3x cheap eBay USB sound card (3 x $1.00)
Soldering iron
6x capacitor

If your amplifier has RCA inputs, you will also need to buy three jack to RCA adapters.

I decided to go for a cable-type of USB hub, because I wasn't certain that the spacing between the inputs on a box-type hub would be sufficient for the sound cards to sit next to each other. In retrospect they are a bit thinner than I had expected from the photos, and I guess it should be no problem to use a box-type usb hub. It would probably even be a cleaner looking solution.

Specs on the sound card: 16-bit audio at a max sampling rate of 48 kHz. This is far from Hi-Fi quality, but for a medium amplifier and speaker setup it's good enough. Using an expensive sound card to play music over a medium quality setup is like serving a Big Mac on a golden plate: it looks really nice, but it will still taste the same, and it's just wrong!

I've done a quick google search to find out whether it's possible to combine sound devices on windows computers, but no helpful results turned up. If anyone succeeds in doing this, please let me know in the comments and I'll add it to the instructable. For now, this is a Mac-only instructable.

Step 2: Sound Card Adaptations: WHY?

In the next step I will be adding output capacitors to the USB sound cards, here I'll discuss why that is necessary:

In order to save money in the production of the sound card, no output capacitors were added. The manufacturer has made a placeholder for them, but instead of soldering the capacitors onto the PCB they just shorted the connections.

If you don't add them, you will hear a very loud pop every time you plug a speaker or a pair of headphones into the sound card. This is because the inbuilt opamp needs a symmetric power supply, so a virtual ground is placed at half the maximum voltage of 5V. Therefore the amplifier output has a DC voltage component of 2.5V. In order to filter this DC-component out, and have only an AC audio signal going to the speakers, a capacitor needs to be placed in series with the output.

You'll want the capacitance value to be sufficiently high, because we're in fact making a high pass RC filter where the resistance is the amplifier input impedance. The frequency response is given in the picture above. This means that a frequency of 0 Hz isn't passed through to the output of the filter. However, if the cutoff frequency (fc) is too high, the lower audible frequencies are also attenuated and there will be no bass present in the signal.

The design tradeoff you have to take into account is the following: a higher capacitance means a lower fc, but also a bigger capacitor. For use with an amplifier, typical input resistance is over 40 kOhms. If you plan to plug a pair of headphones into the sound card, a very high capacitance will be needed, since the impedance can go as low as 16 Ohms.

I went with a 47uF radial electrolytic capacitor. The diameter of 5mm is just small enough for the cap to fit inside the sound card's casing, and the fc is below 1 Hz when used with an amplifier, which is more than low enough! For use with headphones, I suggest not going below 470uF.

Step 3: Sound Card Adaptations: HOW

1) Remove the plastic casing from the PCB.

2) Localize the two pairs of unsoldered holes.

3) Cut the jumper trace between each pair with a sharp object. Do this carefully so that you don't cut other traces as well. If you have a multimeter, use the continuity checker to make sure that the pads are no longer connected.

4) Solder the capacitors in place. The negative leads have to be connected to the output jack. Again a continuity checker would be helpful, otherwise just try to follow the pcb traces. There are +/- markings on the silkscreen of the PCB. In my case the markings were correct, but I've read about sound cards where they are wrong, so don't trust the silkscreen!

5) Close the plastic casing back up.

6) Repeat for the other sound cards.

7) Connect the USB hub to your computer, and the soundcards to the hub.

Step 4: Combining the Sound Cards in OSX

To do this you have to open an application called Audio MIDI setup, located under Utilities. With this program it's possible to combine any number of connected sound devices into a single virtual one.

1) Click the small '+' symbol at the bottom

2) Choose one of both options (with aggregate device you can combine any number of inputs and outputs, with multiple output device it's only outputs)

3) Select the aggregate device, and tick the boxes of the three sound cards. You can also change the sampling rate, the relative volume, the master clock device and whether you want drift control to be activated. (Google for more info, or just test the different settings)

4) Click 'configure speakers', select Multichannel and fill in your setup. To identify which channel is which speaker, click on it and white noise will play through that channel only, click again to turn it off.

After this you are no longer able to use the keyboard buttons to control the volume, but the speakers probably have a volume control of their own.

There it is, a 5.1 sound card that produces acceptable quality audio for less than five bucks!