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October 5, 2006

Building an Aux Input Cable for Bose SoundDock

Update: I've had a number of requests from folks interested in purchasing this cable. If you don't want to go through the trouble of making one for youself, you can now buy one from CableJive.

It's been about 3 months since I hacked my Bose SoundDock to include an auxiliary input. I've gottent a fair amount of response from folks about it, SoundDock owners everywhere are seriously interested in being able to listen to more than just the iPod on the speakers. However, most folks aren't up for pulling the unit apart and taking a soldering iron to it. A lot of folks have wished for a cable that simply plugs into the iPod connector.

Here's how to do it. With the right tools and supplies it doesn't take more than a few hours.

First order of business is getting a female iPod dock connector from Ridax to plug into the SoundDock. I don't believe there's a home-made solution to this, you've just got to wait for Ridax to get the order processed and the mail to snail it's way to your house. While shopping at Ridax you might notice that they now have a SoundDock friendly adapter that takes a 1/8" jack. Perhaps you'll decide that's a better option, in which case you can skip the rest of this exercise.

I ordered two of the white female adapters a few months back. It took them a few weeks to arrive, and then they sat on my desk for a long time waiting for me to get motivated to tinker. The adapter is easily pulled apart and as you can see has a small metal piece with the stretch of exposed pins that you can use to make connections through the female adaptor to the iPod-compatible device.

Once I had the adapters in hand I wrestled with how to figure out how the SoundDock is activated. My first aux input hack required leaving the iPod on the SoundDock for power activation. With this female adaptor hack I needed to figure out how to turn on the SoundDock from the adapter. I figured worse case scenario is that the SoundDock detects a small battery charge and uses that to trigger the power. The easier place to start was some testing to determine if bridging any of the pins triggered the SoundDock to turn on. After a little trial and error, I got the SoundDock to activate by connecting pin 18 and 19 (counting from left to right). The iPod Connector specs indicate that these are voltage pins. For whatever reason, connecting them triggers the on switch for the SoundDock.

Important notice, please ready the update at the end of this post. There's is more to the story for powering on the SoundDock.

To make that connection permanent I pushed the 18 and 19 pins together until they were crossed and making a solid connection. For good measure I stuck a small spec of solder on the two pins to make the connection more permanent. Not easy to solder on such a small scale, but sitcking just a small shaving of solder onto the crossed pins and pressing the tip of the solder iron down did the trick. Of course I tested to ensure that popping it on the SoundDock did indeed activate the unit. With that major issue resolved the rest was fairly straightforward.

The iPod connector specs indicate that pins 1 & 2 are grounds (connected together on logic board of iPod), pin 3 is right line out, and pin 4 is left line out. I had a spare 1/8" jack with a 2 foot cable in my cable box that looked just right for the job. (I thought about using an RCA-type plug but I think the 1/8" jack will be more universal for my use). After putting a small knot in the end of the cable to help keep it secure in the female adaptor housing I stripped back the ground, left and right wires just enough to give space for soldering. I should add at this point that soldering on these tiny prongs is a challenge, especially if you don't have any special soldering equipment (I have a $10 kit from RadioShack). I was able to get fairly nice, solid solder connections but sensed some risk that there could have easily been a drip of solder that would have covered and connected 5 or more of the tiny pins. You'll notice in the picture that I snipped off a few of the unused pins to the right of the audio connection pins just to free up some space.

With the cable soldered in place I put a good blob of clear epoxy over the soldered wires and pins to reduce movement and ensure the wires would stay in position and not accidentally short out.

After giving the glue some time to dry I snapped the unit together and have been listening happily to my laptop via the SoundDock ever since. It's worthy of note that the sound quality from the SoundDock is much improved over the original hack. I suspect it's because when the iPod is attached the audio out from the iPod causes problems with the aux input, even though the iPod isn't actually sending a signal. The extra hiss that I noticed in my first hack is completely gone when using this cable/adapter.

Important update: Some digging around with the multitester indicates that the SoundDock provides 12V, 500 mA from pin 19. Looking at the dock connector specs indicates that on pin 18, the iPod provides a 3.3V output. Using the multitester on my 4G iPod indicates that it outputs 3.3 V, 40mA. I checked using two 1.5 V batteries and indeed, providing ~3V to the SoundDock on pin 18 triggers the on switch. As documented above, I'm sending 12V, 500 mA into pin 18. It works, but may prove harmful over time. To convert the voltage and amperage down to a safe level a resistor should be used between pin 19 and 18. My calculations indicate that this should be a 250 Ohm resistor (drop 12 V by 8.7 V to be 3.3 V with 40 mA). Am headed over to the electronics shop at lunch to pick a range of resistors up and will update with information when I have a chance to try it.

Final Update: After a few trips to Radio Shack, some diagnistoc work, and conferring with a retired college professor/electrician (my father) I have a final update on connecting the current from pin 19 to pin 18. Pin 19 outputs 12V (firewire, used to charge the iPod). Pin 18 accepts 3.3V (for the iPod to power accessories) and uses that to trigger the on switch of the SoundDock. Initially I thought putting a 220 Ohm resistor between the 12V and 3.3V would reduce the electricity by 8.7 volts and be just what is necessary.

Unfortunately it isn't that simple. A resistor only reduces voltage if there is a significant draw in amps (or milliamps). Circuit 18 draws almost nothing, so putting a resistor between pin 19 and 18 made no significant change in voltage. The amount of draw is undetectable by my multitester. It can go down to hundredths of a milliamp and still wasn't registering anything when attempting to test the draw of electricity from pin 18.

The good news about that is that the SoundDock doesn't seem to be actually using the electricity coming in on pin 18 other than detect that there's some current there. So my original attempt to make a direct connection between pin 19 and 18 to trigger the on switch by sending 12V into the 3.3V doesn't seem to cause short-term damage. It's uncertain whether it causes long-term damage, as pointed out in the comments. Until someone verifies the electronics inside the SoundDock, or an engineer from Bose comments here, it's risky to send the 12V from pin 19 directly into the 3.3V on pin 18. It may prove to be damaging in the long run.

For those of us who'd like to play it on the safe side, there's still a way to reduce the voltage by hooking up resistors in serial and tapping out at the right point to get 3.3V.

Without going into a lot of electrical theory the idea is that you want to connect the 12V to the ground through two resistors, selected in a ~4/1 ratio. If you tap out between the two resistors you will get approximately 1/4 the voltage coming off the 12V. At first I chose a 1K and a 220 Ohm, put the 1K on the 12V side and the 220 on the ground site. Tapping off between them provided just about 3.3V. Since the SoundDock requires almost no current I decided to try a 10K Ohm on the 12V side and a 2.2K on the 3.3V side (both 1/4 watt resistors). Even with close to 0 amps coming off the tap between the two resistors the SoundDock powers on. I believe this is the best method (using the 10/2.2K resistors) for channeling power from the 12V to the 3.3V as it provides only a bare minimum current. I've made a few of these now and will take some photos to post on the next one I make.

Making this cable is just a bit more challenging when you have to put the resistors into the small connector housing.

Posted by mike at October 5, 2006 10:13 PM