This hack/mod may be a bit scant on some details and is meant for dedicated hack/makers with a bit of skill under their belt. No details are given for the physical dismantling of the iSub but, screws are under the feet (carefully remove feet) and just follow your nose after that. Use lots of padding – you don’t want to put scratches all over it as you work. All the following is used at your peril or elation 😉
I picked this up from Ebay as a purposeful hack project. Apple no longer supports the iSub, although on my Windows laptop it does appear as an audio device and I can play sound through it.
The iSub by Harmen Kardon was designed for Apple as a USB only subwoofer. It is a fairly iconic looking subwoofer, which has since been replaced with the HK Soundsticks range, using the same looking subwoofer but with two small stereo speaker stacks, all running via standard audio (USB has gone). The electronics in the iSub consists of two parts, one is the USB to audio section and the other is a plain and simple subwoofer amplifier.
The iSub is well constructed, with audio seals everywhere, even on the power socket. The USB had to go and to maintain the air seal integrity, the USB cable would be re-purposed as the audio cable, re-wired for audio in via a 3.5mm stereo plug.
As far as reverse engineering went, I did not go as far as to re-create the full circuit diagram. It appears that the circuit board is multi-layered, so recreating the circuit would be a nightmare! I have found that the electronics in the iSub consists of two parts, one is the USB to audio section (UDA1321, USB to Audio IC + 8582C 2kbit – 256x8bit i2c EEPROM to store the audio settings for the 1321 chip) and the other is a plain and simple amplifier (TDA7256, 30W Amp IC).
Here’s the PCB.
So what I have done…
Soldered a short across the C-E of Q01 – this stops the iSub from being constantly muted, Q01 is actively driven by the USB audio chip, which I disabled by removing the little 3.3V regulator (top right in the picture above).
Then the audio trace from the USB audio circuitry was cut. The direct audio feed will be soldered to the cut track on the left.
Now, using the USB cable to feed the audio. As mentioned, this keeps the air-tight integrity of the iSub and saves drilling holes, etc to feed another cable. The cable diameter is a little large, but I was able to force fit a metal (for strength) 3.5mm stereo jack to the end, in place of the USB connector. The earth/ground is connected to the shield an I chose two random conductors for the L&R audio signals. For a Sub, though, you can just get away with using the Tip conductor of the plug, generally the very low frequency signals in stereo are pretty much the same.
On the PCB, I removed the small inductor array (L01) then fitted two 10k-ohm resistors to combine the L&R signals into one and then wired it to the left of the cut track.
It was then a case of a quick bench test, then re-assembling the iSub.
Final observations: The whole mod works well as an audio amplifier. BUT, I thought that the power amp circuitry may have had some Subwoofer filtering (given the number of electrolytic capacitors around the audio path), it appears not. The subwoofer profiling may have been done on the USB audio chip, or in the Mac OS when it was able to drive the iSub, so after the mod the iSub needs to be driven by a dedicated Subwoofer output from a audio processor or a sound card.
Safety tip : A good supply of ice helps with soldering iron burns.