iSub 2000 mod

apple iSub
The beautiful iSub 2000

For physical dismantling of the iSub slowly remove the rubber feet by lodging a fingernail or razor blade under the footie and lifting up gently, 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, it is truly a beautiful speaker. All the following is used at your own peril.

I picked the iSub up from my Brother-in-law, so kudos to him for affording me the projects main subject. Apple no longer supports the iSub. Works under windows to a degree, but who want’s to go there?

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.

It appears that the circuit board is multi-layered, so recreating the circuit would be problematic. 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.

iSub 2000 icb

So what to do to convert USB to standard stereo…

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).

Speaker attachment

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.

 

Solder on to cut tracks

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.

Wrap things up.

 

It was then a case of a quick bench test, then re-assembling the iSub.

Final observations: The mod works as an audio amp. But, 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.

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