The iSub Hack


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.

Have fun!

Safety tip : A good supply of ice helps with soldering iron burns.

31 thoughts on “The iSub Hack

  1. A very nice job of analyzing the analog/power side of the circuit board, and of describing and illustrating a useful mod. Thank you! I followed your procedure with a few minor changes:

    – I didn’t find it necessary to remove the #2950 3.3-volt regulator that powers the digital side of the circuit board;

    – Since the analog side is a monophonic amp+speaker, instead of using a stereo jack and a resistor network to mix left and right input signals, I installed a single RCA jack in the hole left by the removed USB cable; and

    – To allow the volume to be controlled independently of the signal feeding the separate “main” left and right speakers, I installed a 100K audio-taper pot in a hole drilled in the side of the iSub’s base next to the white “Harman/kardon” logo, and wired it to the new RCA jack.

    I’m not entirely convinced that the three translucent gaskets installed where adjoining parts of the iSub meet (including the one you mention on the power socket) are there to provide an air-tight seal, since the interior of the iSub’s base is open to the atmosphere via the four large holes in the bottom of the Plexiglas dome and the single hole in its top. I’m guessing the gaskets are there to dampen any noise from rattling edges of abutting Plexiglas pieces when the whole unit is being vibrated at various frequencies by the speaker driver.

    I agree with you that the modified iSub needs to be driven by a signal that has already had the mid- and high frequencies filtered out. Right now I’m using an electronic crossover (a tiny Samson “S-xover”) and sending the mid/high stuff through a small 2-channel amp to a pair of bookshelf speakers. I’m still experimenting, but it’s already clear that the optimum crossover frequency setting depends on the type of music.

    One problem the iSub shares with most inexpensive subwoofers is that deep maie voices (e.g., announcers on music radio stations) sound boomy if the sub’s frequency cut-off is set too high. But unless the left and right “main” speakers (the bookshelf speakers in my case) extend well into the mid-bass, a low crossover setting leaves a mid-bass “hole”. Since single voices on the radio are almost purely monophonic (the left and right signals are nearly identical), I’m going to try to get around this problem by adding a resistor network or 1:1 audio transformer to feed the DIFFERENCE between the left and right channels to the iSub.

    • Thanks for the input. I took out the 3.3V reg just to kill the USB circuit as a precautionary measure. I do think they are air tight gaskets, that’s why poorly designed subs have the “doof doof” sound the “oof” coming from high frequencies or whistles as the air is sucked through small gaps, holes or joints. For such a small device, this might not be a problem. I’m glad the mod description was of use!

  2. I’ve looked at this article for a few weeks now, and this will be my weekend project. Now that you have used the hacked iSub for a few months, do you have any additional insights on your hack?

    • Actually, I haven’t been using the modified iSub since the conversion, but one thing I was thinking of doing was putting in a sub-woofer filter sometime in the future. Good luck with the projext!

      • Thanks for posting this simple hack–well, I say simple because you did all of the work. I just plugged it into my iPhone to test and it works…albeit bassy.

        I purchased a $14 “Sub 100hz Low Pass Subwoofer Crossover” off of eBay to ensure the sub only handles bass frequencies. When it arrives, this beautiful subwoofer will return to prominence in my setup.

  3. Hello, thanks for the explanation on how the sbwofer works.
    My subwofer (not satellite) makes huming (strange continuisly noise)
    Do you have an idea where it could comes from ?
    Thanks in advance

    • Not really, likely causes – faulty signal cable from computer to sub woofer or a fault in the power supply section of the subwoofer, good luck!

  4. Hi,
    Great post, I am hoping to get a thrift shop iSub working the way you described. My first problem is that it didn’t have a power supply. Could anyone here help me with the pinout for the power supply? I understand it requires 15V up to 2A – which I am trying to track down. But I am not sure whats what with the three connectors.

    Thanks for any info!

    • James,

      Just measured mine. The third, small diameter pin does nothing. If you look into the plug on the cable with the notch on the outer casing toward the bottom the + is on the right, – on the left, 15VDC @ 2A.
      The original power supply died on mine and I just replaced it with a switch-mode wall transformer spliced into the original power cable.

      – Rob.

  5. Hi, I stumbled onto this page while searching for mods to my 2.1 soundsticks/sub.

    I’m a little confused as to which version I have, as my isub is USB only, but has 2 RCA outputs that hook up the the 2 soundsticks. My sub does NOT have an audio in, only USB. I guess my question is if I do this mod, will it effect the audio that is being sent to the output for the sound sticks?? I’m thinking the crossover has to be post the USB to audio converter, only because I doubt the DA has 3 or 4 outputs, if you follow me.
    thanks for any insight..


    • Sean,

      You have a later version. My iSub was a standalone sub-woofer, with no soundsticks.
      The circuitry in yours may be completely different, I would not attempt this hack relying on the information I’ve given here!

      – Rob

  6. I had given up hope when I searched for such a hack years ago. Sitting in my garage… waiting.
    I gotta say, ROCK ON… took me a couple tries (ok 6-7) to get the wire to solder itself to the cut trace wire and I had to buy resistors for the first time in decades, but hey it works! Sorry for any astonishment, but I haven’t done anything like this in years (with any success).
    I’ve used a Y splitter to connect my existing speakers & the iSub and it all worked fine. Had to increase the sound output to compensate for the splitting. After a very short while I noticed static in the speakers. When I unplugged the iSub it cleared. Later I noticed (when no music was playing) that there is lots of static.
    When I wiggle the power cord it comes/goes.
    Is this a problem with age/lack of use? Presumably there’s nothing that can fix this except finding the sweet spot for the power cord to sit.
    Thanks for posting the hack.

    • You could have a dirty power connector, or more likely a dry solder joint in that area of the board where the power connector is.
      The power supply is a weak point in the design, mine was replaced at some stage.
      You could open her up again and just retouch all the solder joints and/or give the connector a clean with come connector cleaner.

      I’m glad the hack was useful for you!

  7. First, I don’t know if you’re still following this post at all, but I found it fascinating.

    Second, it’s been a long time since I tore into anything of this kind, so there will be an underlying ignorance in my questions.

    What I’d like to do is bypass the entire amp/DAC sections of the iSub and run an external DAC/ 20 watt amp (Topping) directly to the speaker connections–either using your 3.5 mm mod of the USB cable or some other solution that keeps the air tightness of the unit intact. I’d keep the circuitry in the unit in order to not change the acoustical properties of the enclosure (thanks, by the way for the info on air tight–I found the same to be true with Pro speakers).

    This way I can control the iSub volume from the external amp and rely on the external DAC for a digital/analogue conversion that I like.

    I assume that this process also removes any crossover function in the iSub and that leaves me with these questions.

    1. Do I need the crossover in the circuitry to keep the input in line with the rated response of the iSub? And if so, what kind/specs do you recommend?

    2. Is there any reason you would note that I should not do the above?

    My overall goal is to pair this with apple Pro speakers I’ve modified to accept 16 awg wire. (Seems to make a decided improvement).


    Martin Miller


    • Far from me to recommend anything but.. running the speaker directly is a workable idea. I don’t thing you should be too worried about the crossover used in the design, I think it would work if you used an external sub-woofer driver amp.

  8. Great Job!!!!!!!!!!!!! and very impressive.

    Question i do have the 1st generation soundsticks, which is the subwoofer and two speaker sticks that connect to the back of the subwoofer. The unit only has a USB port, will this conversation work for this system so i can use my ipod or ipad. My daughter use these for her computer.

    PLEASE HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  9. Hi. My sound sticks 2 pcb component U20 got burnt. I want to know it’s no. Or specification so that it can be replaced.

    • Sorry, I don’t know anything about the Sound Sticks 2. It uses different circuitry as U20 doesn’t exist in my iSub.

  10. Interesting but the electronics is beyond me.
    Just wondering if I get a 3.5 mm to USB (female) adapter, could I then use this thing
    hooked up to the 3.5 mm port of a laptop? Thank you for your help. I would think that there’s a chance since the power is there (thru the AC adapter). The signal will be fed out of the 3.5 mm audio port of the laptop into the iSub USB input.

  11. hi i just got my hands on one of these just the subwoffer but not sure what model it is (old or new) it has a 3 hole power jack and a 3.5 line runing out of it (its the audio input) and then a short cable that has what looks like a s-video port and a white rca port on the same cable connecter peice. i looked it up on youtube and all the videos about this when i click to play them show a static screen where it has been removed and i looked at like 6-7 different videos same thing happens. My question here is What is that all about what?

  12. Hi there,

    Very interesting. I would like to ask if this hack will make iSub work well on OS X Mountain Lion and Windows 10?

  13. Hey,

    I have the one with 2RCA and a usb out HK soundsticks!

    I’m trying to look for a DAC or something without opening the sub

    Basically I want to use with 3.5mm output

    Please recommend a simple and feasible solution!

  14. Some body of ere has a diagram for connect the DIN 7 pins connector from the Sound stick iii board to satellite speaker. I not have the original speaker and i can’t install another’s because the audio always is mute. How I can control the capacitive touch signals? Thanks..

  15. Is the L01 thing you removed the same as the L1? I’m attempting this mod currently… I have a wire attached to the left of the break, and am wondering where I need to attach it to. There are 4 leads coming out of the USB “connector” do I wire individually to two of those? Or something else?

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