hello all, this is my first post so please bear with me as I am no electrical wizard and would need a speakers for dummies solution to my problem.

I purchased the Saitek PZ09A - Notebook Subwoofer to complement the excellent top end switching unit with digital and analogue out to my amp for when I need to crank up to my 5.1 system speakers. Rather I was looking for are very localised enrichment of the sound and lower volume levels and on my travels. So I thought this Saitek sub woofer, which is the size of a compact disc, with a 3.5 mm and USB power connector was just the ticket has no batteries are required.

However it turns out that there is a fatal flaw because according to the retailer who contacted Saitek the design requires a common ground connector for the speaker output and the USB which obviously my HP laptop (TX2 1015ea) which is the latest model with optical and analogue out via 2 x 3.5 mm jacks doesn't have! So when I connect the speaker as a sub woofer of the main audio output via which on the speaker I get a large humming noise with no output.

So my question is can I purchase or cobble together a common ground could connector for this subwoofer 3.5 mm jack speaker and USB connector? Please remember that I am a complete novice when it comes to electrical matters.
If not does anybody know of an alternative relatively compact and portable subwoofer to complement and work with the speakers that come with most modern laptops? My search on the Internet has been fruitless so any pointing in the right direction would be appreciated.

thank you