Long USB cable

Hi everybody,
I need a 25' to 30' USB cable so I can connect my Imac to my main rig.
Please let me know which cable would you recommend.
Thanks in advance.
I think usb only goes 15 feet.
That is correct, 15' is the max. There are repeaters to extend this, but I'm not sure about the effect on sound quality.
If your imac has optical out of its mini headphone jack, get a good glass optical cable. Talk to underwoodwally.
I was told that Wire World make 23' long USB cable.Also,Transparent
Audio makes 30' USB. Has anybody tried any of these.
I don't know what Wire World makes in USB but I do know that they are one of the 2-3 companies in Audio that I would trust anything they make.
It may go that long if powered.
Post removed 
USB requirements are that the cable be only 26 nSec long, electrically. The reason they give is so that the reflections have enough time to die down. That limits the length to around 15'. We have measured some cables that are a bit over the limit, at 15'. Most are very close to it.

Use longer at your own risk.
Stay with the USB . I tried the mini headphone jack with my I mac and found the sound inferior .
bluejeanscable.com offers hdmi cable to 25 feet.
What does HDMI have to do with USB?
Please excuse my screw up...I had a brain freeze and mixed up USB with HDMI!
I use a 30 meter USB cable from Opticis. They make shorter lengths too. The cable converts USB to fiber optic and then back to USB. Works like a charm. I also use an Opticis 30 meter fiber optic HDMI cable.
I would recommend that you avoid a USB cable over 3.5M long. Do you have a wireless LAN in your home? If so, use a Logitech Squeezebox or Sonos to stream music from yopur iMac server to your system via wi-fi.
From the source: http://www.usb.org/developers/usbfaq#cab1
To Br3098's point, I agree with avoiding "regular" copper wire UBS cables that are more than 3.5m. A longer copper cable would not be within the USB spec and would likely not give you a signal or cause dropouts. But if you use something like the Opticis fiber-optic cable you should be able to go longer than 3.5m meters with no problem. The USB spec requires a limited length wire cable. But Opticis (and other companies that do fiber optic, etc.) have found a way around this with their implementations. Like I said, I have been using a 30m USB cable and a 30m HDMI cable with no problem and no decrease in performance for several years. Wireless is another equally attractive option, but obviously requires a different (maybe easier) implementation and hardware; it depends on how simple you want to go and what best meets your situation.