The Logic Off Topic list

ok see if you can help me troubleshoot this.

i bought two of these cables:
<http://www.cablesforless.com/index.asp?PageAction=VIEWPROD&ProdID=2779>

"Our Custom Made 3.5mm Male to 2-Male 1/4 inch phono plugs audio  
cables are perfect for connecting your speakers and microphones.  
These cables feature Belden's 1800B digital XLR cable and Steren®  
precision machined connectors. To ensure maximum signal integrity and  
protection from RF and electromagnetic interference the cable also  
features a Beldfoil® shield and fully shielded connectors.  
Professional termination using high grade solder ensures optimal  
signal transmission. Designed for optimal sound quality, Belden's  
performance speaks for itself. Compare Belden's specifications  
(below) with those of other digital XLR cables and see for yourself  
how Belden cable is unmatched. Belden Brilliance® teamed with Steren  
Connectors brings audio to life!"

sounds good, right? they both exhibit the following symptoms.

i'm using it in the headphone jack of my powerbook g4 15" and going  
out to a little mixer so i can hear system audio, itunes, etc.  
through my system. this longer cable is replacing a monster cable of  
the same kind (but shorter).

i'm getting absolutely HORRIBLE buzz when i plug it into the  
headphone port. if i jiggle it a bit i can get it to behave but only  
if it sits *just* right.

on a crt imac and an older titanium powerbook G4 the issue is much  
better. i still hear a line hum. i wouldn't want to record like this,  
but it's not a nightmare.

on an ipod plugged into the power adapter it works beautifully.  
however, i CAN get it to buzz horribly if i jiggle the plug as i  
insert/remove it.

eyeballing it with the monster cable tip, it looks exactly the same.

so on one hand, the monster cable works great in all uses.
the cablesforless cable works terribly on the powerbook but nowhere  
else.

now here's a clue: the powerbook G4 has combination analog/digital  
input and analog/digital output jacks. i haven't been able to repeat  
it reliably, but the cablesforless cable seems to be able to trigger  
the digital interface part, as evidenced by a red light shooting out  
of the port.

unplugging the powerbook totally removes the buzz.

i get the buzz when this cable is in EITHER the input or output jack.
tried a different power supply in a different jack in the same  
room... same results both ports (slightly different tone to the buzz)

if i remove the little mixer and plug the 1/4" cable ends into two  
inputs on a practice amp, i get the same issue.

finally, i got another digital audio powerbook g4 15" and the  
connection seems to be slightly more reliable, but still problematic.



anyone have a good handle on what might be happening?