Control Pod BYPASS Plug for Bose Companion 3 Series I & II for ROUND 9-pin port

Then, connect your computer's audio output via a cable with a 1/8" (3.5mm) male stereo TRS (tip-ring-sleeve) plug to the input jack on the Bose amp (labeled "From Computer"). In addition to the Bypass Plug being a variable in tuning the audio quality, you can also further tune by flipping the orientation of the 2-pin power cable plug at the rear of the amplifier.

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