operational amplifier - Line level audio output from headphones level output - Electrical Engineering Stack Exchange
AU-D160 - Professional USB Digital Audio Converter (DAC) with Integrated Headphone Amplifier - CYP Europe
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Amazon.com: Headset Buddy Line-Level Audio Adapter with Built in Attenuation for iPhone, Smartphones, Soundboards and Mixers (iPhone-Mic-Line) : Electronics
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LiNKFOR USB to SPDIF Coaxial RCA and 3.5mm Headphone Jack Converter – LiNKFOR Store
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Amazon.com: PAC SNI-35 Variable LOC Line Out Converter 1.2x6.25x5.9 : Everything Else
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Scosche LOC90 Speaker to RCA Line Output Converter 2-channel speaker-to-RCA line output converter with remote level control at Crutchfield
Kicker 46KISLOC2 2-Channel Line Output Converter with Remote Turn-On Wire | World Wide Stereo
capacitor - Stereo line-level converter: Odd grounding? - Electrical Engineering Stack Exchange
