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Amazon.com: Dual OPA1622 SoundPlus Audio Operational Amplifier with a Low Noise Density of 2.8nV/Hz and a Low THD+N of -119.2dB at 1kHz,a High Output Drive of +145mA/-130mA for Direct Drive Headphone :
Sjöström Audio - QRV01 Headphone amp
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Gold Mini Class A Headphone Amplifier Hi-Fi Desktop Home Stereo Headset Amp | eBay
QSC GX7 Stereo Power Amplifier | Guitar Center
Opamp Rolling, Does It Work? | Audio Science Review (ASR) Forum