Efficient Pocket Headphone Amplifier by OPA2227P - EEWeb
RJM Audio - The Sapphire Headphone Amplifier
Rod Rain Audio DA10 BT5.1 Bluetooth DAC Headphone Amplifier Muses02 Op Amp pe66 | eBay
Single Rail Op Amp Power Supply
Amazon.com: Small Stereo Audio Preamplifier, Headphone Amplifier,Dual Op Amp Headphone Amplifier with 3 Output Ports,for Home Theater,Stereo Speakers : Electronics