Headphone Amplifier 4 Channel Mono/Stereo Monitor Set (100-240V) HA400P Headphone Amplifier Sound amplifier Power - AliExpress
Mic 2U Headphone Amplifier 4 Channel Mono/Stereo Metal Stereo Headphone Amplifier with RCA/6.35mm/3.5mm Input Volume Control
Whirlwind Headphone - Interface, stereo line in, XLR mic in, level controls, 1/4" TRS and 3.5mm headphone jacks, latching mic switch, cough switch THS2 · AVGear
Amazon.com: Headset Buddy Line-Level Audio Adapter with Built in Attenuation for iPhone, Smartphones, Soundboards and Mixers (iPhone-Mic-Line) : Electronics
Ultra-compact 4-channel Stereo Headphone Amplifier Upgraded Mini Audio Amp with Mono & Stereo Switch Power Adapter Professional Mono/Stereo Audio Amp for Studio and Stage - Walmart.com
audio - Stereo line level to mono microphone level - Electrical Engineering Stack Exchange
Mono-to-Stereo 3.5mm 2-Conductor to 3-Cond Adapter
Radial USB-PRO DI for Laptops with Level Control / Mono Summing / Headphone Out
Switchable Stereo/Mono Headphones with Individual Volume Control Per Ear
4 Channel Mono/Stereo Audio Mixer w/LED Level Meter Ultra Low-Noise Line Mixing | eBay
Little Labs Monotor 2-channel Headphone Amplifier | Sweetwater
OWSOO HA800S Headphone Amplifier 8 Channels Mono/Stereo Metal Stereo Headphone Amps Audio Amp. Ultra-compact 8-Channel Audio Stereo Microphone AMP. with Power Adapter - Walmart.com
Williams AV HED 027 Heavy-Duty Folding Mono Headphones - Sound Productions
Amazon.com: Egghead Switchable Stereo Mono Over Ear Headphones 3.5mm (1/8") Plug - Pack of 10 (EGG-IAG-1003-SO-10) : Electronics
operational amplifier - How could I build a headphone stereo to mono converter? - Electrical Engineering Stack Exchange