introducing airmid
A refined Output Transformerless amplifier built around ZMF’s Airmid circuit topology, featuring an Aegis tube-rectified power supply, gyrator-loaded input stage, and actively loaded power pentode output.
The Airmid is the second collaboration between Keenan McKnight-Gwyn and ZMF, Aegis being the first. Known as L0rdGwyn on Head-Fi, Keenan is a prolific DIY tube amplifier designer whose legendary thread has become a cornerstone of the Head-Fi DIY community. His builds, spanning SET amplifiers, OTLs, DACs, and phono stages, are renowned for their engineering rigor and sonic excellence.
Named after the Gaelic goddess of healing, the original Airmid was a hand-built OTL. Only six were ever made, but the design garnered praise for its outstanding detail retrieval, clarity, and spacious presentation. Community members described it as “beautiful and fast,” with a sound that “left me speechless.”
The ZMF Airmid aims to bring a refined, production version of this celebrated circuit to a wider audience for the first time. The production design adapts the original topology with a simplified input stage, power pentode outputs, and a three-position feedback switch, making L0rdGwyn’s acclaimed OTL sound accessible beyond the five originals.
THE CIRCUIT
Aegis tube-rectified power supply / proven ZMF power supply design with wide rectifier compatibility
Gyrator-loaded 6SN7 input stage / active loading for low distortion and high detail without cathode bypass
Directly coupled input to output / no coupling capacitor between stages for purest signal transfer
Actively loaded power pentode output / current sink loaded cathode follower for low impedance drive
Input Stage DC Heater Supply / ultra-low noise floor for the input stage
Time-delayed startup protection / safe power-on sequencing for tube longevity
Optional global negative feedback / three settings to match your headphones and preferences
Preamp outputs / use the Airmid as a tube preamp for your speaker system or other amplifiers
THE SOUND
Airmid opens up the music around you. Its presentation is spacious and diffuse. Instruments and voices spread out naturally rather than crowding together, so you hear the room as much as the performance. That sense of space never comes at the expense of weight, as notes land with real impact and clear definition, and dynamic swings feel alive and immediate.
Active loading keeps distortion low, which lets detail come through cleanly at any volume. Fine textures, decays, and the small cues that make a recording feel real are all there, resolved with precision but never pushed forward or made fatiguing.
No two listeners are alike, so Airmid lets you choose. Set the feedback switch on High to lower output impedance from 73Ω to 21Ω for the lowest-distortion sound and a firmer grip on lower-impedance headphones. For demanding planars, add the Schiit Imp for $99. It makes a 20Ω planar look like a 160Ω load to Airmid, so the amp drives it easily and its tube sound stays intact. Your sound, your way.
