Airmid OTL Amp

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.

Spacious. Dynamic. Detailed.
A spacious, diffuse sense of space with impact and definition, low distortion from active loading, and a highly detailed, resolving presentation.

The Airmid was created by Keenan McKnight-Gwyn, known as L0rdGwyn on Head-Fi, 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 of which only six were ever made, praised 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 is a collaboration between L0rdGwyn and ZMF Headphones 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 original builds.

Walnut Faceplate, Foundry Copper Vein Chassis and Covers, with Aged Copper Knob, Accent Plate and Power Button. Coordinates perfectly with the Aegis!

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

Three-Position Feedback Switch
Off: no feedback, 73Ω output impedance, maximum dynamics and punch.
Moderate: medium feedback, 38Ω output impedance, balanced dynamics and refinement.
High: high feedback, 21Ω output impedance, smoothest sound and lowest distortion.

Off gives you the most punch and life. High lowers output impedance from 73Ω to 21Ω for the smoothest, 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.

Dimensions: 14.25" W x 12.25" D x 8" H. 29.5 pounds.

The stock tube set is included with every Airmid.

Optional add-on tube set: "select" Vintage 5U4G, Vintage 6SN7 and TAD KT-88 Black Plate, $200 (US and Canada only).

Rectifier - GZ34, 5R4, 5U4, GZ37, 274B. Many others compatible (not low-current types, e.g. 5Y3).
Input - 6SN7; 12AU7 and 6J5 with adapter. Additional input options to be confirmed.
Output - EL34, EL37, KT66, KT77, KT88, 6550, 6L6. High-current power pentodes, bias current fixed at 60mA.

Tube compatibility list: Click here!