ZMF Airmid

ZMF Airmid

OTL tube amplifier without output transformers – the second collaboration between ZMF and L0rdGwyn

After the Aegis, the Airmid is ZMF’s second collaboration with Keenan McKnight-Gwyn, known in the headphone community as L0rdGwyn. It is based on a hand-built OTL amplifier named after the Gaelic goddess of healing, of which only a handful were ever made. For the production version ZMF adapted the circuit, with a simplified input stage, power pentodes in the output stage and a switch for the feedback. The Airmid is built by Cayin.

OTL stands for output transformerless: the Airmid works without output transformers, the output tubes drive the headphones directly. The power supply with tube rectifier is taken from the Aegis. The input uses a gyrator-loaded 6SN7; according to ZMF, this active load keeps distortion low and detail high without a cathode bypass capacitor. Input and output stages are directly coupled, with no coupling capacitor in between. In the output stage, two power pentodes work as cathode followers loaded by a current sink, so the headphones are driven from a low impedance. The input stage heaters run on DC for a low noise floor, and a delayed start-up protects the tubes.

A switch on the front sets the feedback in three steps and thereby changes the output impedance: 73 Ω without feedback, 38 Ω at medium, 21 Ω at high. ZMF describes the settings like this: no feedback for maximum dynamics and punch, medium for a balance of dynamics and refinement, high for the smoothest sound with the lowest distortion and a firmer grip on lower-impedance headphones.

The Airmid also invites tube rolling. Suitable rectifiers are GZ34, 5R4, 5U4, GZ37 and 274B, but not low-current types such as the 5Y3. The input takes the 6SN7, or a 12AU7 or 6J5 with an adapter. The output stage uses high-current power pentodes: EL34, EL37, KT66, KT77, KT88, 6550 or 6L6; the bias current is fixed at 60 mA.

Headphones connect via 4-pin XLR, 6.35 mm or 4.4 mm; a switch selects the active socket. The source connects via a pair of RCA inputs. Two preamp outputs (RCA) turn the Airmid into a tube preamplifier for a loudspeaker system or another amplifier; a selector switches between headphones, preamp 1 and preamp 2.

An OTL amplifier shows its strengths above all with high-impedance dynamic headphones, such as the 300 Ω ZMF Atrium or Auteur Classic. For lower-impedance headphones ZMF recommends the high feedback setting, and for demanding planars an additional impedance adapter (ZMF names the Schiit Imp), which makes a 20 Ω planar look like a 160 Ω load to the Airmid.

Who is the Airmid for?

For anyone looking for a tube amplifier without output transformers for high-impedance dynamic headphones who also wants to use it as a tube preamplifier for their system. If you mainly listen to low-impedance planars, the transformer-coupled ZMF Aegis is the better choice.

Technical specifications