The Lux91 MonoMax

Mon, 02 Nov 2026 00:00:00 +0000

The Lux91 MonoMax.

In many ways the Lux91 is where all the concepts discussed in my article Some Thoughts about Single-Ended Valve Amplifiers come together, and it is their high point:

Direct-coupled cathode follower drive gives real improvements in both power output and overload recovery.
A single pentode gain stage gives an optimum harmonic profile.
A stiff, regulated power supply keeps rail impedance to a minimum.

In the terms of my position paper The Audio Heretick’s Progress, it is an illusion engine designed for audible fidelity, not for a set of FOM: figure of merit. A single number, such as THD+N, SINAD or jitter in picoseconds, used to sum up signal fidelity, with the claim that a better number means better sound. It relates to audible fidelity only at the extremes. The Audio Heretick's Progress: the FOM →.

The specific “technology modules” and the mechanical and transformer platform are long gone, but the principles behind the design remain fully valid. The design files I kept, or was able to recover after DIY HiFi Supply, are available for download: the original 2010 manual, the Active Bias System and the DHT heater supply schematics and PCB layouts, and the Film Capacitor Universal Power Supply manual.

For a 2026 refresh of this design, as an example, the 5687WB cathode follower could be replaced by a MOSFET such as ST’s STD4N90K5 (900 V, 3 A, very low capacitance), and a 6SN7 cascode could be used instead of the 310A pentode.

The Lux91 MonoMax was sold by DIY HiFi Supply in Hong Kong from 2010, as a kit and assembled. The circuit, the manual and the technical writing about it were mine, including what appeared under the company’s name. This article is put together from my 2010 manual, my answers for the 6moons review and my forum posts about the amplifier.

Lux91 on Decades of Musings on HiFi

The Lux91 MonoMax