Fi X4

About the Fi X4

The Don Garber Fi X4 is a unique, direct-coupled single-ended triode (SET) amplifier featuring two Tung-Sol 46 power tubes, two 6SF5 driver tubes, and a 5Y3 rectifier tube. It produces approximately 1.5 watts of undistorted output power, ideal for use with ultra-efficient loudspeakers. The amplifier's design is notable for its very short signal path, with zero passive components in the signal chain, and an X-shaped chassis layout that isolates the power supply, filter sections, and amplifier section. The direct coupling eliminates the need for DC blocking capacitors by raising the output tube's grid voltage to match the plate voltage of the driver tube. The Fi X4 uses Hammond 125E output transformers by default, with an option for upgraded Magnequest DS050 transformers at a higher price. It is compact and relatively lightweight, measuring about 7.5 inches tall by 9 inches wide, with tubes protruding sideways. The amplifier is known for its smooth, dynamic sound with rich midrange, good bass, and non-fatiguing treble, making it well suited for horn speakers or other high-efficiency designs. The power tubes run hot and the unit has no cover, so care is advised. The Fi X4 was Don Garber, the designer, passed away in 2017, and the amplifier is considered a classic among SET enthusiasts.