Trilogy 931i Headphone Amplifier

The 931i is Trilogy's headphone amplifier — the 931, upgraded in 2019. The original brief was to keep the essential elements that made the 933 successful at a significantly lower cost. It is a fully discrete design with no generic integrated circuits, so Nic Poulson controls every crucial parameter of the active circuit and optimises it for this application alone. The small, light drivers in headphones sit very close to the ear and are ruthlessly revealing.

The audio circuit operates in pure single ended Class A with active constant current sources. Each channel has a single continuously biased output device, so the output is free from crossover distortion and the device is held at a constant temperature. An oversized linear power supply gives the active circuitry good operating headroom. Two stereo line inputs feed an audio grade ALPS volume control, and branded components are used throughout.

The output devices dissipate enough heat to need a heatsink. An extruded off-the-shelf part would have served; Trilogy uses a custom CNC machined one instead. Semiconductors are sensitive to mechanical vibration and their characteristics vary with temperature, so attaching the output devices to a high mass with a low resonant signature and large thermal inertia is worth the cost. Assembly is done entirely by hand in Trilogy's UK workshops. Standard finish is a soft natural silver, with lustrous paint finishes optional.

• Topology: fully discrete, no integrated circuits; pure single ended Class A with active constant current sources; one continuously biased output device per channel
• Output power: 800 mW into 60 ohms; 200 mW into 300 ohms
• Output impedance: 10 ohms or less
• Inputs: 2 stereo line, 4 RCA phono sockets; 50 kohms or greater
• Volume control: audio grade ALPS
• Gain (max): 18 dB
• Frequency response: 20 Hz – 20 kHz +/- 0.5 dB
• Distortion (THD): 0.05% or less, 10 mW into 300 ohms
• Signal to noise ratio: 85 dB A weighted or better
• Phase: phase correct (non inverting)
• Dimensions / weight: 140 x 237 x 50 mm / 1.7 kg; 16 W