In this video, Benchmark takes apart the HPA4 headphone amplifier and LA4 preamplifier to explain how the two models are built and what separates them. The Benchmark HPA4 and Benchmark LA4 share the same preamplifier platform, while the HPA4 adds Benchmark’s discrete THX-888 headphone amplifier.
Much of the video focuses on the relay-controlled analog signal path. Input selection, muting, gain adjustment, and volume attenuation are handled by high-precision relays rather than a conventional potentiometer. The 256-step attenuator adjusts level in precise 0.5 dB increments across a wide range, with separate circuits for the left and right channels.
FPGA-Controlled Volume Adjustment
Benchmark uses an FPGA to control the timing of the relays. This allows the circuit to use make-before-break switching, reducing clicks and pops in the audio while the volume changes. The physical clicking heard from the chassis comes from the relays themselves, not from the audio output.
The HPA4 includes separate attenuators for its headphone and line outputs. This allows both outputs to operate simultaneously at independent volume levels. The LA4 uses the same line-level architecture but omits the headphone amplifier and its dedicated attenuator board.
THX-888 Headphone Amplification
The HPA4’s discrete THX-888 amplifier uses feed-forward error correction to reduce distortion when driving demanding or inefficient headphones. Benchmark also includes protection relays that shut the amplifier down if the output is shorted, overloaded, or driven into clipping.
The video also covers the fully balanced signal path, dual-mono board layout, internal power conversion, input and output isolation, and bidirectional 12-volt trigger connections. It offers a useful look at the engineering behind two preamplifiers designed around precise analog level control and low-noise operation.