Benchmark ADC1 USB
A half-rack box that converts analog signal to 24-bit/192 kHz digital and, with one small addition, sends it straight to a laptop over USB 1.1.
Overview
The ADC1 USB is Benchmark's two-channel analog-to-digital converter. It is a half-rack reference unit that takes balanced analog in and gives back digital at up to 24-bit/192 kHz. It was an upgrade to the original ADC1, and the change was one thing: USB. Pro-Audio-Review (July 2008, p.23) described it as a 2-channel 24-bit 192kHz A/D converter. The price was $1,775, and the unit was made in the USA.
Benchmark stacked the headline numbers on purpose. Distortion is rated at an ultra-low 0.0003 percent, and dynamic range is quoted at greater than 120 dB A-weighted. That is enough for an engineer to trust the box to capture what the microphones heard without adding its own signature. called the converter accurate, with excellent imaging and bass control, and noted that in direct comparison the USB and optical outputs sounded the same - no degradation, no sputtering, clicks, or pops.
The USB implementation is deliberately unglamorous. It runs USB 1.1, which streams up to 96 kHz/24-bit natively without special drivers, and works on both Windows and Mac OS X. Benchmark's reasoning was to sidestep the driver issues that can dog USB 2.0 or FireWire, and to lean on a specification with near-universal compatibility and more than enough bandwidth for the job. In practice you plug it into a laptop and record.
Beyond the USB path, the converter handles the full 24-bit range at sample rates from 32 to 192 kHz. The back panel is generous for a half-rack: balanced XLR left and right analog inputs, single-wire 176 kHz and 192 kHz digital output, optical digital output up to 192 kHz, BNC and S/PDIF outputs to 192 kHz, and an auxiliary BNC that either parallels or delivers an alternate sample rate. The main, auxiliary, and USB outputs can each run at an independent sample rate simultaneously. There is also SMUX2 and SMUX4 output for two 24-bit channels at up to 192 kHz over an Alesis ADAT optical port, plus a BNC sync clock input and a BNC clock output.
The front panel is packed. Each channel gets a first-stage gain switch offering 0, +10, or +20 dB ranges, a ten-turn input trimmer pot, a variable/fixed gain switch, and a large 41-step detented rotary gain control. Overall variable input gain spans -6 to +39 dB, so the unit can interface directly with a wide range of devices. Levels are read on a nine-segment, dual-range LED meter with peak-hold, switchable between -6 dB and -1 dB steps. A separate nine-segment LED display indicates the selected sampling frequency. Word length is fixed at 24 bits on the main output but adjustable on the auxiliary output.
Clocking is handled by Benchmark's UltraLock circuit, the jitter-immune system the company built its converter reputation on. It is designed to keep interface jitter and poor external clock performance from other sources out of the conversion. The unit ships in silver or professional black, with or without rack ears, and includes a BNC-to-coaxial adapter, an IEC power cord, and an informative manual.
Reviewers wished for two things: an off/on switch on the front panel, and a sample-rate display as legible as the separately-lit, legended LEDs on the Lavry converters. The ADC1 USB's LEDs do show sample rate for all the digital outputs and the reference clock input, but the reviewer found it less obvious than the Lavry approach.
The USB 1.1 choice dates the unit to its era. The design spends the money on clean conversion and jitter rejection and keeps the interface simple, with no driver to fight.
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