Benchmark MPA1
A half-rack, transformerless two-channel mic preamp built to the same obsessive standard as Benchmark's reference converters.
Overview
The MPA1 is Benchmark's two-channel microphone preamplifier. It uses the same "system 1" half-rack frame as the company's DAC1 digital-to-analog converter. That tells you the intent: a studio tool meant to sit next to reference-grade conversion, not a colored "money channel" preamp. Benchmark's own advertising described it as a two-channel mic preamp designed to Benchmark standards of "transparency, ultra-low distortion, and ultra-low noise performance," and pitched it at stereo pairs, ribbon microphones, and any job where purity and precision are the goal. Marketing copy is not a verdict, but the topology backs it up. The MPA1 is a transformerless design built around a discrete Class A amplifier.
The spec sheet reads like a measurement contest. Frequency response is quoted at 10 Hz to 200 kHz, ±0.5 dB — bandwidth that extends well past anything the ear can hear, which is the point: what happens inside the audio band should be uneventful. THD+N is stated at <0.0005%, equivalent input noise at -128 dBu, and maximum output level at +26 dBu. Output impedance is a low 50 Ω. On paper this is a preamp that will not impose its own signature on a source, and the period review coverage agreed with that characterization in practice rather than just on the bench.
Mix (August 2009, p.50) reviewed the MPA1 and came away impressed by its transparency, describing acoustic guitar and vocal takes that were detailed and three-dimensional without sounding sterile. Reviewer Matt Bishop noted the unit's tone was full and balanced from top to bottom, and that while it did not color the sound, it avoided the clinical feel associated with many "clean" preamplifiers. That is the hardest trick for a transparent design to pull off, and it is the reason the MPA1 has a following among engineers who want accuracy rather than character.
Key Features
Stepped-Resistor Gain Control, 0–74 dB
The MPA1's most distinctive feature is its gain control. Instead of a continuous potentiometer, it uses a stepped-resistor design with detented positions, giving 74 dB of gain in 2 dB increments. The knob is organized in four stages - +0, +24, +48 and +74 dB - and the gain range is switched automatically by relays when the control passes above or below its first or last position, as Mix (November 2008) noted in its AES coverage. Benchmark's advertising called the arrangement a "virtual 38-position switch" covering the full 0-74 dB span. The payoff is recall. An engineer can return to an exact gain setting and know it will match, which matters for stereo pairs and repeatable sessions. Mix (August 2009, p.50) called the recallable detent settings a great feature, particularly for a preamp so adept at stereo miking, and found the precision gain steps useful for pushing a vocal right up to the edge of the converter.
The fact sheet quotes a gain range of +20 dB to +70 dB, while the manufacturer's own documentation and period coverage describe 0-74 dB in 2 dB steps. The stepped-control figure is the one the documentation supports in detail.
Transformerless Discrete Class A Topology
Inside, the MPA1 is a transformerless design using a discrete Class A amplifier. There is no input or output transformer to saturate or color the signal, which fits the ultra-low-distortion, ultra-low-noise brief. Input impedance is 1.5 kΩ, a sensible load for the microphone pairs the preamp was aimed at. Per Mix (August 2009, p.50), it picked up every nuance of an acoustic guitar without cluttering, and handled background vocals with a spectacular top end and well-behaved low mids. The reviewer's summary was blunt: amazing sonic performance, flexible design. The caveat was that true-to-life transparency may not suit buyers looking for a preamp that adds color.
+48V Phantom Power with Multistage Protection
Phantom power is +48V. The phantom section includes a multistage protection circuit designed to absorb the surges caused by shorts and by plugging or unplugging microphones with phantom engaged. It is unglamorous engineering, the kind that separates a working tool from a spec-sheet hero. Mix (August 2009, p.50) singled the protection circuit out as one of the design's innovative twists.
40 Hz Highpass Filter and Polarity Invert
Controls beyond gain and phantom include polarity inversion and a highpass filter set at 40 Hz. That is an unusually low corner for a mic preamp, and a deliberate one. Mix (August 2009, p.50) found the 40 Hz filter useful for clearing room and mechanical noise without cutting the low-end extension of sources that actually have bottom. The same review noted that the half-rack form factor, with unit couplers and blanks, allows a variety of rackmount configurations, handy for field recording kits as well as studio racks.
HPA2 Headphone Amplifier and Front-Panel Output
A headphone output with its own volume control sits on the front panel. Mix (November 2008) identified the headphone stage as Benchmark's signature HPA2 amplifier. That makes the MPA1 usable as a standalone monitoring point when tracking away from the console.
Half-Rack Chassis, 10 lbs
The MPA1 measures 1.75" H × 19" W × 10" D and weighs 10 lbs. That is heavy for a half-rack unit, and it matches the heavy, tightly assembled casing Mix (August 2009, p.50) described. Power is 100-240V AC at 50/60 Hz, so the supply is universal. The unit shares the DAC1's frame design, which helps anyone building a matching Benchmark rack.
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