Store demo unit with minor cosmetic marks.

The Warm Audio WA-MPX is a single-channel tube microphone preamplifier that reproduces the AMPEX 351-style preamp, the tape-era tube preamp behind countless classic recordings from Elvis Presley to Nirvana. With three vacuum tubes, custom CineMag USA transformers, and up to 90 dB of gain through a 300V circuit, the WA-MPX delivers everything from pristine clean headroom to harmonically rich tube saturation with selectable tape emulation.

Tube Circuit and Gain Structure

The WA-MPX runs two 12AX7 tubes and one 12AU7 through a hand-wired 300V signal path. In normal operation, the first 12AX7 provides 70 dB of clean tube gain. Engaging the High Gain switch brings the second triode into the circuit, boosting available gain to 90 dB for low-output passive ribbon and dynamic microphones. The Tone switch toggles the CineMag input transformer impedance between 600 and 150 Ohms, altering the frequency response and harmonic interaction between the microphone and preamp.

Tape Saturation and Filters

The Tape Saturation switch engages a discrete analog circuit that adds harmonic distortion, gentle compression, and non-linear frequency response to emulate the sound of analog tape machines running at 15 IPS. An 80 Hz high-pass filter removes low-end rumble before the gain stage, and a 2 kHz low-pass filter rolls off upper-mids and highs for a focused, lo-fi character when creative tone shaping is the goal.

Specifications

• Type: Single-channel tube mic/line/instrument preamplifier
• Topology: AMPEX 351-style, fully discrete, hand-wired
• Tubes: 2x 12AX7, 1x 12AU7
• Transformers: CineMag USA (custom large-core, input and output)
• Gain: 90 dB (with High Gain engaged)
• Circuit voltage: 300V
• Filters: 80 Hz high-pass, 2 kHz low-pass
• Tape saturation: Selectable, emulates 15 IPS reel-to-reel
• Controls: Input gain, output level, input select, 48V, polarity, tone, high gain, HPF, LPF, tape saturation
• Inputs: XLR mic, TRS line, TS instrument
• Outputs: XLR, TRS
• Metering: VU meter
• Power: 115/230V, IEC
• Rack size: 2U, 19"
• Dimensions: 483 x 89 x 222 mm (19 x 3.5 x 8.7")

In the Box

• Warm Audio WA-MPX
• IEC power cable