The Locomotive Audio Model 40AM-2 is a two-channel, transformer-coupled octal tube microphone preamp, instrument DI and line-level colour processor. Inspired by 1940s American broadcast designs, it combines two independent 6SJ7-based tube channels with five-position Input and Response controls for everything from warm, weighty tracking to assertive tube-and-transformer saturation.
 
The Locomotive Audio Model 40AM-2 brings the bold, dense and naturally compressed character of early American tube amplification into a flexible two-channel 2U studio processor. Its circuit takes inspiration from classic RCA BA-11A and Gates SA-70 designs, but is not a simple reproduction. Locomotive adds independent mic, line and DI operation, selectable transformer-loading configurations, variable tonal response, high-pass filtering and practical tracking functions—turning the unit into both a serious recording front end and a powerful analogue colour processor.
Each channel has its own pair of octal 6SJ7 tubes. The first tube operates as a pentode, while the second is configured as a triode. Together, they deliver up to 52 dB of gain per channel, combining strong low-mid authority with broad harmonic density, softened transients and a physical sense of scale. The 40AM-2 is designed for engineers who do not want an invisible preamp: it is for sources that benefit from more body, thicker centre image, fuller tone and the controlled compression that comes from driving a well-designed tube-and-transformer circuit.
 
Locomotive winds the premium transformers in-house. MuMetal-shielded input transformers help minimise hum sensitivity, while nickel-and-steel output transformers provide the output stage’s weight, tonal density and robust drive capability. The four tubes and transformer-coupled architecture give both channels the same vintage-inspired signature, allowing them to be used independently in dual-mono operation or together on stereo sources.
 
The five-position Input selector allows each channel to be matched to a microphone, line source or instrument. It provides 500 Ω and 1.8 kΩ microphone configurations, Line Low and Line High settings, plus a high-impedance 1.6 MΩ DI option. The microphone 500 Ω setting and the Line Low setting each add approximately 4.5 dB of additional gain. These lower-level configurations are useful for low-output microphones, ribbon microphones, quieter sound sources and creative overdrive; the other settings provide different transformer loading and gain relationships for cleaner or more open operation.
 
A separate five-position Response control modifies the frequency response and harmonic footprint of each channel. It makes the 40AM-2 considerably more adaptable than a fixed vintage-style preamp: choose a response that supports the source’s body, midrange articulation and top-end openness, then refine the amount of tube colour through the Gain and Level controls. This gives engineers a direct way to use the 40AM-2 as a source-matching tool rather than treating its vintage sound as one unchangeable character.
 
The independent Gain and Level controls are the primary drive controls. Moderate Gain delivers large-scale, warm tube amplification with useful detail. Raising Gain pushes the first 6SJ7 stage and the transformers more strongly, increasing saturation, harmonic complexity and natural dynamic compression. The Level control lets you compensate at the output, so you can choose the desired amount of internal tube drive without simply making the next device in the chain louder.
The 40AM-2 includes dedicated XLR connections for microphones and line-level sources. Separate line inputs make it safe to route converter outputs, DAW stems, virtual instruments, synths and drum machines into the preamp without the danger of sending phantom power to connected line equipment. In Line mode, the unit becomes a two-channel tube re-amp and analogue colour stage: process a stereo bus, a pair of stems or two independent mono sources through the 6SJ7 tubes and custom transformers.
 
Both channels include a front-panel high-impedance DI input for direct guitar, bass, keyboards and other instrument-level sources. +48 V phantom power, a 20 dB pad, polarity inversion and an 85 Hz high-pass filter add essential tracking flexibility. The pad is useful on high-output microphones, loud close-miked sources and hot line signals; the high-pass filter helps control rumble, stand vibration, room buildup and proximity effect before the signal reaches downstream conversion or compression.
 
The 40AM-2 provides two fully independent channels rather than a fixed stereo-linked circuit. That makes it useful as a stereo front end for overheads, piano, acoustic guitar or synthesizers, but also as a dual-mono unit for separate vocals, bass, guitars, percussion or DAW stems. With up to +24 dBu output level, robust 2U steel construction and a lifetime manufacturer warranty, it is a distinctive analogue front end for studios that want authentic octal-tube authority with modern usability.
 
Key features
Dual-channel, 1940s American-style octal tube preamp
Independent dual-mono architecture in a 19-inch, 2U chassis
Inspired by RCA BA-11A and Gates SA-70 tube-amplifier designs
2 × octal 6SJ7 tubes per channel; 4 tubes total
First 6SJ7 configured as pentode, second as triode
Up to 52 dB gain per channel
In-house-wound MuMetal-shielded input transformers
In-house-wound nickel-and-steel output transformers
Five-position Input selector on each channel
500 Ω and 1.8 kΩ microphone impedance settings
Line Low and Line High source configurations
High-impedance 1.6 MΩ DI mode
Approx. +4.5 dB additional gain in Mic 500 Ω and Line Low positions
Five-position Response control per channel
Independent Gain and Level controls
Dedicated microphone and line XLR inputs
Front-panel Hi-Z DI input per channel
+48 V phantom power per channel
Switchable 20 dB pad per channel
Polarity inversion per channel
85 Hz high-pass filter per channel
Three-colour output LED metering
Maximum output level: +24 dBu
Suitable for microphone preamplification, DI, line re-amping and analogue colour processing
Handmade in the USA
Lifetime manufacturer warranty on parts and workmanship
 
Applications
Dual-mono tube tracking for vocals, bass, guitar and instruments
Stereo recording of piano, overheads, drum rooms and acoustic ensembles
Thick, harmonically dense lead vocal processing
Bass DI with tube authority, low-mid body and controlled saturation
Characterful electric and acoustic guitar recording
Large, colourful drum and room-mic capture
Adding depth to synths, keyboards and drum machines
Processing DAW stems through tube and transformer stages
Stereo re-amping of mixes, buses and virtual instruments
Building a vintage-inspired dual-channel recording front end
Source-specific tone shaping with Input and Response controls