Locomotive Audio Model 40AM-1 is a 500-series preamp that delivers the thick, compressed tone of 1940s American broadcast units such as the RCA BA-11A and Gates SA-70. Two octal 6SJ7 valves, one operated as a pentode, the other strapped as a triode, provide up to 52 dB of gain, driven by in-house-wound MuMetal-shielded input transformers and nickel-and-steel output transformers.
A versatile five-position Input switch selects transformer windings for mic or line sources, with the 500 Ω mic input and the low line input each adding approximately 4.5 dB of extra gain. A five-position Response switch tailors overall frequency balance and harmonic content: position 5 lifts highs and lows for a rich sound, position 1 emphasises the mid-range with gentle saturation, and position 4 offers a neutral starting point, while positions 2 and 3 provide intermediate curves.
A twenty-decibel pad prevents overload, and an eighty-five-hertz low-cut filter removes unwanted rumble. The high-impedance DI input of the Locomotive Audio 40AM-1 accommodates instruments directly. A Level control trims output and interacts with the Response switch so that attenuation can be offset by readjusting tonal balance.
A three-colour LED meter (green, yellow, red) indicates signal level for instant visual feedback. The design combines vintage circuit topology with modern flexibility, delivering authentic valve weight on anything from vocals to instruments while allowing precise sculpting through transformer selection, response shaping, filtering, and gain staging.
Features
- Input transformer options: five-position switch selects mic or line windings
- Channel gain: two octal 6SJ7 valves provide up to 52 dB
- Response switch: five settings adjust frequency balance and harmonics
- Pad: –20 dB to prevent input overload
- Low-cut filter: 85 Hz for rumble control
- DI input: high-impedance for instruments
- Level control: output trim with interactive tonal compensation
- LED meter: three-colour indication of signal level
- Transformers: MuMetal-shielded inputs, nickel and steel outputs wound in-house