Locomotive Audio Model 40AM-2

Introducing the 40AM preamp, your gateway to achieving that coveted vintage 1940s sound. Drawing inspiration from iconic models such as the RCA BA-11A and Gates SA-70, this preamp combines design elements from each into one ultimate creation. Packing a powerful punch but versatile enough to be used on a wide range of sound sources, the 40AM will help produce recordings that embody the warmth and character of a bygone era.

While it is possible to drive into distortion by overloading the input or hitting maximum output headroom, the 40AM's true beauty lies in the subtle nuances available by adjusting any combination of its features. Whether you’re choosing between input transformer windings or utilizing the 5-position Response switch, you can easily tailor the sound of any source. With added features like a 20 dB pad, 85 Hz low cut filter, and a high impedance DI, the 40AM is an essential addition to your rack, allowing you to sculpt the sound you desire.

Transformers

Premium MuMetal shielded inputs and nickel & steel outputs are wound in-house!

Tubes

Two octal 6SJ7 tubes (one used as pentode & one strapped as triode) amplify the signal with 52 dB of max gain

Output level LED

A 3-color LED indicator shows signal level.

Green -> Yellow -> Red

Input knob

The 40AM features a versatile 5-position Input Switch, allowing users to select their preferred input transformer winding configuration in both MIC and LINE modes. This influences the frequency response and the signal level sent to the first 6SJ7 tube. Both the (500Ω) MIC INPUT and (LOW) LINE INPUTconfigurations provide approximately 4.5 dB of additional gain.

Response knob

A 5-position Response Switch gives you complete control over the frequency response and harmonic footprint of any material processed through the 40AM. Position 5 enhances high and low frequencies, delivering a rich sound while minimizing distortion. Conversely, position 1 softens the highs and attenuates the lows, emphasizing the mid-range and adding subtle harmonic character. For the most neutral sound, we suggest position 4 as a starting point. The Response switch allows you to tailor the sound to your taste.

Level knob

The Level knob provides users with the flexibility to make precise signal level adjustments. It's important to note that it interacts slightly with the response knob based on its setting. As the signal is attenuated, users may notice a subtle darkening of the sound. However, by working in tandem with the response knob, adjustments can be made to compensate for any changes in tone.

Inspiration

Combining vintage circuits requires time and patience to achieve the perfect balance. However, when executed correctly, you can harness the strengths of each to create an exceptional hybrid. The RCA BA-11A pentode input paired with its triode output and feedback structure of the Gates SA-70 come together to produce the rich, compressed sound of the 40AM, beautifully complementing its 86B and WT-72 siblings.