Locomotive Audio L72P-1 | 500s Tube Preamp

The Locomotive Audio L72P-1 is a single-channel 500-Series tube preamp that combines the characteristic sound of classic German V72 console amplifiers with the flexibility of modern studio technology. Its sound is characterized by a powerful and stable bass foundation, dense and creamy mids, and an open, pleasantly smooth high-frequency range.

To reproduce the sound and transfer characteristics of historic V72 modules as accurately as possible, Locomotive Audio developed its own reproductions of the input transformer, the output transformer, and the anode choke. These three components are wound in-house and form the crucial sonic core of the L72P-1.

Authentic V72 character in a 500 Series format

The historic V72 amplifiers were originally developed for German radio broadcasting and were later used in studio consoles by EMI and Decca, among others. The L72P-1 is closely modeled after this classic circuit, but expands upon it with a wider gain range, a separate line input, and adjustable output attenuation.

While original modules typically operated at a fixed gain of about 34 dB, the L72P-1 offers up to 54 dB of gain. The gain is adjusted in precise 4-dB increments, allowing for reproducible adaptation to different microphones and signal sources.

In-house transformers and anode chokes

The V72’s distinctive character is largely the result of the interaction between the input transformer, the anode choke, and the output transformer. Since the amplifier circuit itself produces relatively low levels of harmonic distortion, these magnetic components play a decisive role in determining the tonal character and dynamic response.

To achieve this, Locomotive Audio analyzed historical circuit diagrams and original modules and redesigned all winding components from scratch. Among other things, the company examined inductance, winding capacitance, DC resistance, phase response, frequency response, and square-wave response. The result is a modern design whose transmission characteristics closely mirror those of its historical counterparts.

The multi-chamber input transformer ensures a tight and controlled soundstage. The three-chamber anode choke with a nickel core provides the output stage with high impedance, enabling high output levels while maintaining low distortion. The meticulously shielded output transformer uses multiple layers of copper shielding between the windings to achieve clean high-frequency reproduction and controlled phase response.

Tube circuit with reliable double triodes

Instead of the rare EF86 or EF804 pentodes—which are often prone to microphony—found in historic V72 modules, the L72P-1 uses two 6922/E88CC double triodes. Both tubes are operated in a cascode circuit, whose behavior has been specifically tailored to match the characteristics of the original pentode stage.

This design combines the organic character of a tube circuit with greater availability of suitable replacement tubes and higher operational reliability. According to the manufacturer, other compatible dual triodes, such as the 12AU7, can also be used, offering additional options for fine-tuning the sound.

S-Transformer Based on a Historic Studio Model

The L72P-1 comes standard with the so-called “S” version of the input transformer. This variant is modeled after the studio modules that became well-known, among other things, for their use in EMI and Decca mixing consoles. Compared to the original broadcast version, it has a slightly more linear high-frequency response.

An internal jumper allows the load impedance of the input transformer to be switched between 400 kOhm and 200 kOhm. This makes it possible to further adjust the high-frequency and transient response to match historical modifications of the S version.

Flexible control of gain and output level

A five-stage output attenuator reduces the final output level in 2-dB increments. This allows the tube and transformer stages to be driven harder as needed, while keeping the level sent to downstream devices or converters under control.

The L72P-1 is therefore not only suitable as a transparent to characterful microphone preamp, but also as a sound-shaping tool for pre-recorded signals. Via the dedicated line input, tracks, subgroups, or complete mixes can be routed through the tube and transformer stages to give them more body, density, and analog depth.

A 20-dB pad circuit is available for microphones with high output levels. An additional 34-dB attenuation allows for the processing of line signals.