Locomotive Audio L72P-2 | Dual-Mono Tube Preamplifier

The Locomotive Audio L72P-2 is a two-channel tube microphone preamp in a 19-inch format that combines the characteristic sound of historic German V72 console amplifiers with modern features. Both channels feature independent tube and transformer circuits and deliver a powerful, controlled bass response, dense and creamy mids, and an open, pleasantly smooth high-frequency response.

The L72P-2 is modeled after the legendary amplifier modules that were originally developed for German radio and later used in studio consoles at EMI, Abbey Road, and Decca, among others. Locomotive Audio has not merely emulated the circuit’s sound; rather, it has completely redesigned key components—such as the input transformer, output transformer, and anode choke—based on historical models.

Two independent V72-inspired tube channels

The L72P-2 integrates two complete L72P preamplifiers into a rugged 19-inch rack-mount enclosure. Each channel offers a maximum gain of 54 dB, adjustable in precise 4-dB steps.

Compared to older V72 modules, which typically operated at a fixed gain of about 34 dB, the L72P-2 can therefore be adapted much more flexibly to different microphones and signal levels. Even dynamic microphones and ribbon microphones with low output voltage can be used without an additional inline preamp.

In-house transformers and anode chokes

The characteristic sound of vintage V72 amplifiers is largely the result of the interaction between the input transformer, the anode choke, and the output transformer. Since the actual amplifier circuit produces relatively low levels of harmonic distortion, these magnetic components are primarily responsible for the tonal character and dynamic response.

Locomotive Audio manufactures all transformers and anode chokes in-house. During the development process, the company analyzed and reproduced, among other things, the inductance, winding capacitance, DC resistance, phase angle, frequency response, and square-wave response of historical components.

Each channel has:

• a six-chamber input transformer
• a three-chamber anode restrictor with a nickel core
• a multi-shielded output transformer
• four copper shields between the windings of the output transformer

The anode choke presents a high impedance to the output tube, thereby enabling high output levels with low distortion. The meticulously designed output transformer supports clean high-frequency reproduction as well as controlled transient and phase response.

Reliable dual triodes in a cascode configuration

Historical V72 modules used EF86 or EF804 pentodes. Since low-noise and low-microphonic examples of these tube types are now only available in limited quantities, Locomotive Audio uses modern double triodes in the L72P-2.

Each channel uses two 6922 or E88CC tubes. These are operated in a cascode configuration, whose behavior has been specifically tailored to the characteristics of a pentode stage. In this way, the L72P-2 combines the sound character of the historic circuit with better availability of replacement parts and greater operational reliability.

Alternatively, according to the manufacturer, compatible dual triodes such as the 12AU7 can also be used, which opens up additional possibilities for customizing the sound.

S-Input Transformer Based on a Historic Studio Model

The L72P-2 comes standard with the so-called “S” version of the input transformer. This variant is modeled after the studio modules that became well-known, in part, through their use at Decca and EMI.

Compared to the original broadcast version, the S-transformer has a slightly more linear high-frequency response. According to the manufacturer, the difference is approximately 0.25 dB at 15 kHz and 0.5 dB at 20 kHz. The powerful and dense low-midrange character is retained in both versions.

Internal jumpers can be used to switch the load impedance of the input transformers between 400 kOhm and 200 kOhm. This allows the high-frequency and transient response to be further adapted to historical circuit configurations.

Separate microphone, line, and instrument inputs

In addition to the microphone inputs, the L72P-2 features dedicated line inputs on the rear panel. This allows previously recorded tracks, subgroups, synthesizers, or complete mixes to be safely routed through the tube and transformer stages.

The separate line XLR inputs prevent accidentally activated phantom power from being passed on to connected line-level devices. In addition, high-impedance instrument inputs for guitars, basses, and other unbalanced sources are available on the front panel.

Flexible sound shaping via gain and output attenuation

Each channel features a five-step output attenuator that allows the final output level to be reduced in 2-dB increments. This makes it possible to drive the amplifier stage harder while keeping the level sent to converters or downstream studio equipment under control.

At moderate gain levels, the L72P-2 delivers an open, stable, and comparatively low-distortion sound. At higher gain settings, the sonic characteristics of the tubes, transformers, and anode chokes become more pronounced, resulting in greater density, weight, and harmonic structure.

Extensive features per channel

Both channels can be operated completely independently. In addition to gain control and output attenuation, functions for phantom power, phase reversal, level reduction, and high-pass filtering are available.

A 20-dB pad allows for processing microphone sources with particularly high signal levels. An additional 34-dB attenuation is available for line signals. The switchable 85-Hz high-pass filter reduces low-frequency noise, footfall noise, or unwanted proximity effect.