Introducing Hank Stone’s Passive Transformer Direct Box

Hank Stone Audio Engineer and Designer

A Game-Changer for Remote, Studio, and Stage Applications

Hank Stone Audio Engineer and Designer
Hank Stone Audio Engineer and Designer


If you’ve spent any time in a recording studio or setting up live sound, you’ve undoubtedly encountered a direct input (DI) box. These handy devices are essential for connecting unbalanced instrument outputs to balanced microphone preamp inputs, ensuring that the sound remains clear and free from distortion or noise. Hank Stone has taken this concept to the next level with his newly designed passive transformer direct box, ideal for remote, studio, and stage applications.

Key Specifications:

  • Input Impedance: ~200k/800k Ohms (depending on mic preamp)
  • Transformer Turns Ratio: 12:1
  • Attenuation: 22dB/38dB (instrument/speaker input)

How a Passive Direct Input Box Works

To understand why Hank Stone’s passive DI box is a game-changer, it’s helpful to first grasp the basics of how passive direct input boxes function.

DI Basics

A direct input (DI) box serves the crucial role of connecting unbalanced instrument outputs (like those from guitars or keyboards) to balanced microphone preamp inputs. This connection is necessary for both live sound applications and studio recordings, ensuring that instruments are properly interfaced with pro-audio equipment.

Most DI boxes, including Hank Stone’s, feature:

  • An unbalanced ¼” input
  • A balanced XLR output
  • A ground-lift switch to eliminate hum and buzz caused by ground loops

There are two types of DI boxes: active and passive. Active DI boxes require power, typically from +48v phantom power provided by the preamp, due to their active components like transistors and integrated circuits. Passive DI boxes, on the other hand, use no power and rely solely on passive components, primarily a transformer.

The Problems DI Boxes Solve

DI boxes were created to address three main issues when connecting instruments to professional audio equipment: voltage, impedance, and grounding.


Instrument outputs can vary widely in voltage, from millivolts to about 10 volts, whereas microphone preamp inputs are designed for the very low levels typical of microphones, usually in the millivolt range. Without a DI box, the high voltage from an instrument can overwhelm and distort the preamp.

The transformer inside a DI box reduces the instrument’s voltage to a level suitable for the preamp. Hank Stone’s DI box uses a transformer with a 12:1 turns ratio, meaning the output voltage is twelve times lower than the input. This reduction ensures that the instrument signal is at a manageable level for the preamp.


Impedance matching is critical for maintaining sound quality. The general rule is that the input impedance should be at least ten times the output impedance of the connected device. Instruments typically have high output impedances, which can cause problems when connected directly to the low input impedance of microphone preamps.

The transformer in a DI box also transforms impedance, but it does so by the square of the turns ratio. With a 12:1 ratio, Hank Stone’s DI box effectively matches the high impedance of instruments to the low impedance of microphone preamps, ensuring a clean, undistorted signal.


Grounding issues are a common source of hum and buzz in audio systems. Instruments use unbalanced connections, which do not isolate the ground from the signal. When an unbalanced instrument is connected to a balanced input, ground currents can flow through the signal path, causing noise.

Transformers provide galvanic isolation, meaning they transfer the signal via electromagnetic induction without a direct electrical connection. This isolation prevents ground loops and eliminates the noise they cause.

Why Hank Stone’s Passive DI Box Stands Out

Hank Stone’s passive transformer direct box excels in all three areas:

  • Voltage Management: The 12:1 transformer reduces instrument voltage to levels that are safe and manageable for preamps.
  • Impedance Matching: The effective impedance transformation ensures that the instrument’s high output impedance is properly matched to the preamp’s low input impedance.
  • Ground Isolation: The transformer provides galvanic isolation, preventing ground loops and ensuring a noise-free signal.

Additionally, the input impedance of Hank Stone’s DI box (~200k/800k Ohms) can adapt to different mic preamps, making it versatile for various applications. Its attenuation capabilities (22dB for instrument input and 38dB for speaker input) make it suitable for a wide range of signal levels, from delicate instruments to powerful speaker outputs.


Hank Stone’s passive transformer direct box is a robust and elegant solution for connecting instruments to professional audio equipment. By addressing voltage, impedance, and grounding issues with a single, passive component, it ensures high-quality, noise-free audio in any setting—whether in the studio, on stage, or in remote recording environments. For anyone serious about sound, this DI box is an indispensable tool.

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