Walrus Audio Mantle

Mantle
Analog Bass Preamp // D.I.

The Mantle is a premium preamp and DI for electric bass, created in close collaboration with Ian Martin Allison. Designed to give your instrument an impeccable sonic foundation, the Mantle brings studio-grade refinement to any environment - whether you’re stepping onstage, tracking in the studio, or shaping your tone at home.

At the heart of the circuit are our proprietary, custom-wound Sapphire Transformers, engineered to deliver the authoritative low-end and midrange push reminiscent of a classic Neve circuit, paired with an airy, articulate top end inspired by revered API preamp architecture. Together, they provide a harmonically rich canvas that elevates your playing while preserving every nuance of your instrument. 
The Mantle empowers you to sculpt your signal with intention, offering musical gain, smartly tuned EQ, tremendous headroom, and transformer-balanced outputs. Every component and stage of the signal path is built to ensure whatever comes next - amp, interface, or mixer - receives your bass in its most confident, fully realized form.

“I am an absolute DI fanatic, owning probably 50+ DIs and preamps. And while they’re all great in specific ways, they all either lack something critical or try to stuff in too many features, making for an underwhelming or even overwhelming user experience! This is exactly why I wanted to make Mantle. I needed this product to exist. I’m thrilled to say that now it does. And it is absolutely killer.”  
                                                                                                                   -Ian Martin Allison
Features:

• High-impedance JFET Input Buffer for articulate, touch-sensitive response and exceptional clarity. 
  
• Up to +23dB of transformer-based gain offering clean, magnetic, harmonically rich amplification with premium depth and detail. 
  
• Custom-tailored Bass & Treble controls co-designed with Ian, utilizing a sophisticated blend of shelving, bandpass, and selectable LPF/HPF behaviors. 
  
• Transformer-balanced XLR output ensures expansive headroom, uncompressed dynamics, and commanding low-frequency extension. 
  
• High-headroom +24V internal power supply for clean dynamics and effortless low-end. 
  
• Active/Passive Pad & Ground Lift switches for enhanced compatibility to allow for more headroom if needed when using an active bass. 
  
• Flexible routing with 1/4" in/out and balanced XLR direct output for modern routing needs.

Signal Path Overview

Input → JFET Buffer → Low-Pass/High-Pass Filters or Dry Path → Input Transformer (-6dB to +23dB) → Boost EQ → Output Transformer

The Mantle’s architecture is meticulously voiced to maintain clarity, delivering musical transformer coloration, and giving you control over both the front end and final output of your tone.

Transformers:

The Mantle features our proprietary custom-wound Sapphire Transformers. With more than a year of research, listening, and refinement, the Walrus engineering team has set a new benchmark for their designs with new input and output transformers reflecting our commitment to providing creators with cutting-edge tools that inspire innovation and creativity. 
Based on Neve designs, the input transformer is capable of handling large signals without added distortion while maintaining a clean, strong, and quiet signal. High-quality core material and copper windings were used to achieve the gain needed for pristine performance across the frequency spectrum. 
The output transformer is based on the output stage of an API 312. This serves two functions: It balances the signal quietly and effectively while also providing rich harmonic content to the amplified signal. Like the input transformer, the style of core and customized winding method result in a strong signal that can be transmitted over long cable runs without signal loss.

Connections:

• 1/4" TS Unbalanced Input (Hi-Z): 
  Optimized for electric bass and buffered by a high-quality JFET stage to maintain impedance, detail, and instrument integrity.
  
• 1/4" TS Unbalanced Output: Send your shaped signal to an amp or additional pedals.
  
• Male XLR Balanced Output: Transformer-balanced for elevated DI tones that feel at home in high-end studio and live environments.
  
• Power: 9VDC, 500mA center-negative. Internally boosted to +24V for massive headroom and clean, authoritative bass response.

Controls:

Bass & Treble (Boost/Cut Filter System)

The Mantle’s EQ is a sophisticated dual-behavior design, far more expressive than standard shelves. Both Bass and Treble controls feature eleven-position rotary switches for a satisfying studio feel, easily reproducible settings, and a true flat response at noon (position 6).

Boosting (positions 7–11):

• Begins as a shelf, gradually transitioning into a broad, musical bell as you increase gain.
  
• Each increment adds +3dB, up to +15dB, without drifting into muddiness or harshness.

Cutting (positions 5–1):

• Introduces a steep -18dB/oct HPF on the Bass control at selectable points: 
  30Hz, 50Hz, 80Hz, 100Hz, 120Hz
  
• Introduces a steep -18dB/oct LPF on the Treble control at selectable points: 8kHz, 6kHz, 4kHz, 2kHz, 1kHz

This design allows precision mix shaping or broad musical coloration, without sacrificing elegance or fidelity.

Gain Control
The Gain control adjusts the amount of transformer-based gain applied to the signal. The gain circuit comes directly from the input transformer, creating magnetic manipulation at its finest. This control offers a gain range from -6dB to +23dB across the control range. 
With the combination of the JFET buffer and transformer gain, the result is a tone that feels alive, dimensional, and deeply expressive.

Switches:

Active / Passive (-10dB Pad)

• Engaged: Introduces a -10dB input pad, ideal for active basses or instruments with high-output preamps. Prevents overloading the input stage.
  
• Disengaged: Bypasses the pad for full-strength passive bass output and maximum sensitivity.

Ground Lift

• Engaged: Separates the XLR output ground from circuit ground.
  
• Purpose: Eliminates hum and ground-loop noise when connecting to unfamiliar or complex live/studio setups.

Specifications:

• Power Requirements: 9VDC, 500mA, Center-Negative, 2.1mm. Power supply not included.
  
• Internally boosted to +24V for higher headroom.
  
• The use of an isolated power supply is recommended for powering all Walrus Audio pedals.
  Daisy chain power supplies are not recommended.
  
• Input Impedance: 1M Ω
  
• Output Impedance: 150 Balanced, 150 Unbalanced
  
• Noise floor: Balanced -101dBV, Unbalanced -94dBV
  
• THD @ 1kHz 1Vrms: 0.04%
  
• Frequency Response: 20Hz - 20kHz
• Gain Range: -6dB → +23dB
  
• EQ Boost Range: 0dB → +15dB
  
• Exact Size of the die-cast enclosure (with knobs) is 6.09” x 4.7” x 2.28” (154.78mm x 119.35mm x 57.98mm)