SPL Crossover Mk2 red

SPL Crossover Mk2: fully analog active filtering

Integrating a subwoofer into a fully analog high-end system poses a concrete problem: almost all filters on the market require digital conversion. Going digital again within an analog chain means sacrificing part of what one was trying to preserve. SPL takes the opposite approach with a two-way stereo active crossover that separates frequencies without ever entering the digital domain, and whose scope extends far beyond bass reinforcement.

Separating frequencies without going digital

An active crossover operates before the power amplifiers: it splits the line-level signal into two bands and sends each one to its amplification stage. The Crossover Mk2 is organized into two independent sections. The low-pass section feeds the subwoofer(s), while the high-pass section feeds the main speakers. Each has its own crossover frequency, independently adjustable between 20 Hz and 3.6 kHz.

The benefit of keeping everything analog follows simple logic. In a high-end system, the signal travels without conversion from beginning to end. Inserting a digital filter means adding analog-to-digital and then digital-to-analog conversion where this type of stage had specifically been excluded. The Crossover Mk2 processes the voltage signal without a clock or converter, keeping the chain analog from end to end. This arrangement requires an insertion point between the preamplifier and power amplifier, or a preamp output.

VOLTAiR technology and its 120 volts

The VOLTAiR designation refers to the shared electronic platform used by SPL’s Professional Fidelity range. The principle is to operate the audio circuits at a 120-volt supply voltage, split into ±60 V, whereas conventional operational amplifiers operate at around ±15 V. This voltage quadruples the available headroom before clipping. SPL claims approximately 12 dB of additional dynamic headroom compared with a ±15 V design.

Such voltage would destroy standard components. SPL therefore developed its own operational amplifiers, the 120V SUPRA, capable of handling these ±60 V. The result is reflected in the measurements: 0.0008% harmonic distortion on the high-pass output, 138 dB dynamic range, and frequency response from 10 Hz to 100 kHz at -3 dB. The designation can sometimes be confusing: these 120 volts have nothing to do with mains voltage; they apply solely to internal signal processing.

Eighteen crossover frequencies and two slopes

Frequency adjustment is handled by two selectors. The first, X-FREQ, chooses one of three operating ranges: 50 to 120 Hz, 300 to 720 Hz, or 1.5 to 3.6 kHz. The second, X-OVER, selects one of six values within the chosen range. Together, they provide eighteen crossover points, independently adjustable for the low-pass and high-pass sections.

These values are stepped rather than continuous: you choose from a grid instead of freely turning a potentiometer. In return, each setting can be reproduced identically from one session to the next. The slope can be switched between 12 and 24 dB per octave on each of the two sections. A gentle slope (12 dB) allows greater overlap between the two paths and softens the transition; a steep slope (24 dB) more clearly separates the bass from the rest, at the expense of more pronounced phase rotation around the crossover point.

A fixed 20 Hz high-pass filter completes the package. It removes inaudible frequencies below this threshold from both paths, preventing the amplifier from wasting power on unwanted infrabass frequencies and relieving the speakers.

Aligning bass with the main speakers

Making a subwoofer and main speakers work together requires their signals to reach the ear in phase. The Crossover Mk2 offers continuously adjustable phase control covering 360°. A three-position switch sets the direction of rotation: from 0° to 180° in one case, from 0° to -180° in the other; the potentiometer then sets the exact value. Third position: off, and the phase circuit is completely removed from the signal path.

Bass level is adjustable from -10 to +10 dB via an Alps RK27 potentiometer. This adjustment is primarily used to match the subwoofer to the speakers, but nothing prevents using it to add body to a recording lacking in bass.

The low-pass output operates in mono or stereo. In mono, the filter sums the two channels and feeds a single subwoofer; in stereo, it drives two separate subwoofers, one per channel. And if you prefer to let the main speakers run full range, the high-pass filter can be disabled, restoring the entire spectrum to the main channels.

What the Mk2 adds over the original version

The first version of the SPL crossover was limited to the 50-120 Hz range, with six positions and a fixed 24 dB-per-octave low-pass slope. Its use was confined to connecting a subwoofer below full-range speakers. The Mk2 extends the crossover range up to 3.6 kHz and increases the number of frequencies from six to eighteen.

This expanded range changes the purpose of the Crossover Mk2. Extending up to 3.6 kHz makes it possible to separate bass from midrange/treble, thus enabling speaker bi-amplification rather than merely adding a subwoofer to an existing system. Both slopes are now switchable on each section, and the phase adjustment gains its three-position switch. However, the Mk2 drops the two mechanical VU meters found on the original version: the choice favors adjustment range over visual level monitoring.

Balanced connections and German manufacturing

Each input and output is available in two formats: balanced XLR and unbalanced RCA, all gold-plated. A selector chooses the active input, XLR or RCA, while the outputs deliver the signal simultaneously through both connectors, eliminating the need for adapters regardless of the rest of the installation. The balanced connection offers 82 dB common-mode rejection, useful with long cable runs or in electrically noisy environments.

The AMP CTL connector handles 12 V triggering. It puts up to four compatible devices into standby or wakes them at the same time as the Crossover Mk2, and the crossover itself can be controlled by an SPL preamplifier such as the Director Mk2, the Elector, or the Phonitor x. In standby, power consumption remains below 0.3 W.

As for manufacturing, SPL assembles the Crossover Mk2 at its German factory in Niederkrüchten. Sensitive components are through-hole mounted, soldered through the printed circuit board rather than on its surface, a choice that allows each part to be selected by ear. The power supply is linear, with a shielded toroidal transformer. The chassis measures 278 × 100 × 300 mm and weighs 4.6 kg, and is available in black, red, or silver.

Technical specifications

Overview

• 2-way stereo analog active crossover
• Second generation of the SPL Crossover
• Designed for integrating subwoofers and multi-amplified systems
• Proprietary SPL VOLTAiR 120 V technology
• Made in Germany

Key features

• High-fidelity analog filtering without digital conversion
• Independent crossover frequencies for the Low Pass and High Pass sections
• 18 selectable crossover frequencies
• Switchable filter slope: 12 or 24 dB / octave
• 20 Hz low-cut filter (Low Cut)
• Variable 360° phase adjustment
• Mono or stereo Low Pass operation
• High Pass filter can be disabled
• Subwoofer level adjustment from -10 to +10 dB
• Balanced XLR and unbalanced RCA inputs and outputs
• Simultaneously active RCA and XLR outputs
• 12 V Trigger management (AMP CTL)

Crossover frequencies

• Independent adjustment for Low Pass and High Pass
• 18 available frequencies:
  • 50 / 60 / 70 / 85 / 100 / 120 Hz
  • 300 / 360 / 420 / 510 / 600 / 720 Hz
  • 1.5 / 1.8 / 2.1 / 2.6 / 3 / 3.6 kHz
• 3-position X-FREQ range selector
• 6-position X-OVER selector

Settings

• Filter slope:
  • 12 dB / octave
  • 24 dB / octave
• Low-cut filter (Low Cut): 20 Hz
• Phase adjustment:
  • Variable from 0° to 180°
  • Variable from 0° to -180°
  • Phase circuit can be disabled
• Low Pass output:
  • Mono
  • Stereo
• High Pass filter can be enabled or disabled
• Bass level adjustment: -10 dB to +10 dB

Audio performance

• Maximum input / output gain: 32.5 dB
• Frequency response (-3 dB): 10 Hz – 100 kHz
• Harmonic distortion:
  • 0.0008% (0 dBu, 1 kHz, High Pass)
  • 0.0009% (0 dBu, 30 Hz, Low Pass)
• Crosstalk (1 kHz): -70 dB
• Signal-to-noise ratio (A-weighted):
  • -106 dBu (High Pass)
  • -105 dBu (Low Pass)
• Dynamic range:
  • 138 dB (High Pass)
  • 137 dB (Low Pass)
• Common-mode rejection (XLR): 82 dB
• Output impedance: 75 Ω

Connectivity

• Inputs:
  • 1 balanced XLR pair
  • 1 unbalanced RCA pair
• Outputs:
  • High Pass XLR
  • High Pass RCA
  • Low Pass XLR
  • Low Pass RCA
• XLR / RCA input selector
• AMP CTL In Trigger input (12 V)
• AMP CTL Out Trigger output (12 V)

Power supply

• Linear power supply with shielded toroidal transformer
• Analog voltage: ±60 V
• Relay and LED power supply: +12 V
• Mains power supply:
  • 230 V AC / 50 Hz
  • 115 V AC / 60 Hz
• 230 V fuse: T 0.5 A
• 115 V fuse: T 1 A
• Maximum power consumption: 50 VA
• Standby power consumption: < 0.3 W

Dimensions and weight

• Dimensions (W × H × D): 278 × 100 × 300 mm
• Net weight: 4.6 kg
• Weight with packaging: 5.85 kg