Home / SPL AUDIO PASSEQ Passive Mastering Equalizer
Product Overview
The King of Passive Inductive Equalization
While legendary classic passive equalizers (like the vintage Pultec units) are highly celebrated for their smooth, musical tone, they have historically been limited by a restricted set of frequency options and high internal noise floors. The SPL PASSEQ completely shatters these limitations by reviving the golden-era inductive filter topology and supercharging it with state-of-the-art German engineering.
The PASSEQ stands as the most powerful passive EQ system ever built, being the first of its kind to supply three dedicated, completely separate frequency ranges for both the boost and cut networks. Featuring a unique parallel structure where resistors, custom single-wound coils, and premium capacitors interact organically with the audio signal, it handles broad tonal strokes with a velvety, relaxed character that active circuits simply cannot replicate.
Product Description
120V Rail Makeup Gain Stage:
Because passive filter configurations have no internal amplification, they shape sound exclusively by attenuating frequencies; to boost a frequency, all other adjacent bands must be lowered structurally. This results in an overall drop in signal level that can require up to 20 dB of makeup gain. Traditional tube or solid-state makeup amplifiers often introduce significant background hiss or color the sound aggressively.
The PASSEQ solves this natively by feeding its output recovery path directly into SPL's signature 120V DC high-voltage rail platform. Operating at a voltage ceiling that is double that of discrete designs and four times that of traditional IC op-amps, the PASSEQ easily accommodates massive +32.5 dBu input thresholds. This ensures that the recovery gain stage remains dead-silent, completely open, and entirely transparent, allowing the authentic sonic imprint of the passive inductive components to shine through.
Granular Multi-Band Control and Optimized Q-Factors:
The dual-channel architecture features 12 step-switched frequencies per band, resulting in 36 boost and 36 cut settings across the unit. Crucially, the boost and cut frequency choices do not overlap identically, providing 72 unique target bands per channel for highly intricate mastering adjustments. For this generation of the PASSEQ, every single frequency selection has been fine-tuned, and the component matrix has been rebuilt with custom single capacitors optimized per band. This grants each individual frequency its own uniquely calculated, highly musical Q-factor curve.
Extended Air Band and Low-Mid Control:
The frequency ranges have been thoughtfully widened to satisfy the strict demands of modern high-resolution audio workflows:
The Low-Frequency Band (LF): Delivers phenomenal control over the bottom end, allowing engineers to lift lowest sub-bass energy or cut muddy rumbles without introducing blurriness or transient distortion.
The Mid-Frequency Cut (MF-): Has been extended downward all the way to 200 Hz (as opposed to the historical 1 kHz limitation), making it highly effective at cleaning out low-mid woofiness, tubbiness, and boxy resonances.
The High-Frequency Boost (HF+ Air Band): Extends up into the stratosphere with dedicated 25 kHz and 35 kHz shelf choices, opening up an incredibly lush, silky sheen that breathes acoustic life into an entire master track.
Key Features
120V DC AUDIO RAIL SYSTEM: Runs on high-voltage operating rails for exceptional analog headroom, immense dynamic clarity, and low distortion.
PASSIVE INDUCTOR FILTERS: Recreates the charming, smooth sound signature of classic vintage 1950s/60s inductor gear without historical noise defects.
72 FREQUENCIES PER CHANNEL: Features 36 distinct boost and 36 non-overlapping cut frequency selections for limitless sound-shaping curves.
INDIVIDUALLY CALIBRATED Q-FACTORS: Every selectable frequency point possesses a dedicated, component-optimized bandwidth for musical coherence.
EXTENDED HIGH-END AIR BAND: Added 25 kHz and 35 kHz frequency selections to dial in beautiful, sparkly top-end open spaces.
EXPANDED MID-RANGE CUT: The midrange attenuation curve goes all the way down to 200 Hz to surgically remove low-mid workspace clutter.
DETENTED STEPPED POTENTIOMETERS: Equipped with premium 41-step detented pots for Gain, Output, and HF+ Q to guarantee fast and precise recall.
±10dB OUTPUT LEVEL MASTER: Large, stepped ALPS output faders allow fine gain-staging control into subsequent hardware processors.
INTEGRATED AUTOMATIC BYPASS: Built-in auto-bypass function with a variable timing interval permits critical, objective A/B listening assessments.
GAS-SEALED HARD RELAYS: Dedicated individual illuminated hardware bypass switches route signals flawlessly through premium gold relays.
