The Passeq complements our parametric Equalizers Qure and PQ 2050 by a classic design with passive coil filters and perfectly fulfills the highest expectations in all areas of audio processing, from recording through mixing to mastering.

The superior sound qualities and musical characteristics of passive filters ideally meet increasing demands in creating manifold sound colors full of character.

Unique Features

• The most powerful passive EQ ever made—144 (!) passive filters (72 per channel) in one EQ.
• Individual coils per filter.
• Single core coils, which means that every one is wound individually on its own dedicated core. This excludes sonic degradation from mutual influences while at the same time improving THD values.
• 120V makeup amplifiers based upon SPL unique SUPRA-OPs with 150dB dynamic rage and 200 v/ms slew rate.

Special Features

• Individual design and component specificity for each filter.
• Custom made coils for critical mid-frequency ranges.
• Boost and cut crossovers mesh perfectly so that with the high number of frequencies one can dependably command the most elaborate set of response curves that to date any passive EQ has offered.
• Transformers from Lundahl with perfectly matched sonic characteristics provide for balanced I/O stages.

Other Features

• XLR contacts from Switchcraft.
• Switches and potentiometers from Elma and ALPS (including ALPS “Big Blue” with 41 steps).
• Internal, linear power supply featuring a generously proportioned toroidal transformer and voltage selection (110-120V, 60Hz or 220-240V, 50Hz).

With 72 passive filters per channel (36 x Boost, 36 x Cut), the Passeq surpasses all previous designs of this type by a wide margin. Each channel is divided into three Cut and three Boost bands, each offering 12 switchable frequency ranges. The Cut and Boost ranges are not identical; crossovers are designed to work with like a precise mechanic cogwheel so as to allow the engineer access to the largest possible number of optimal, wide-band S-curves with variable slopes.

Advantages of Passive Filtering

• Typically coil inductance in virtually all active filters is achieved through simulation. True passive coil filters, on the contrary, can only deliver the genuine, characteristic sound associated with inductive components.
• Distortion elements typical of active filters are ruled out by passive filter design.
• For any number of reasons stemming from design and component advantages over active filters, passive filters achieve a very natural aural quality and through their harmonic treatment (THD, distortion, phase response, etc.), offering at the very least, a clear sonic alternative, which our ears often perceive as an extremely attractive one.
• All passive filter components (variable resistor, capacitor and coil) work in concert to produce this beautiful sonic result. An important part of this process is played by coil saturation and condenser loading characteristics. The resulting difference in latency from characteristically extremely fast reaction of active filters provides for more pleasant, musical sonic qualities. We tend to perceive these attributes in terms of an increased suppleness and transparency, with perceptibly improved, silky highs and robust basses.

Filter Types

The Passeq employs two distinct filter types: One of these functions much like traditional shelving filters, while the other, as a peak filter, and together, they provide the combined characteristics of wide-band control in low and high ranges with more specific frequency range control in mids. This selection minimizes mutual influences between low, mid and high bands while providing a more selective control in the mids is often useful.

Mid boost and cut, as well as HF boost filers have been set up in a peak (bell) configuration, while the hi cut, low cut and low boost filters function in a shelving configuration. The HF boost band offers variable values from Q=1 to 0.1.

Single Core Coils

Until now design approaches have involved individually wound coils, but multiple coils have nonetheless been placed on a single core. The Passeq design places each coil on separate cores. This eliminates any possible unwanted mutual influence transmitted through common-core windings and thus, among other improvements, results in better THD values.

120V Make-up Amplifiers

With passive filtering comes an unavoidable drop in signal level that requires makeup amplification, and with the Passeq, here SPL’s extraordinary Supra-OPs, with their unique analog 120-volt technology, come into play. With a 116 dB signal-to-noise ratio and +34dB of headroom, the SUPRA-OPs offer a stunning 150dB dynamic range, placing them in an unsurpassed leadership position in either analog or digital signal processing. The tremendously fast SUPRA slew rate of 200V/ms allows for a highest possible precision in filter output signals, particularly in the all-important arena of transient response. These amplifiers effortlessly and without coloration or degradation, transmit all the desired filter characteristics and sonic results an engineer has sought out and in the process, pushing beyond the limits of what has been technically possible to now.

SPL SUPRA Op Amps

The specially designed and for-audio optimized SUPRA OPs are constructed in three stages with high performance, extremely low noise transistors from the modern HF technology sector.