The most powerful passive Equalizer ever made

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.
A further noteworthy Passeq feature is its individual sonic adaptation of each inductive filter through separate coil/condenser/resistor combinations: In stark contrast to earlier filter design and construction, each Passeq filter is optimized for the frequency assigned.
To insure the best possible signal warmth, richness and musicality in processing, coils for critical voice frequencies are custom-made for the Passeq.
This achieves the widest possible range and tonally appealing sound color palette from any passive EQ.

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 Makeup 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.

Tech Talk: SPL SUPRA Input Stages

SPL SUPRA components
The central component of the PQ is a fundamentally new amplification design: discrete, custom made Class A audio operational amplifiers which run on a 120V operating voltage (+/- 60V). This amounts to over three times the operating voltage found in most high quality audio gear (+/-15-20V) and about twice as much as the highest voltages used in the best units currently available.
This extremely high voltage allows the circuitry to process an astonishing dynamic range of ca. 150dB and an amazing +34dB of headroom, virtually eliminating overloading of individual filter stages—even when processing extremely high-level signals.
For the first time, transistor circuits with such an impressive degree of stability and freedom from harmonic distortion can be realized.

Input stages of the SUPRA components
The development of the SUPRA components focused on high loop amplification, extremely low phase shifting and THD, combined with maximum amplification and a frequency response up to 100kHz.
A main and obvious advantage of the discrete SUPRA components is the exclusion of parts often found in industrially manufactured standard components that are not necessary for audio processing.
The SUPRA input stages are designed as balanced differential stages and comprise six matched high voltage transistors switched in parallel.The concept of the input stage is based on the established principle that currents of not correlated noise sources in shunt circuits add up—which decreases the overall noise of the input circuitry. The input stages are free of coupling capacitors to exclude additional capacitor noise. The balanced operational voltage of +/-60V is delivered from a linear -80dB high voltage power supply.

Intermediate stages of the SUPRA components
The audio signal is further routed to a differential stage and from there through further processing stages to the Class-A output stage. All passive components have been tested to yield the highest possible fidelity.

Output stages of the SUPRA components
Extremely low noise, high voltage output transistors are set up with a high quiescent current and excess heat is dissipated via special cooling plates.

Lundahl Transformers
The Swedish firm of Lundahl is recognized world wide for the superior sonic qualities of its hand made transformers. SPL has used Lundahl transformers for many years, typically for optional in- and output stages of various products. In the case of the Passeq, there is no question of whether solid state or transformer-based input and output stages are the better choice: Because of their excellent and similar sonic qualities, Lundahl transformers are a clear choice to complement to the Passeq’s EQ circuitry.

I/O transformers are classical analog components in many “vintage” machines. In addition to increased operational safety due to the isolation from incidental I/O electrical interference they offer, transformers also introduce their own element of sonic “warmth” that is today too often inadequately attributed solely to tube circuitry.

The sonic quality from Passeq’s Lundahl transformers may be described in comparison to straight electronic I/O circuitry as: Bass and fundamentals are rounder, fuller, and exhibit more “punch”, while higher frequencies and harmonics sound silkier and more present, yet without leaving the impression of being overly emphasized or singled out. Moreover, they add the subtle impression that mix elements are better localized.
The reasons for this are the tendency of transformers to reduce uneven harmonics (which give the impression of harshness in a sonic canvas) and to act with some latency compared to electronically balanced stages. In particular, fundamentals and low frequencies benefit from this.

Layout of Operational Elements
Initially one might be struck by the circular arrangement of the Passeq’s control elements. As unusual as this first appears, the more understandable and clearer these elements become when one looks closer.

Along with the fact that we simply like this design from an aesthetical view, this layout makes even more sense with respect to the idea of the passive EQ concept itself: In a passive design, filters for boosting and cutting a frequency range are physically separated from each other. Reflecting this fact, the elements left of the central output control perform level cuts, while controls to the right of this central regulator serve as signal boost controls. Cut and boost switches are positioned next to the appropriate frequency band selector and frequency bands are arranged from low to high from the standpoint of both physical and frequency range layout—all in all a clear overall functional picture though without much in the way of boring routine.

The Full Range
The Passeq is the first passive EQ which provides three separate frequency ranges for both amplifier and cut stages. One famous, if not the most famous, passive design was the Pulteq EQ from the decades of the 1950’s and 60’s. This EQ sported two frequency bands (low and high frequencies, or LF and HF), and had only a few switchable frequencies to offer. In contrast, the Passeq has 12 switchable frequencies per band, totaling 36 boost and 36 cut frequencies. Boost and cut frequencies are NOT identical, thus the resultant 72 frequencies per channel offer an enormous choice for the most elaborate EQ curves (please refer to the next chapter, “Frequency Layout”).
The Passeq offers for the first time passive filter control possibilities extending throughout the relevant audio frequency range—and that with an unheard of abundance of filter choices.

One Coil per Filter, one Core per Coil
Each Passeq filter is individually constructed for its intended frequency, that is, each coil, condenser and variable resistor (var. resistor=boost or cut control) ensemble is sonically tuned to its intended frequency range. Thus each filter has its own musically sensible audio color appropriate to its own frequency.
In turn, each coil is also wound on its own separate core to avoid mutual and degrading influences which stem from past designs where multiple coils were wound on a single core. Not the least, the construction of each filter on its own particularly high grade core also provides for excellent THD values.

Allocation of Frequencies
The greatest Passeq design challenge was in determining the choice of frequencies, which in contrast to parametric EQ designs, are fixed or nonadjustable. One could accept standardized values from such as the so-called ISO frequencies, but such measurements stem too much either from conventional measurement standards or those from room corrections rather than choices of what may be musically more sensible.

In assigning the Passeq's frequency ranges it was inevitable that we would rely on the nearly 30 years of experience of SPL’s chief developer, audio engineer and musician, Wolfgang Neumann. To enhance further our achieving this musical objective many audio experts and musicians were consulted regarding their favored frequencies. Among the many, David Reitzas, Michael Wagener, Bob Ludwig, Ronald Prent and Peter Schmidt offered valuable advice. From this point of departure we managed to determine that there is definite agreement among professionals about their preferred musical frequencies, and these differ clearly from the standard ISO choices.

The results also showed that the closely meshed boost and cut frequencies are important and sensible. Through them one can on the one hand focus more precisely on a certain frequency, and on the other, offer the option of influencing the Q factor (which is typically rather small in passive designs) by creating so-called S curves.

An Example: Assume you wish to boost in the mids around 320Hz, an instrument or voice level while at the same time avoiding a boost to the frequency range below it due to the small Q factor (high bandwidth) of the filter, and perhaps even lower it. In this case, let’s say you choose the LMF-MHF boost band and increase the chosen (320Hz) frequency range by about 3dB. At the same time, you chose a 4dB reduction in the LF-LMF cut band. The close proximity of the chosen frequencies allows you achieve an increase in the slope between the two. This is "S slope EQ-ing" at its best, and in this discipline, the Passeq is a world champion in both options and results.

Frequency Bands: LF-LMF Cut and LF Boost
The low cut frequency range extends from 30Hz to 1.9kHz and will be ref­erred to in this text as LF-LMF (Low to Low-Mid frequencies). In con­trast, the low boost (LF Boost) band en­com­passes a range of 10 Hz to 550 Hz. The maximum available in­crease in this LF Boost band is (+)17dB, while the maximum reduction of the LF-LMF Cut band is (-)22dB.

Optically these filter bands may be represented as having a shelving characteristic with an 6dB slope. Passive filters do not allow for direct alteration of the slope gradient because this quality is pre-determined by component selection and not, as with active filters, by an variable value.

The lowest frequencies begin here with 10Hz, then follow with 15, 18, 26, 40Hz, and so on. At this point one might think that such a lavish set of frequency choice in this range might be a bit overdone, as there is acoustically a rather limited amount of audio material of any real significance below 26 Hz. However, these choices are anything but arbitrary. These frequencies represent a consistent -3dB point of a sloping down response curve. That is, the gentle 6dB slope also allows frequencies above 10 Hz to be processed. As mentioned in other parts of this text, special condenser/coil/resistor filter networks have been designed for each frequency range. The choice of one or the other inductances produces differences in sonic coloration even when limited differences between frequencies such as 10Hz or 15Hz play a subordinate role. Along with this differing phase relationships may come into play and affect tonal color. Because modern productions often demand a definite number of choices in an engineer’s options for achieving an optimal result in bass emphasis, the Passeq has been designed with a very complete set of low frequency options to insure realizing these goals.

MF-MHF Cut und LM-MHF Boost
The Midrange bands elevate the Passeq to a complete combination of filter options that classic passive designs do not offer. Both midrange bands exhibit peak filter characteristics, that is, when viewed from the boost band, the frequency curve appears as bell-shaped slopes above and below the chosen frequency range. The slope or Q-value is, again, not variable, but attuned through the choice and configuration of the passive filter's components for a maximum in musical efficiency, relying in the Passeq on its developer, Wolfgang Neumann's years of musical experience. The middle bands' peak structure is chosen for a clean separation of LF and HF bands. Were the choice here to be for a shelving filter design, too many neighboring frequencies would be processed, with resulting undesirable influences extending into LF and HF bands. Along with this is the simple fact that a midrange peak filter characteristic is accompanied by a more easily focused center point processing of critical voice and instrument fundamental frequencies.

The MF-MHF Cut Band overlaps the LF-LMF Cut band by approximately an octave, with its lowest frequency extending from 1kHz. The LF Boost and LMF-MHF boost bands are set up in a similar fashion, with the lowest LMF-MHF boost band frequency set at 220Hz and thereby 1-1/2 octaves under the highest LF boost band frequency. The maximum reduction of the MF-MHF Cut and LMF-MHF boost band extends from -11.5dB to +10dB.
The overlapping band characteristics give a good idea of the available degree of precision in frequency adjustment: For example, one can boost in the LMF-MHF Boost band at 220Hz while in the LF boost band, 240Hz can be followed by 320Hz in the LMF-MHF boost band: The next step could be at 380Hz in the LF boost band, followed by 460Hz in the LMF-MHF Boost band and 550Hz in the LF-Boost band ...

MHF-HF Cut and HF Boost
Passeq’s high frequency bands have a different layout for the cut and boost ranges: The MHF-HF Cut Band exhibits a (wideband) shelving characteristic, while the HF Boost band exhibits a variable Q, peak filter characteristic.
As seen above, one can also note and intensification in choice of frequencies in the high range. Here the same reasons apply as in prior cases: Individually designed and constructed coil-condenser-resistor configurations result in slightly differing sonic coloration. Thus beginning at 10kHz there are seven additional switchable frequencies. The available variable Q (ranging from Q=0.1 to Q=1.0) allows the engineer access to an enormously flexible range in high frequency boost options.

HF Boost Quality Settings with the Proportional Q Principle
With the proportional or variable Q principle, boost control settings would apply only if the HF Boost Q were to be set at Q=1.0 (control set fully clockwise). Were the value to be reduced (thus increasing the bandwidth), the boost would also be reduced. This can lead to a situation wherein, for example, a HF Boost Q-setting of 0.1 and a boost of 3dB would result in effectively no audible boost in the chosen frequency—at this value the Q-value resides at about 0.3dB. With this Q value, don’t hesitate to turn turn up the HF band boost control to its full 12.5dB setting—this resultsin an actual overall increase of around 3.5dB. Narrower Q settings, for example, to 0.6, result in further level boosts again.
The advantage of Proportional Q as compared to Constant Q designs rests with the musically superior way it functions. The wave energy which resides below the bell curve remains essentially the same and in the process, retains the balance of high frequencies in relation to the entire frequency spectrum as one experiments with varying Q values. While it is true that one must think independently of the scaled HF boost dB values in such cases (because these only apply to a value of 1), the result is a simpler, more musically sensible and worthwhile way to work that does not require continual additional corrections.

MHF-HF Cut
The MHF-HF Cut band is similar to a shelving filter that can reduce higher frequencies in a wide bandwidth. It is appropriately wide, beginning with 580Hz and extending to 19.5kHz, a range of over 5 octaves and overlapping the lowest, LF-LMF Cut Band by just about two octaves. With it one can lower a very wide bandwidth and with the peak mid range filters further reduce—or raise—specific ranges. The process can result in the creation of very interesting curves. Here the maximum cut is -14.5dB, while the maximum boost reaches +12.5dB.
The Passeq is not limited to any one particular kind of application, and, for example, is also especially well suited to processing individual instruments in recording sessions. In such cases the wide downward reaching MHF-HF Cut band may be play an exceptional role. Individual instruments can easily be cut upwards, either to give them a more compact sound or when higher frequencies might be supplied from different microphone—or because the mix simply suggests it.