Vintage Digital & Analogue Studio Effects

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SPL PassEQ Passive Mastering Equalizer

SPL PassEQ Passive Mastering Equalizer

In the SPL PASSEQ they have revived the concept of the passive equalizer that was used in the 50’s and 60’s. The charming sound of this vintage technology combined with the modern 120V technology results in a very special equalizer.


The SPL PassEQ Passive Mastering Equalizer is a dual-channel, passive equalizer. The SPL PassEQ Passive Mastering Equalizer – Model 1650/1654 is a new revised edition of the well known SPL Passeq – Model 2595. Both units are based on our 120V Rail Technology. The new PASSEQ Mastering Equalizer features the same high-quality characteristics as the previous model, though it was revised and optimized to fulfill all requirements of a Mastering Equalizer.

The SPL PassEQ Passive Mastering Equalizer of the first generation was the first passive equalizer which provides three separate frequency ranges for the boost and the cut stages. With 12 switchable frequencies per band, totaling 36 boost and 36 cut frequencies, the Passeq was the most powerful passive EQ system ever made. The new PASSEQ Mastering Equalizer features the same amount of cut and boost frequencies, though the choice of frequencies was changed and the Q-factor was individually newly adjusted for every selectable frequency with an optimal coil, condenser, resistor combination.

Each boost and cut frequency now got an individual optimized Q factor. Especially the mid bands are much more harmonious for working on program material than they were on the previous model. When we were fine-tuning, through component selection, within countless listening sessions, the focus was on receiving musically sounding curves, which are clearly on par with legendary devices like the Pulteq EQ from the decades of the 1950’s and 60’s, but without all disadvantages of a 60-year-old original, like a high noise level and most of all a very limited choice of frequencies.

All passive filter components (variable resistor, capacitor and coil) deliver excellent sonic characteristics. An important part of this process is played by coil and condenser loading and saturation characteristics. The resulting inertia in contrast to the characteristically extremely fast reaction of active filters is the reason for pleasant, beneficial sonic characteristics. We tend to perceive these attributes in terms of an increased suppleness and transparency, with perceptibly improved, silky highs and warm and punchy basses. Another highlight of the new SPL PassEQ Passive Mastering Equalizer is the HF+ Band, which was expanded with the frequencies 25kHz and 35kHz, sounding so incredibly good, that you don‘t want to turn it off anymore.

Thanks to the big detented output control, you can increase or attenuate the output level by 10dB, to reach following devices with the perfect desired level. This is important, especially when it comes to mastering. A fader ranging from 0dB to limitless, like the previous model provided, would not be helpful in this case. With the PASSEQ you can also adjust the parameters Gain, Q (HF +) and Output in 41 steps with the detented potentiometers. All other parameters can be adjusted with 12-position switches. Thanks to these switches and detented potentiometers, a precise recall is very easy to realize.

In addition to that, the handling of the SPL PassEQ Passive Mastering Equalizer was improved through small corrections of the positioning and naming of the control elements. Like all new SPL Mastering devices, the new SPL PassEQ Passive Mastering Equalizer provides an Auto Bypass. Of course each of the two channels can also be manually activated or deactivated with the big illuminated pushbuttons. Thanks to the Auto Bypass, this can also be realized automatically in a freely selectable time frame.

Filter Types
There is basically only one type of filter used in the PASSEQ: The bell-filter or peak-filter. But since the center frequency of the peak-filter, concerning the HF and LF bands, is only marginally within the perceptible hearing range, these filters auditorily and visually rather correspond to shelf-filters.

Shelf Filters
A shelf filter increases or decreases the energy of all frequencies above or below a chosen frequency. Depending upon the direction of processing one refers to high frequency (HF) or low frequency (LF) shelf filters. Beginning with the threshold frequency, the frequency band is boosted or cut much like a shelf. The maximum boost or cut achieved at the point furthest from the threshold frequency. The threshold frequency is usually about 3dB less (with the overall increase set to maximum). This gives the typical rising form of the shelf filter’s response curve.

Peak Filters
A peak filter boosts or cuts a chosen frequency‘s energy with a maximum amplitude and a definable frequency range around this frequency with a fall off of up to 3dB to both sides. The chosen frequency with the maximum amplitude is called center frequency—it takes place in the middle at the peak of the response curve. The response curve forms a bell, thus peak filters are also often referred to as bell filters.

The width of a frequency range or band is musically defined in octaves. The technical counterpart to this is the “Quality” of a filter, and the abbreviated “Q” is the most common value for the bandwidth of a filter.
A high Q value means a narrow bandwidth while a smaller Q factor corresponds to a wider one:

  • Bandwidth 2 Octave: 0.7 Q
  • Bandwidth 1 1/3 Octave: 1 Q
  • Bandwidth 1 Octave: 1.4 Q
  • Bandwidth 1/2 Octave: 2.8 Q



Analog inputs & output; XLR (balanced)

  • Maximum input & output gain: 32.5 dBu
  • Input impedance: 20
  • Output impedance: 75 Ω
    Common mode rejection: -60 dBu
  • Frequency range (-3 dB): 10 Hz – 200 kHz
  • Crosstalk (1 kHz): -108 dBu
    THD + N (+30 dBu, 1 kHz): 0.0012 %
  • Noise (A-bewertet): -95.2 dBu
  • Dynamic range: 134.5 dB

Internal Linear Power Supply with Shielded Toroidal Transformer

  • Operating voltage for analog audio: +/- 60 V
  • Operating voltage for relays and LEDs: + 12 V

Mains Power Supply

  • Mains voltage (selectable, see fuse chamber): 230 V AC / 50; 115 V AC / 60 Hz
  • Fuse for 230 V: T 500 mA
  • Fuse for 115 V: T 1 A
  • Power consumption: max. 15 VA

Dimensions & Weight

  •  Width x height x depth): 482 x 132 x 300 mm
  • Unit weight: 10.2 kg
  • Shipping weight (incl. packaging): 11.9 kg

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SPL PassEQ Passive Mastering Equalizer

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