Z-Systems' z-Qualizer EQ is a 6-band mastering-quality digital equalizer. It is the budget-conscious cousin of the company's flagship stereo mastering
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Z-Systems' z-Qualizer EQ is a 6-band mastering-quality digital equalizer. It is the budget-conscious cousin of the company's flagship stereo mastering EQ, the z-Q2, and features the same low-distortion EQ algorithms, 24-bit/96 kHz capability, and complete parametric control. By scaling back the z-Q2's size and interface features, Z-Systems has crammed world-class EQ processing into a half-rackspace box.


The z-Qualizer has a modest look, with an unpainted steel case and a basic black front panel dotted with five 1/2-inch knobs (see Fig. 1). Three rotary dials labeled Gain, Frequency, and Q are located below the central data screen. Each knob performs its dedicated function when the unit is in the primary EQ mode. The knob set performs other multiuse duties when you select auxiliary modes.

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FIG. 1: The rotary knob on the front panel''s left side accesses the Z-Systems z-Qualizer''s eight modes. The chosen mode dictates the roles of the pots located beneath the display.

A yellow-green LED indicates stereo (S) or dual-mono channel status in the EQ modes. During dual-mono use, an L or R blinks to alert users that the left and right channels are being controlled independently.

A Signal Present LED in the screen lights to indicate an external digital clocking source at the input, but the display does not indicate either an audio signal or a gain level. The z-Qualizer doesn't have a gain meter, current sampling-rate indicator, or clipping meter in any mode.


The rotary knob on the left side of the front panel controls z-Qualizer's various modes of operation. Each of these eight modes has its own status light, panel display, and roles for the three central knobs.

The selectable EQ function modes are located in the upper positions of the main pot (L is left-channel only, R is right-channel only, and S is stereo linked). Listed clockwise, the other five modes are Bypass, MIDI, M-S (middle-side or mono-stereo) and Dither, Save, and Load.

Bypass disables all EQ band settings while keeping previous stereo or dual-mono parameter adjustments in memory. One smart and user-friendly feature of the z-Qualizer is its ability to store a complete set of left, right, and stereo settings simultaneously. Consequently, separate dual-mono EQ curves are not changed or reset if you engage the stereo EQ mode while scrolling to the bypass position.

Loading and saving EQ presets is easy. Just select a numbered memory location (01 through 99) and click on a knob. You can't name z-Qualizer presets, but they are kept in memory if the unit is powered down. They can also be saved and accessed from a MIDI sequencer.

Whereas most of z-Qualizer's functions are self-explanatory, using the unit's M-S or Dither menus mandates a trip to the manual. For dithering, you can select from among eight types: dithered or undithered 24-, 20-, and 16-bit types, and 16-bit POW-r dither types 2 and 3 (which are usable only at 44.1 or 48 kHz sampling rates). The documentation has details about the recommended uses for these algorithms.

M-S processing is unrelated to dithering and appears as yes/no commands on the remaining two central knobs in that mode. M-S is basically an encode/decode operation that allows separate EQ and gain control over center-panned and side-panned information in the stereo spectrum. Removal of vocals in a 2-channel mix is the most common application of that process.


The rotary pot on the front panel's far right side governs the selection of four separate and fully parametric EQ bands, two shelving EQs, and an overall gain control. When adjusting EQ or channel gain, one knob-click represents 0.1 dB up or down within a range of ±3 dB. Progressively larger gain increments are set for EQ and overall gain, and the maximum ranges are +12 dB to -95 dB.

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FIG. 2: The z-Qualizer's rear panel has a pair of AES/EBU input and output jacks, MIDI In and Thru jacks, and a power connection.

The frequency settings are preset at 1/6-octave intervals and are the same on all four bands. Q ratios range from 0.4 (very wide) to 8.0 (very narrow), with a generous array of 22 intermediate increments.

The z-Qualizer's back panel contains only a pair of AES/EBU input and output jacks, a pair of MIDI In and Thru jacks, and a connection for the external 9 VDC wall wart (see Fig. 2). The unit doesn't have a power on/off switch, analog I/O, or S/PDIF connectors. The underside of the module has three threaded sockets for mounting the chassis on a rack tray.


At my Headless Buddha Mastering Lab facility, I typically use the 5-band digital equalizer program in my TC Electronic M2000. I have grown attached to the convenience and features of the M2000's EQ program and have learned to appreciate some aspects of DAW-based EQ software. But after just one mastering session, it was easy to appreciate the z-Qualizer's pristine sound quality and range of control. It is especially conducive to subtle mastering tweaks, thanks to its 0.1 dB gain steps and the surgical precision of its Q settings. Z-Systems' EQ algorithms reminded me of the versatile and sweet-sounding filters in the AMS Neve Digital Film Console at Skywalker Sound.

The z-Qualizer also has remarkably low latency and no problems with zipper noise or audible clicks when changing gain or EQ on the fly. The 6-dB-per-octave high- and low-shelving EQs are gentler than what I'm used to and a lot easier on the ears. When I auditioned a wide range of source material, I never heard evidence of typical shelving-gain distortion such as fuzzy or brittle highs or unfocused bass.

In critical upper-midrange frequencies around 3 kHz, the unit was always free of distortion or harshness. At times, the z-Qualizer was so smooth that it was unsettling. I made numerous bypass comparisons to confirm that the Z-Systems digital magic was indeed taking place!

That brings me to some issues I have with z-Qualizer's feature set; the least attractive one is its bypass mode. For mastering applications, I regard instantaneous bypass as crucial, and I prefer a simple, industry-standard bypass button for this oft-repeated task.

Repeated visual confirmation of the bypass position, followed by the necessary return to the desired EQ bank, became an annoyance due to z-Qualizer's mode knob, which was sometimes unresponsive. In addition, the built-in bypass switching delay of as much as 1.5 seconds — programmed to eliminate switching artifacts — was a distraction.

Although the 0.1-dB-per-click-stop gain increment is standard for mastering, the action of the z-Qualizer knobs is not conducive to quick, coarse changes. A fast twirl of the EQ gain knob skipped over several of the associated pot's soft-click stops, producing a net gain change of only 0.3 to 0.4 dB. In such cases, five or six such turns are required to affect a 2 dB gain change. That characteristic of z-Qualizer's pots makes it difficult to audition a manual bypass on a single band.

Some users also might find the absence of an S/PDIF jack to be a disadvantage. And anyone planning on using the z-Qualizer regularly will want to invest in a rack tray. This model needs to be at eye level and in a rack for ease of viewing and 360-degree access to the knobs.


Z-Systems deserves an award for putting the power of its high-end mastering EQ at the fingertips of small-studio owners. Besides its mastering uses, the z-Qualizer is an ideal addition to any DAW front end. The unit's features also make it an excellent choice for normalizing or premastering at the end of any digital signal chain. Overall, the sonic quality of the z-Qualizer is subtle, pristine, and utterly transparent.

Although the z-Qualizer's performance was slowed by switching delays and sluggish controls and it lacked some conventional features, its value and sonics are a bargain. No competitor with z-Qualizer's attributes is available in its price range. My reviewer's crystal ball tells me that this affordable stereo mastering EQ is going to be popping up all over the personal-studio landscape soon — starting right here in my own mastering lab.

Myles Boisenis the CFO (chief frequency overseer) and dishwasher at Guerrilla Recording and The Headless Buddha Mastering Lab in Oakland, California.

Z-Qualizer Specifications

Inputs/Outputs 24-bit AES/EBU Gain Control -95 dB to +12 dB Filter Types (4) parametric, (2) shelving Center Frequency Resolution1/6-octave ISO 28 Hz-20 kHz Filter Bandwidths Q = 0.4-8.0 Shelf Filter Slopes 6 dB/octave (first order) Channel Separation effectively infinite Dither Types 24-/20-/16-bit proprietary floating-point, 16-bit POW-r Dither Dynamic Range > 144 dB THD+N < -135 dB Processor Type 32-bit floating-point DSP Sampling Rates 32/44.1/48/88.2/96 kHz Power Supply 9 VDC @ 500 mA Dimensions 8.5" (W) 5 1.75" (H) 5 8" (D) Net Weight 3 lbs.



digital equalizer



PROS: Same world-class, low-distortion EQ algorithms used in top mastering gear. Is 24-bit/96 kHz compatible. Stereo or dual-mono modes. Broad range of gain, frequency, and Q parameters. Affordable price. M-S processing. Has 99 preset locations. MIDI control. Generous selection of dither types. Compact half-rackspace case.

CONS: Bypass mode is unwieldy. Some controls are slow to respond. Unit needs to be rackmounted. No gain, sampling rate, or clipping indication. No preset naming. Lack of S/PDIF I/O is a disadvantage for some applications.