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Zu Audio Undertone Mk.II

Zu Audio Undertone Mk.II



Undertone is for those searching for that missing majesty and space, for systems that need that added tone in the foundation of big/deep music and soundscapes—and without that slow and foggy one-note sound of a modern under-built subwoofer. It is a down-firing subwoofer with active signal processing and built in amplification. It accepts line-level feeds (RCA left and right summing), sub feed (RCA) and speaker-level (five-way binding posts, left and right transformer isolated and summing).


Undertone is designed specifically to compliment playback systems that have great sound down to about the kick drum, 50Hz* or so, but lack a sense of glory and tranquility that deep bass done right can give. The stereo and home theater systems that best fit Undertone are those that typically feature large monitors or medium to large floor-standing loudspeakers—high resolution and dynamic systems that gives you a solid impression of bass but lack sincere detail of body and weight.

Undertone Mk.II (mark-two) is designed to further compliment high-resolution, wide dynamic range loudspeakers that could use help in handling the dynamic demands of the bottom most octave. Undertone’s excellent integration of sub-contrabass content is stunning and very resolving of the subtle tones, textures and dynamics that live and breath in that deep bass domain—music or film. If you are looking for an agile, deep and powerful subwoofer system that is flexible, dynamically agile and incredibly musical consider Undertone.

(Yes, a stereo pair is even better than just one. Didn’t Confucius say, good things come in pairs….)

Undertone Mk.II Ad Copy & Spec’s Download

Undertone Mk.II features the same electronics and awesome Eminence LAB-12 [A] subwoofer driver and tuning we developed for our Definition Mk.IV [Rev-B] loudspeaker, but strips out the high cost of the machined aluminum alloy electronics enclosure and massive base. Undertone is all about giving you state-of-the-art subwoofer performance for a very reasonable amount of cash. For those that have heard our Definition Mk.IV loudspeaker, the performance is pretty close, getting you about 90% of the quality of deep bass sound and 100% of the power.

* Yes, Undertone can be used higher than 50Hz but above 70Hz you are falling outside its design targets.


Undertone Mk.II features a new and improved built-in amplification, specifically the line-level stages which have lower noise levels, improve signal resolution, and are designed to more intimately drive the UcD amplification modules. Specific changes are:

  • Lundahl loudspeaker-level input transformers. This addition lowers system noise, balances the input and removes ground reference, improves system transient response and fidelity generally, and allows any main amplifier to be paired with the Undertone Mk.II subwoofer system.
  • Premium, 1% precision Vishay resistors on line-stage. These high quality parts improve system precision and accuracy, with values to work in conjunction with the Lundahl inter-stage transformer to give improved performance in subwoofer/main loudspeaker blending.
  • Zu Event-level internal interconnecting cable harnesses reduce electrical noise further through significantly improved RF rejection characteristics.
  • Replacement of all internal fiber fills with premium open-cell polyurethane foam for improved internal damping and management of acoustic standing waves.
  • AC mains transformer is imbedded top and bottom in a very low durometer damping compound to reduce mechanical AC transformer noise from the mounting plate as well as the signal processing and amplification circuits. This mounting also improves the operational life of the AC mains transformer.
  • Diamagnetic stainless steel hardware is used throughout the Undertone Mk.II subwoofer system.
  • Sweeter knobs. Kilo International machined from billet 211 alloy knobs.
  • Cabinet to amplifier fasteners are stainless steel, both inserts and mating screws. (Note: in the event of removal, stainless to stainless fasteners need a touch of lubricant or anti seize to eliminate the chance of thread galling.)


Device: Down-firing, sealed, 12” subwoofer system with active signal processing.
Height: (with spikes) 25” [635cm]
Footprint: 15 x 15” [38 x 38cm]
Weight: 68 pounds [31kg] each
Bandwidth: 14 – 80Hz
Line-Level Input: yes, RCA (left and right summing)
Sub Line-Level Input: yes, RCA
Speaker-Level Input: yes, 5-way binding posts (L/R summing)
Tunability: yes, very
Signal Processing Bypass: yes, switched for those that want to integrate more elaborate signal processing but desire to take advantage of the excellent amplification of Undertone.
Driver: Eminence LAB-12 [A]
Plate Amp: Zu Subwoofer Plate Amp Mk.III
Amplification: Hypex UCD 400W
Interstage Transformer: Lundahl LL-1585
Line-Level Interconnecting Cable: Zu Event
AC Mains Input: standard C-14 [IEC 60320]
Voltage input: 100–120V or 230–250V factory set
Fuse: slow-blow T3.15A @240V or T6.3A @120V
Hypex UcD amp module made by Hypex, Netherlands
Eminence LAB-12 driver made by Eminence in Kentucky, U.S.A.
Balance of the system designed and made by Zu Audion in Ogden, Utah, U.S.A.


Undertone is made from North American high-density composite wood plate (comp-60) and is machined on state-of-the-art CNC mills in Utah. Undertones finished in wood use real wood veneers that are pressed at the plywood factory and is not laid-up post cabinet assembly. This ensures the finish will not lift or warp, and it yields the flattest surfaces and tightest seams. Titebond-III is used for layup for the highest joint strength and a lifetime of unfailing, moisture-proof service.


Standard paint and wood finishes are top coated by UV protecting premium finish systems. All finish work is done in Zu’s custom finish shop in Ogden. Other finishes—anything you can dream—are available through the Zu custom shop, inquire for pricing.


Old-school, in all the best ways. Undertone is a medium compliance, sealed enclosure system where the restorative driver forces are shared between the driver’s suspension (spider is primary) and the air in the cabinet. Undertone continues our tradition of making quick, detailed “sealed” subwoofer systems.


Undertone’s electronics really perform and offer just enough tunability to be useful and not overwhelming. The amplifying module is the highly regarded Hypex UcD 400. Undertone’s electronics were designed to accept unfiltered left and right RCA line-level inputs, home theater sub-woofer signal, and unfiltered loudspeaker-level feeds (tapping only signal). Undertone is also able to integrate with high efficiency loudspeakers like ours without having to jump through hoops with gain and volume matching. Some design highlights include:

  • Hypex UCD 400 V5 amp module.
  • ALPS pots for volume and signal processing control.
  • Lundahl interstage transformers on speaker-level inputs.
  • Zu Event interconnecting cable harness assemblies.
  • 3M IDC adhesive locking terminals.
  • Panduit disconnect terminals.
  • Neutrik RCA jacks (sound great and as tough as it gets).
  • Extra large toroidal transformer and overbuilt linear power supply.
  • Machined 1/4” [6.3mm] 6061 aluminum amp plate with laser engraved information.
  • Kilo International machined knobs.
  • High-copper brass machined 5-way binding posts on speaker-level inputs.
  • Gain structure of preamp section is designed to accommodate both low and high-efficiency loudspeakers.