Almost two years ago I reported being very impressed by the sound sculpturing capabilities of the Zo Personal Subwoofer. digiZoid has now released version 2, and brought some significant upgrades to the pocket-friendly headphone amp. With so many quality dedicated digital music players (the Cowon C2 or the Colorfly C4, for instance) and high end smartphones already offering pretty decent audio reproduction, is it worth laying out extra cash for sonic enhancement technology that sits between device and earphones? I've been giving a review unit a good testing over the last week or so in an attempt to answer that very question.

It's claimed that the Zo takes lifeless source audio and injects some much-needed color into the soundscape, transforming a run-of-the-mill music player into something matching the kind of performance offered by high-end audio systems. Lofty claims indeed, but as I found with the first version of the powerful audio accessory, they're not without substance and the second generation Zo has even more to offer the discerning listener than its predecessor.

What's in the box?

The contents of the Zo 2 box are the same as the previous version. There are two audio cables to connect the device to an audio source via its 3.5mm jack – one long and one short. There's a USB charging cable, a Smart Start Guide and a color reference guide for the new additions to the LightBar (more on this shortly), and of course the Zo itself.

The new version is exactly the same size and weight as the 2010 model, at 2.75 x 1.5 x 0.38-inches (70 x 38 x 9.6 mm) and 0.94 ounces (26.6 g) respectively. The first noticeable change is the soft-touch matte coating of the high-impact resistant polycarbonate outer shell instead of the scratch-prone shiny surface of the first Zo, and the LED window is now a transparent plastic affair rather than a mirror-like panel. The pictorial audio in and out guides on the top have changed to an image of combined eighth notes from musical notation to indicate the port for connection to the audio source and a speaker symbol for the headphone/line-out.

Preparation is everything

To best judge any differences in audio delivery and enhancement between the two versions, I gathered together the same movie and music tracks as used for my first review. At the risk of repetition, these were: the classic musical fantasy movie from 1980, The Blues Brothers - still a firm favorite. The audiophile-pleasing Money from Pink Floyd's Dark Side of the Moon album (at a bit rate of 320kbps and an audio sample rate of 44kHz) set the tone for the musical menu. Blues guitar feast The Ballad of John Henry by Joe Bonamassa (256kbps/44kHz) as an apéritif, followed by a main course dish consisting of a reworking of ZZ Top's Precious and Grace by Queens of the Stone Age featuring Billy Gibbons (128kbps/44kHz) and Coke by the Drum (128kbps/44kHz). Finally, my insatiable sweet tooth got pleasantly satisfied by Stanley Jordan's Forest Gardens (256kbps/44kHz).

With the stage nicely set, the time arrived for first switch on.

Ringing in the changes

As I mentioned in the opener, digiZoid has blessed the new Zo with a host of enhancements and improvements. The unit is switched on by pressing in the thumbwheel once, which presents the first change to the device. Just like the original version, the new personal subwoofer offers 32 levels of pre-tuned contour profiling courtesy of the company's SmartVektor technology but pressing the thumbwheel a second time takes you into previously uncharted territory.

The Zo 2 now features two gain modes. There's a High Gain headphone out setting that significantly boosts the volume output from the Zo 2, so much so that I reduced my player volume by half to achieve the same level of output which obviously has player battery life benefits. Then there's a Low Gain line-out mode used when connecting Zo 2 to an audio source with a non-adjustable output level such as a Line-Out Dock.

When in Low Gain mode (compatible with output impedance from 16 - 80 ohms, maximum input voltage of 2.8 Vpp), the volume of the player is fixed and the Zo 2 is used to control the output level by selecting from 32 volume levels which causes the blue LED to get brighter as the volume is increased until it changes to purple as a warning that the next notch up will be the High Gain mode.

To enter the High Gain mode (compatible with output impedance from 16 - 250 ohms, max input voltage of 1.47 Vpp), the thumbwheel needs to be held in the up position for eight seconds, immediately identified not only by the LED changing to shocking pink but also by a hefty increase in volume – as much as 20dB of gain at ~30Hz. The first time the Zo 2 is turned on, it's set to High Gain headphone mode. Depressing the thumbwheel takes you back to the contouring mode.

With the original Zo, it was possible to bypass the contour circuitry when the unit was switched off and so compare the before and after, so to speak. This facility has been removed in the Zo 2 to make it possible to incorporate a Line Out feature, so if you want to compare enhanced and non-enhanced audio you'll need to spend some time connecting and disconnecting cables. I also found that the process of switching the unit on and off was a little on the noisy side with the original, changes to the circuitry of the second version have led to improvements in this regard, too.

Internally speaking

The circuitry has been given a bit of a refresh as well. The new subwoofer incorporates a 16MHz 16-bit Ultra Low Power Microcontroller from Texas Instruments on a gold-plated, four layered printed circuit board in place of discrete logic for better system management. There's a Class-A headphone amplifier and ultra-low noise pre-amp, and audio input circuitry has been enhanced to reduce EMI/RFI susceptibility.

The included Li-Polymer battery recharges in less than an hour (reduced from two hours with version one) via the included USB cable and offers playback of up to 17 hours. digiZoid says that the battery should be good for about 700 recharge cycles before it will need to be replaced, at which point users can get in touch for a new one (with full instructions provided on how to remove the old and fit the new). The Zo 2 now features a Low Battery Indicator and will automatically exit High Gain mode when the battery starts to get low in order to conserve power.

Start the audio bombardment

Although I opted to watch the same movie as before, and listen to exactly the same music, using the same players, I expanded my scope by adding a few more devices to the mix and, after discussing a previous issue with digiZoid's CEO Paul Berg, I was also able to use the Zo 2 to control the bass profile of my guitar amplifiers to great effect.

While the original Zo increased the playback volume a little, the High Gain mode of the latest version is a very welcome and most impressive addition, most notably when used with our car stereo. Since recently changing vehicles I find myself missing the performance of our previous car's sound system.

Under normal circumstances, if I increase the volume to do justice to Disturbed's latest tunes or add a fair splattering of bass EQ for New Model Army's earlier works then the car's speakers start to groan and protest – so much so, in fact, that I generally give up and suffer low volume or flatten the EQ. So I was more than pleasantly surprised by the distortion-free volume boost that the Zo 2 gave to the in-car entertainment, significantly increasing the output without even the slightest complaint from the car's puny speakers.

Well, that's not strictly true. If contouring was increased to the very end of the red zone then the car's system did start to show the slightest sign of a wobble. However, I must stress that the fault lies with the speakers and amp in the car and not with the Zo, which has become a most welcome addition to the cabin and a lot cheaper and much less hassle than ripping out and replacing the resident speakers.

Elsewhere, testing proved just as impressive as with the original. Where other portable headphone amps are nothing of the sort, the Zo 2 has a slim profile that hardly took up any extra room at all in my jacket pocket. Although digiZoid has improved the LightScale resolution to show a distinct color change for each sound contour setting and tune each one for more equally spaced intensity changes, finding a desired profile setting has more to do with the ears than the eyes.

The device also saves the last chosen profile setting at switch off, so if you've been using the Zo 2 with headphones and then move to the car's stereo system without altering the volume first you're in for quite a blast when you switch on. As with before, to get the best from the Zo 2's contouring capabilities, users will need to set any EQ settings on the source audio device to flat and cancel any sound processing technologies like Dolby or SRS.

Just a few important audio specification tweaks to mention before closing. With a full battery, in High Gain mode and at highest contour setting, the Zo 2 is said to have a Total Harmonic Distortion of 0.004 percent (at 1kHz), Intermodulation Distortion of 0.010 percent (at 250Hz and 8kHz), a Dynamic Range of 87 dB and Stereo Crosstalk of 89.2 dB. With the contouring reduced to its lowest level, the frequency response is reported to be 20Hz - 20kHz ± 0.15 dB.

digiZoid told us that three changes have been made to the contour profiles. The lowest (green) profile is said to have a flatter frequency response, so it acts more like just a headphone amp. The highest (red) profile gives slightly higher bass intensity and with each incremental step in contour profile, the change in bass intensity is less than before (especially for the lower 1-8 profile levels).

So, to answer my opening question – although the suggested retail price of US$119.95 might seem a touch on the expensive side, I'd have to say that this is money well spent.

Source: digiZoid

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