Digizoid ZO Portable Subwoofer Review

dzzomain Digizoid ZO Portable Subwoofer ReviewDigizoid – or, digiZoid, as they write it – is a relatively young company from Arizona that specializes in sound enhancing techniques. Their patent-pending technology called Smartvector promises to improve several aspects of an audio signal originating from any source – be it from a portable MP3 player, be it in a recording studio used during mixing/mastering, be it while watching a movie on a home cinema setup, be it in a live DJ setup.

Since Digizoid are very secretive about their technology, it is easier to say what Smartvector is not: it is not a run-off-the-mill bass booster, it is not an EQ, it is not some psychoacoustic algorithm, it is not digital. Smartvector operates in the analog domain; it recovers the signal’s dynamic range, expands the spatiality (soundstage), and extends the low-frequency cutoff of a speaker/driver, making it deliver lower note extension than generally possible. I don’t know how they do it, but it doesn’t affect the THD (total harmonic distortion) of the signal, so they certainly don’t go the cheap route of harmonics enhancers/exciters.

Digizoid’s first commercial product utilizing Smartvector technology is the ZO, a portable amp, or “personal subwoofer”, as they call it. The ZO uses only a portion of Smartvector, named Lofreq. Unlike a still-theoretical Fullspec variant, affecting the whole audible frequency range, the ZO only operates on frequencies up to about 1 kHz.

Being a fan of natural sound reproduction – contrary to ‘neutral’ – I have to say that the ZO is the best thing I’ve heard so far in portable sound enhancements. Until now Cowon’s BBE and Mach3Bass have been the cream of the crop to my ears, as far as putting some excitement in ‘polite’ phones is concerned. The ZO however is the new king of crisp, precise, yet bassy sound, if you ask me.

Don’t be put off by the “personal subwoofer” slogan. The ZO is not some cheap boombox replacement for trunk rattlers. It is a very refined sounding tool to make audio more enjoyable – without damaging any part of it. Read on if it’s the thing for you.

  • Digizoid ZO Specs
  • Physical: 2.75 x 1.5 x 0.38 in (7 x 3.8 x 0.9 cm), 0.94 oz (27 g)
  • Power: run time up to ca. 12h, full charge 2h (100%), rapid charge 1h (80%)
  • Input: 3.5mm stereo, 2.0Vpp max. (0.5Vrms), 15 kOhm
  • Output: 3.5mm stereo, 840mW@25 Ohm (per channel), 190mA@3.7V, 0.2 Ohm
  • Frequency response: 20Hz-20kHz +/- 0.25dB (min. contour)
  • Noise level/dynamic range: -87.9dB/87.6dB (A)
  • THD: 0.003%
  • THD+N: -80.9dB (A)
  • IMD @ 10kHz: 0.015%
  • IMD+N: 0.014%
  • Accessories: USB charging cable, 12cm 3.5mm-to-3.5mm cable, 90cm 3.5mm-to-3.5mm cable, printed manual

Design, Specs

Until you turn it on, the ZO is quite an unassuming little black box. It is a bit narrower than a credit card while having about the same length. Of course it’s a bit thicker. It pairs well with even the tiniest and slimmest portable players, such as a Sansa Clip+ or Cowon i9. It is made of shiny Lexan plastic, and thus it’s a bit of a fingerprint magnet. I was told the next version of the ZO will have a rubberized shell.

On one side there’s the silver Digizoid logo, on the other there’s the minimalist ZO lettering. Above that is what they call a “Lightscale” indicator, showing the processing strength. Lightscale means there’s a green and a red LED mixed in varying intensities: green means low processing, orange means medium, red means high. It gives a rough estimate of the current level in the 32-step sound processing spectrum the ZO provides.

On the top are the 3.5mm in- and output jacks. On the sides we find a regular mini USB jack for charging, and a rocker that turns the device on or off when pushed, next to changing the Smartvector processing intensity. This is not a volume control – the ZO isn’t made to work with line-outs on portable players, it works with volume controlled headphone-outs. The upcoming second version of the ZO might have a volume control. Personally, I’m not missing a volume control at all, since all the devices I attached the ZO to can change the volume on their own – but for some applications, like a line-out on an iPod or CD player, it sure could come in handy.

The ZO charges from empty to full in about two hours, a one hour quick charge gives about 80% capacity. A blue LED besides the USB port lights up during charging and goes off when the battery is full. The ZO can be used while connected to USB, but that depends on the quality of the USB port used for charging. Some cheap AC chargers or laptop USB ports might be too noisy, resulting in audible distortions and hiss. With quality chargers there’s no issue however. Battery life of the ZO is acceptable-ish, somewhere between 6 and 12 hours, depending on processing strength. It’s not great, but I get about the same battery life out of the ZO as I get with the Sansa Clip+, so that’s a decent match. The next ZO might have a bit better battery life.

Even when the battery is dead – or one just wants to turn the ZO off – there’s still sound coming through from the source. The ZO’s bypass functionality is a very welcome feature. It’s basically the same sound as the lowest processing setting, just without the ~6dB gain / ~20dB dynamic range amplification (more about the ZO’s dynamic range later).

Speaking of noise – the ZO doesn’t seem to be properly EMI/RFI shielded; it does pick up some electromagnetic interference when it’s close to a cellphone – on my GSM 900MHz phone, at least. Paired with a laptop however I had no interference issues when both Wifi and Bluetooth are turned on.

As for other noise, the ZO doesn’t play overly nicely with the impedance-matching output of my netbook’s Realtek HD sound card. There’s quite some background hiss going on – which can be easily fixed by using an in-line volume control (instead of the OS’s software volume control) or an impedance adapter between the sound card and the ZO. I’ve been using a 250 Ohm Ultimate Ears adapter, which works well without compromising sound quality in that setup. Other amps might hiss with such sources as well, so that’s nothing out of the ordinary.

With other portable sources, such as a Sansa Clip or a Cowon J3, the ZO behaves generally well. There I a very slight background hiss that’s barely audible with most regular dynamic driver IEMs and completely inaudible with most full-sized headphones. With some ridiculously efficient and über-sensitive 120dB/mW+ multi-armature IEMs like the Shure SE530 or Ultimate Ears UE11 the ZO’s background hiss however is clearly noticeable and seriously annoying. It’s the same hiss on all enhancement level settings, from 0 to 32, even when no source is plugged into the ZO. Increasing the volume on otherwise dead quiet sources, such as the Cowon J3, increases the hiss on the ZO as well with such phones attached to it. Contrary to the “input fix” I mentioned above, an impedance adapter on the ZO’s output won’t do much good. It will get rid of the hiss, but added impedance usually changes the sound quality of such phones to the worse. Even with single moving-armature phones like the Ortofon e-Q5, added impedance changes their sound signature dramatically (loss of bass, increased treble).

Either way, other multi-armature IEMs such as the Westone 4 or Earsonics SM3 behave much better with the ZO. They still hiss a tiny bit, but in their case it’s generally negligible. I think this tells more about the sub-par R&D of Shure and Ultimate Ears than about the ZO. The Westone 4 and SM3 are a newer generation of multi-armature IEMs, with better designed, noticeably more refined and easier to handle crossovers.

To be fair, I only know of two amps that are able to drive the aforementioned UE11 and SE530 without hiss and background noise, the Headstage Arrow and the Corda Headsix. Every other portable amp I know drives those annoyingly difficult phones with as much hiss or even more than the ZO – from the inexpensive FiiO E5 to the overpriced Minibox E+. This means – despite my nitpicking – that the ZO is about average as far as hiss with badly designed phones goes. It’s far from being the worst, but in an ideal world it certainly could perform better. It’s not Digizoid’s fault anyways – clearly Shure and Ultimate Ears are to blame for designing such botched circuitry.

Another issue – one that personally bugs me more than some slight hiss with some of my regularly used IEMs – is that the ZO generates quite a voltage spike at turning it on or off. It’s a very loud, sharp click/pop. This isn’t good for both, headphone drivers and human ears. Luckily, Digizoid is going to fix this in the upcoming new version of the ZO.

Enough with the rants, back to the quality stuff. The ZO is powerful enough to drive basically any high impedance or low sensitivity phone you throw at it. It doesn’t break a sweat driving some 300 Ohm Sennheiser HD650 or some inefficient DBI Pro-700 or Koss KSC75. As far as amplification goes, the ZO adds about 4-6dB of ‘regular’ gain to the input signal, which translates to ‘twice as loud’ in most cases, similar to what the FiiO E5 or E7 manages.

However, the ZO isn’t about this pedestrian volume boost that any other old amp brings to the table. What makes the ZO special is that it can recover up to 20dB of dynamic gain. Dynamic gain is something you usually find mentioned in the description of the professional $6000 Massenburg Model 8900 controller and similar, not in portable end-user devices. It means the ZO can recover/increase the dynamic range of the input signal, and present it to human ears in a ‘musical’ manner that makes sense to the hearing apparatus. This is especially obvious with badly mastered, ‘brickwalled’ contemporary pop music that lacks dynamic range and ‘excitement’ by restlessly pushing the 0dB ceiling. Instead of a constant onslaught of maximum SPL on the ears, the ZO is able to recover breathing room for the audio signal, providing it more punch, space, and depth.

If I could request one additional feature from Digizoid for their upcoming v2 ZO, it would be automatic on/off, like the Headstage Arrow amp has: you turn your MP3 player on, the ZO turns itself on as well – you turn your MP3 player off, the ZO turns itself off after, say, two minutes. That would turn the inconvenience of carrying an additional device around into a nice set-and-forget experience.


A lot of stuff boosts bass. There are good and bad EQs, good and bad sound enhancements based on psychoacoustic principles. There are several approaches to the issue, some based on adding harmonics, some based on nifty FFT, some simply tweaking the frequency domain, such as the usual loudness contour curves. What Digizoid’s Smartvector does is a secret, I don’t know the nitty-gritty. Their trademark application reads, “Electronic products for the manipulation of the frequency, time, and amplitude characteristics of audio signals, namely, audio processors,” which really doesn’t tell me much. Their online guide mentions “isophonic science […] sound contouring system […] with matched acoustic response to the human ear,” which is equally cryptic.

The most apparent change to the sound one notices with the ZO is indeed boosted bass – but not only in the usual sense that the already available frequencies get louder. Instead, the existing bass also gets deeper, seemingly defying laws of physics and electronics. Usually, a given speaker can’t reproduce frequencies lower than its margin of efficiency allows, it can only move so much air before it stops working properly and starts to distort. With the ZO however even extremely bass-deficient phones, such as the Koss KSC75, AKG K701, or any Etymotic model, can become natural sounding in the lower audible spectrum. However, the ZO doesn’t create bass when there is none in the source material. Recordings of string quartets will basically stay untouched by the Smartvector processing, they won’t be turned into a rave party.

Well, that added bass sure sounds excellent in every aspect. It’s by the book – tight, punchy, textured, roomy, deep, not intruding the midrange. I’ve did some RMAA tests with the ZO, which shows that the main peak of the ZO is around 20Hz to 60Hz, in varying intensities depending on the processing level. However, not knowing what the ZO actually does, I don’t want to draw any conclusions from those tests. I just know it sounds great, with basically any phone you throw at it. With bass light phones like the Phonak PFE or the Westone 4 one can turn the ZO up to full level to make them really kick (while still retaining their crystal-clear analytic abilities), with bass heavier phones like the Radius DDM2 or Sennheiser IE8 a processing setting in the lower quarter of the ZO gives these phones’ bass some more tightness and punch.

It doesn’t end here, bass isn’t everything the ZO provides. While the current model only supports the Lofreq part of Digizoid’s Smartvector technology, affecting frequencies up to about 1kHz, it noticeably fattens the midrange, making it more lush, palpable sounding – without muddying or veiling anything. Stereo imaging is widened by the ZO as well – think of it as a kind of ‘anti-crossfeed’. While crossfeed helps in making old 1960/70ies hard-panned recordings listenable on headphones without getting an aneurysm, the ZO makes well recorded modern tracks even more enjoyable by widening the soundstage. For complete sonic witchcraft, I’m of course using the ZO in combination with Rockboxed audio players, running a crossfeed implementation based on Jan Meier’s design. In my home setup I’m using Refined Audiometrics’ HDPHX VST plugin to achieve the same.

Another positive side effect of Smartvector processing is that phones sound subjectively better at lower listening levels with it. One slogan on the ZO’s packaging reads, “Blow your mind. Not your ears.” – which is certainly fitting. With that overall bass and added dynamics and lushness one really doesn’t have to turn things up to eleven. Your ears will thank you for that in years to come.

No matter how I tried, I couldn’t get the ZO to distort, even at its highest processing level, while blasting some filthy dubstep tunes with ridiculous amounts of bass. It seems safe to say that as long as the source player doesn’t distort, the ZO won’t either. With matched levels it performs like one would expect from a high priced piece of studio gear.

I can’t wait to hear what an upcoming Fullspec Smartvector implementation will sound like, compared to the current Lofreq-only ZO. There is a preliminal demo recording (MP3, 5MB) available here, but as I understand it, it can’t actually show what the real hardware is capable of. One needs to hear the device, not a recording of it.

Such a Fullspec implementation of Smartvector would probably have to be tweaked like current generation BBE Sonic Maximizer sound enhancement hardware, with two separate dials for bass and treble enhancements (labeled “Lo Contour” and “Process” on BBE’s 19” rack units). At least that’s what I would wish for, being able to tweak bass and treble separately – catering to phones with recessed treble and forward bass, like the AIAIAI TMA-1, as well as the opposite, such as the KSC75 or AKG K701.

One thing that hopefully will be changed in the upcoming version of the ZO is the weighting of the processing scale. On its scale of 0 to 32, the first few steps make the most difference in the resulting sound, while from step 16 to 32 there is barely any noticeable change. I would appreciate it if Digizoid made the first few steps of the scale more resolving – there could be several more steps between 0 and 1 already, allowing one to fine-tune the processing for bass heavier phones. I would appreciate a truly linear scale increase instead of the almost logarithmic lower quarter, and the barely changing upper half of the ZO’s processing range.


The ZO does what most other portable amps aren’t able to do: it indeed makes the sound subjectively better. It has no detrimental effects on the source audio, and it is able to beef up any signal if one wishes so. This is more than can be said about much higher priced ‘audiophile’ amps that are hyped in certain circles. The ZO might not have a brushed steel chassis or overpriced boutique capacitors and opamps – but contrary to those products, it actually affects the perceived sound quality in a positive, audible way – besides the usual increase in loudness.

I’ve reviewed a few amps so far, and while some of those actually fix some flaws with the outputs of portable players (hiss, bass roll-off, etc), or incorporate a regular bass booster or crossfeed – the ZO is the first one that adds a veritable wow factor to the sound. No, not the awful SRS WOW kind – a real WOW, coming from the guts, that makes me grin uncontrollably.

I would highly recommend the ZO to people looking to beef up their phones/players, people that like visceral yet natural, clear sound. This thing really goes in the right direction. It doesn’t matter much what genres of music we’re talking about, the ZO works with everything I threw at it. Purists might not like that approach, but as an audio engineer I would go out on a limb and say this is actually better than ‘how the artist meant it to be’. Either one likes to tweak one’s personal sound preference, or one doesn’t. The ZO surely can deliver the goods, if one wishes so.

Karen and Paul, the people behind Digizoid, really seem to stand behind their product and are dedicated to go a new, different way in audio enhancement technology. I hope it catches on, and companies start implementing their Smartvector technology in various products. The market is a crowded one – with BBE, SRS, Samsung DNSe, Sony DSSE, Creative Xmod, and dozens of others – but once you heard the ZO you likely won’t settle for less anymore.

The ZO seriously spoiled my on-the-go listening habits. It’s hard going back to listening without it. Don’t take my word for it – you have to hear it yourself.


  • Best bass enhancement I’ve heard to date
  • Improves stereo imaging and ‘fattens’ the overall sound without muddying it
  • Increases the dynamic range of the input signal
  • Amplifies the input signal as well, works great with wimpy MP3 players
  • Clean and precise sound – no detrimental effects when bypassed
  • Makes phones sound subjectively better at lower listening levels
  • Small and lightweight
  • Inexpensive and more useful than ‘audiophile’ amps


  • Loud click/pop at turning on/off (will be improved in an upcoming version)
  • Battery life could be a bit better (will be improved in an upcoming version)
  • Processing range steps could be better weighted
  • Background hiss with some sensitive in-ear phones and/or sub-par sources
  • Insufficient electromagnetic shielding, picks up cell phone interference


Distributors can be found on Digizoid’s website. Suggested retail price of the ZO is $119, but one can get it for $70 or so, if one looks around a bit.



shotgunshane on June 16, 2011 11:24 PM

Absolutely great review dfkt. Very well written. I really like how you are able to explain what the zo does for music in simple, straightforward tems. It would be great if digiZoid could link to your reivew on their site.

Jorge on June 16, 2011 11:35 PM

Very interesting! I’ll keep an eye on it and probably buy the fullspec version :)
Great review, thx for sharing

ziocomposite on June 17, 2011 2:12 AM

I love my Zo and your review is a great one. Gave me chills lol *while listening to Zo* =P

tds101 on June 17, 2011 4:15 AM

Great review Martin, and I’ma damn glad I got one of these – makes my headphones give me that bass-luv!!! Seriously, I’m very glad for the recommendation, best $$$ I’ve spent in quite some time.

Now, how about a quick post about those dbi pro-700′s in the forums when your able,…pwease???

MarvintheMartian on June 17, 2011 6:08 PM

Very interesting! Just when I couldn’t think of any gear I really wanted to buy anytime soon, I read this….and idea on when the v2 will be released?

ironsnakemx on June 18, 2011 8:59 AM

Holy s…. i can just imagine what this thing will do to my RE0 …… gotta have it ASAP

Confispect on June 18, 2011 12:20 PM


The Saxmaster on June 28, 2011 12:32 PM

This sounds like that something-or-other I’ve been looking for. Nice review.

Mark Gleeson on July 5, 2011 8:46 AM

I really enjoyed your article and the description of the digiZoid. Would this product also make an appreciable difference to an MP3 player playing through a Bose Sound Dock during workshops I facilitate?

Thanks for your time (and article),

Martin Sägmüller on July 6, 2011 2:05 PM

I personally haven’t tried the ZO with small speakers, but I assume the effect should be similar as with headphones. You should definitely get deeper, punchier bass out of your dock.

swirlibra on August 12, 2011 5:53 PM

You can’t buy these anywhere. Whatever happen to this product???

Juraj on October 3, 2011 12:00 PM

Does anybody know, is here in Europe seller ?

User1 on November 26, 2011 5:28 AM

Sounds like a nice piece of hardware. Like that it’s so small.

Since the ZO2 is out, I guess we’ll be seeing a review?

pasan on December 12, 2011 3:41 PM

When referring to the AKG K701, are you speaking from actual testing/experience with the ZO or are you just generalizing? I ask because I have a ZO2 on order and can’t decide between a K701 or DT880/250.

Fred on December 21, 2011 8:16 AM

I have a ZO1, but mine is a tad funny…
Once the Batt in the ZO is dead the bypass no longer works. I guess the Bypass uses energy too.

Dave on December 28, 2011 3:21 PM

Hello and thank you for the great reviews! I recently got a pair of BrainWavz B2s to replace my M3s that had a tragic run in with my puppy. I love the clarity and definition but some of what I listen to definitely misses the low end of the M3s. I had been looking at the fiio E11(I find my iPod Classic G6 thin & noisy anyway plus the EQ only adds mud). Would you recommend the ZO2 over the E11? Does the ZO2 have enough amp going line-level in? How does amp quality compare? Thank you.

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