Country of Origin


From mosquitoes to mojitos

Reviewer: Srajan Ebaen
Financial interests: click here
Main system: Sources: Retina 5K 27" iMac (4GHz quad-core with Turbo, 32GB RAM, 3TB FusionDrive, OSX Yosemite. iTunes 14.4), PureMusic 3.02, Audirvana 3, Qobuz, Tidal, Denafrips Terminator+ clock-synced to Gaia; Avatar CDP; Soundaware D100Pro SD transport; Preamp: Vinnie Rossi L2 Signature; Power amps: Kinki Studio EX-B7 monos; Headamp: Kinki Studio; Phones: HifiMan Susvara; Loudspeakers: Audio Physic Codex; Cube Audio Nenuphar; Aurai Audio M1 [on loan]; Cables: Complete loom of Allnic Audio ZL; Power delivery: Vibex Granada/Alhambra on all source components, Vibex One 11R on amps; Equipment rack: Artesanía Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands; Sundry accessories: Acoustic System resonators, LessLoss Firewall for loudspeakers, Furutech NCF Signal Boosters; Room: 4 x 6m with high gabled beam ceiling opening into 4 x 8m kitchen and 5 x 8m living room so no wall behind the listening chairs
2nd system: Source: Soundaware D300Ref SD transport; DAC/pre: Denafrips Terminator; Preamplifier: icOn 4Pro Model 3 SE;
Amplifier: Crayon CFA-1.2; Loudspeakers: sound|kaos Vox 3awf, Zu Submission subwoofer; Power delivery: Furutech GTO 2D NCF; Equipment rack: Hifistay Mythology Transform X-Frame [on extended loan];  Sundry accessories: Audioquest Fog Lifters; Furutech NFC Clear Lines; Room: ~4x6m
Desktop system: Source: HP Z230 work station Win7/64; USB bridge: Audiobyte Hydra X+; Headamp: COS Engineering H1; Phones: Final D8000; Powered speakers: Fram Audio Midi 150
Upstairs headfi system: Source: Soundaware A280 SD transport; Integrated amplifiers: Schiit Jotunheim R or Bakoon AMP-13R; Phones: Raal-Requisite SR1a; Loudspeakers: Acelec Model One
2-channel video system: 
Source: Oppo BDP-105; DAC: Kinki Studio; Preamp: Wyred4Sound STP-SE II; Power amp: LinnenberG Liszt; Loudspeakers: German Physiks HRS-120; Room: ~6x4m
Review component retail: varied

UVB. Undue exposure to ultraviolet light is bad for the skin. So sun-blocker skin creams rate their efficacy by sun protection factor. An SPF of 30 blocks 97%, an SPF of 50 blocks 98% and SPF 100 tops off another percent of UV protection. Hurray, skin cancer averted. Well, little chance of skin cancer in Ireland unless you work road construction during our short summers. But Ireland also has another kind of disturbance. It's not dangerous but irritating nonetheless. And no, I don't mean mosquitoes. Something still buzzes though: DC.

It's direct current on the power line. It's often cause for intermittent or constant transformer hum in hifi kit. This morning I was rudely reminded. I'd moved the Zu Submission subwoofer back into the big rig. Plugged straight into the wall it hummed as is its annoying want. So I stole my single i/o Vibex DC blocker from the upstairs Dynaudio 18S sub. It shut up the Submission as it had many times before. But its absence upstairs now caused the Dane to buzz. It originally hadn't when slurping AC straight from the wall. Because just then I had the Vibex slacking off in the utility closet, I'd tethered it to the 18S just in case. This morning clearly was such a case though I hadn't a clue why.

Thankfully my Puritan Audio Labs PS106DC there includes its own blocker. The utility closet's hifi trunk held a required 6m cord. The forward side-wall sub could reach the Puritan well behind the rack on the front wall's center. Voilà, perfect silence from the Dansk 2×9.5" sub. And that's it for the lunar broadcast on this now very fine morning. Should your hifi kit hum—occasionally for seemingly no reason; or more annoyingly, always—there's a very good chance that a DC blocker will kill it. I leave it to you to shop around to suit your budget. Now you simply know the fix. By extension, if acquisition of a power filter or just AC distributor is on your to-do list, why not get one which includes a DC blocker? I find the Puritan at left very effective. It's far from silly money or typical high-end bling. At right, Isol-8 also from Blighty not Blingy have their own. There are many others. If you suffer hum gremlins in your hifi which aren't ground loops, now you know what to try.

For a tech explanation, we let Nic Poulson of Isol-8 do the honors: "Many of the myriad appliances connected to the supply, at home and in industry, from electronic dimmers to computer power supplies, use the AC energy available in a mains cycle unevenly. The net result is the effective addition of DC to the mains network. This effect is inevitable and also inherently unpredictable. In practice the waveform can arrive at the end user clipped, distorted and asymmetric. Simplistic analyses of unmatched peak voltages are not the whole story. Any AC waveform which does not have equal energy in both positive and negative phases will contain a DC component regardless of what the peak voltages are or what the wave shape is. The AC transformers commonly found in audio equipment cannot by nature tolerate significant DC without being compromised. As all transformers convert power through the medium of magnetism, any DC present will inhibit their capacity to transform AC by partially or completely saturating the magnetic circuit. It is this vulnerability to DC that causes acoustic hum and significantly reduces the transformer's power capacity. Even measured at comparatively low levels we have found this to have a significant negative impact on the following power supply and circuit performance which in turn can degrade sound quality."

The upshot is obvious. Even if you can't hear outright hum, your hifi transformers could still struggle or deliver less than 100%. Best eliminate cause. Put on your sun screen, then raise that mojito. Good-bye to pesky mosquitoes. Good riddance also to their eggs. Those are invisible when nothing hums yet still do something. It's why I've run a comprehensive Vibex DC filter on the downstairs system for years already. None of its components hummed, sub excepted. But the sound sure got better when the DC blocker preceded the iMac, reclocker, DAC and preamp. And as you just heard, with our subwoofers a DC blocker is mujo mondo mandatory. That's so even though they use class D and SMPS so no big just very small¹ transformers. To that argument, my shoulder shrug simply is: Whatever. Works. Wins.


¹ Without a standard line transformer, a switch-mode power supply converts AC line power to a direct DC voltage which then converts into a higher-frequency AC signal used by the regulator circuit to produce the desired current/voltage. The voltage is then rectified, filtered and regulated which results in higher efficiency. Thus a switch-mode power supply only requires a very compact lightweight transformer to lower/raise the voltage. Particularly in subwoofers, they're simply not immune to hum. A linear power supply applies the AC line voltage to a power transformer to lower/raise the voltage before applying it to the regulator circuit. The size of its transformer is indirectly proportional to the operating frequency. For the 50/60Hz line frequency which is much lower than an SMPS' operating frequency, this means a larger heavier power supply and bigger power transformer.