in ,

Removing the grain (Part 2 – Zu Soul MKII and ClarityCap MR)

  • The great crossover potential. A loudspeaker isn’t simply two (or more) drivers fitted inside a cabinet with each directly fed by the full frequency range. It’s a little more complex than that.

    On the inside of a speaker, usually behind the binding posts, sits a circuit that divvies up the musical signal between woofer(s) and tweeters(s); in doing so it filters the signal so that each driver only gets the information it needs. The frequency at which these filters are applied is often called the crossover. That’s the theory.

    In an athletics track relay race, the baton isn’t handed over from one runner to another instantaneously with a hard stop passing to a hard start. The receiver must pick up speed for the handover after which the passer can slow down.

    Similarly, the reality of crossover design means these low- and high-pass filters aren’t designed to hard-cut (like a cliff face) at the crossover frequency, they’re sloped so that information either side of the crossover point falls gradually (like a hill).

    Crossover filters are implemented with a combination of capacitors and inductors (coils). The slope of the ‘hill’ is determined by the complexity of the crossover – more capacitors and inductors means a higher-order crossover and steeper attenuation that reduces the overlap between the output of the woofer and tweeter.

    First order, 6db/octave crossovers feature a capacitor in series with the tweeter and an inductor in series with the woofer.

    (L = inductor and C = capacitor)

    Second order, 12db/octave crossovers feature: 1) a capacitor in series with the tweeter and an inductor in parallel with the tweeter; and 2) an inductor in series with the woofer and a capacitor in parallel with the woofer.

    More complex crossover circuits with more capacitors and coils are required for 12db/octave and 18db/octave filters.

    And just like amplifiers, a loudspeaker’s crossover circuit’s topology matters most but the quality of components also impacts sound quality.

    In its simplest form, a capacitor comprises a polypropylene film (dielectric) layered onto a conductive material (metal) that’s then rolled up like a carpet and inserted into a (mostly) cylindrical case. Influencing the capacitor’s ‘sound’ are its construction methods: 1) the thickness of the conductor; 2) the thickness of the dielectric and 3) the tightness of the wind.

    One Australian loudspeaker manufacturer who’s fully across qualitative differences between capacitors is the Gold Coast’s Mike Lenehan. His ML-1 standmount is offered in a range of flavours, each of which begins with the same drivers, box and bracing but ends with different internal wiring and – you guessed it – crossover components.

    The standard version ML-1 (AU$2750) uses ERSE capacitors. The ML-1 Plus-R version sports “Duelund Alexander 900 volt bypass capacitors, Duelund Carbon Phenolic 15 watt resistors in high frequency circuit” as well as internal RibbonTek wiring to add another $750 to the sticker. At the top of ML-1 range sits the Reference Edition (AU$5200) where “Duelund VSF CopperFoil capacitors and Duelund Carbon Phenolic body 15 watt resistors” as well as a spring steel-plated baffle doubles down on the price of the base model.

    Denmark’s Duelund Audio has a formidable reputation as being (possibly) the most talented player on the audiophile capacitor field but you definitely pay for what you get: their VSF Copper for example will set you back $360/pair. The VSF Silver Foil and VSF Black versions are both $POA and if you’ve a thirst for the reality of street pricing, check out Duelund Cast PIO/Copper variants.

    On Duelund performance I’ll defer to manufacturers like Lenehan and DIYers with bigger wallets. These Dutchmen get flying mention here to illustrate how deep this rabbit hole can go.

    For the majority of listeners crossover capacitor tinkering will probably land in the too hard basket. Unless you’ve the chops solder in new components to the crossover circuit in your loudspeaker, opportunities to learn first-hand what a capacitor upgrade can do will be scarce. Cap-curious non-DIYers must fall back on loudspeaker manufacturers with post-purchase upgrade options (like Mike Lenehan).


    Utah’s Zu Audio are a bit different. I’ve reviewed their entry-level Omen floorstanders and, more recently, their Soul MKII. I still own the latter; their ultra-reveal on upstream changes makes for a knockout reviewer’s choice.

    The Soul MKII isn’t like a traditional two-way speaker. It comprises an in-house designed 10” Nano-tech (ZuCX/ND-8 FR) driver that’s augmented by a coaxially positioned tweeter. The crossover as detailed above is nowhere to be found.

    Zu founder Sean Casey one described crossovers are “tone killers” which is presumably why no filter is applied to the 10”-er that runs full range in the Soul MKII. Running a tweeter full range will ultimately kill it so here a high-pass filter network ensures the tweeter only receives frequencies necessary for it to pick up where the Nano-tech driver falls away.

    “The high-pass filter as used on the tweeter electrically knees in at roughly 18kHz; 12kHz is the acoustic cross over point between the full-range driver and the tweeter assembly”, explains Casey.

    This filter network comprises a single capacitor – that’s it. An ERSE Pulse X comes as standard in the Soul MKII but when right-hand man Gerrit Koer let it slip at this year’s Newport Beach show that an upgrade option was available that even my Mum could install, I wanted in.

    Arriving in the mail a month later came a box containing a pair of 1.0µF ClarityCap MR-based filters. They’re larger than the stock ERSE so if nothing else you can SEE where your money goes. Zu Audio sell a matched pair direct from their factory for US$100 (depending on availability) but such is the simplicity of each capacitor’s termination, rolling your own would be a snap for even the most reluctant DIYer. Zu tolerance matches each pair to within 0.5% but PartsConnexion will do the same for a lousy buck.


    Unscrewing the driver from the cabinet, unclipping the stock filter and then clipping in the new one takes less than ten minutes per speaker. Easy.

    The pay off? A smoother, less grainy treble and a smidge more ease in delivering micro-dynamics. Result! And that’s within only a few hours of burn-in. I have it on good authority from a Los Angeles-based Zu Audio die-hard that further improvements will reveal themselves as playback hours move into the hundreds.

    Not enough? For those who want to take their Zu’s filter network to the next level, an additional bypass capacitor is one way forward.

    On this Casey opines, “You can also run a good paper-in-oil in parallel with the 1.0µF ClarityCap MR. I would go with a 0.1µF there – anywhere between 0.1µF and 0.05µF is good. A 100:1 ratio is pretty common for a bypass cap. However, you are getting into the weeds a bit there. This area is usually reserved for those that live in their mom’s basement and have way too much time on their hands—you really do have to burn all this shit in and then spend a lot of time trying combos out. Two caps that are closer in their ratio can make for some odd sounds, we don’t recommend anything closer than 10:1 and unless perfectly matched do not recommend 1:1 pairs.”

    What about costlier capacitors?

    “I also like the Jupiter Copper caps (~US$200/pair) and the Duelund VSF Blacks. The Duelunds are way too much money for Soul though. Save your cash and get more wax. Or Omen Def.”, says Casey.

    Lastly, check out Humble Homemade Hifi’s phenomenally thorough testing of capacitors in loudspeaker filters – a resource that’s seen ongoing updates since 2002!

    Further information: Zu Audio | PartsConnexion | Duelund Audio

    Written by John

    John currently lives in Berlin where he creates videos and podcasts for Darko.Audio. He has previously contributed to 6moons, TONEAudio, AudioStream and Stereophile.

    Darko.Audio is a member of EISA.

    Follow John on YouTube or Instagram

    Astell&Kern AK120 II review

    Tidal streaming comes to Logitech Squeezebox and AURALiC Aries