For the sake of audio
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PostPosted: Tue Oct 24, 2017 8:09 am 
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Joined: Thu Jul 26, 2007 5:45 pm
Posts: 1161

Maybe some here like good sound on the go, on the cheap?

A recent e-mail from Massdrop alerted me to an interesting IEM. I occasionally read, but never post at Headfi and naturally Headfi had a massive nearly 24000 post thread on these IEM's.


On paper they look decent, 4-Driver, 3-Way, and response to 40kHz in a solid Alu shell for something "ultra-high end" without the usual 500-1K USD pricetag.

As I carry already four different ones (Meze, Xiaomi Piston 3 & Xiaomi Hybrid & Alclair) I figured getting one more was natural. Also, as these are super sensitive at around 128dB/1V I figured if nothing else I could use them to test practical SNR in our gear (and that of others). I figured any needed mods should be doable, as you can open the back and access wiring etc..

After I received them I checked they worked and noticed their signature was overall even but with a very nasty, edgy and exposed treble. Burn in did not much to help. This squares with a lot of impressions at Headfi, though it was not as bad as the measurement graphs posted at headfi suggested.

The Bass/Sub-bass is excellent, the mids clean and overall balance (minus the high treble) among the best I have come across. With my ears wearing comfort is acceptable. But the very exposed, peaky treble completely spoiled things for me. Changing the tips to my preferred Comply ones with earwax screens (more on that later) made it better, but to my ears this was still not bearable.

Adding acoustic foam inside the tips helped the edgy treble but changed the overall sound too and not for the better.

I spend some time reading on Headfi and THEN reading up especially on the Balanced Armature drivers used for the treble.

There are a few things going on.

The chinese manufacturer of the IEM's failed to account that the BA drivers they use are designed to be linked to the ear by a tube and require acoustic resistive termination to offer a flat response. By pointing them directly into the ear, by pointing two of them directly into the ear, they create what some have called "treble knifes".

The drivers will have without the tube a strong resonant peak at 9kHz (about 10dB above average SPL) with two drivers adding another 6dB SPL, so we have an estimated 146dB peak at 9kHz over what is needed, then falling off...

Measurements show this as well, but they are somewhat misleading, as all the measurements shown at headfi are made with either IEC711 couplers or non-standardised couplers of similar volume, which produces a FICTIONAL 11kHz resonance peak which also boost the 9 kHz peak more than it actually is. Another typical example of people making measurements of which they fail to understanding limitations and possible measurement errors and publishing them uncritically without the limits and errors stated. As much as I appreciate the ease of modern testing (I have ordered one of these cheap chinese copies of the GRAAS IEC711 coupler with Mike), we again see that a little knowledge is dangerous.

Using the comply tips with earwax guard helps as this introduces an acoustic resistance and drops the height of the 9kHz peak (measurably) as well as broadening (de-q) the peak.

Some reports on Headfi suggest that adding 75 Ohm in series with the headphone balanced the treble, after some checks I largely agree. This resistor will pull the treble spike down by around 5dB. However the 75 Ohm kills the efficiency. This way 15dB sensitivity are lost from 128dB/1V to 113dB/1V, arguably noise from headphone amps is also reduced 15dB and 113dB peak on an iPHONE is probably WAY LOUD ENOUGH, so for most people this may actually be "a good thing", but not for me and the treble is still a trifle hot for my taste this way.

After some pen-damping on the existing measured curves scraped from Headfi to correct out the IEC711 coupler resonance, I reverse-engineering the actual "crossover" circuit and resulting likely slopes/responses etc. based on the datasheet for the BA Drivers from Knowles - I suspect at the price these IEM sell the actual BA drivers are French Croatian (Fa Ke) rather than Knowles USA but they measure like misapplied originals would, or close enough to not matter.

There is an appx. 7.5 Ohm series resistor on the 6.4mm "midrange" Driver, no crossover whatsoever on the 10mm "Woofer" and a appx. 1uF series capacitor for the dual BA "Tweeter" which is paralleled. One possible trick would be to reconnect the BA drivers in series, instead of parallel, this will drop SPL by 6dB and increase impedance four fold, but it looked incredibly fiddly to me. I might try this on a new pair, using someone else's pairs of hands to solder.

After comparing the curves to the latest Olive/Welti target for headphones I decided to add an 18 Ohm resistor in series with the 1uF capacitor which constitutes the "crossover" of the high frequency drivers. This produces a similar attenuation of the excessive treble (around 7dB vs. 5dB) as the 75 Ohm resistor without killing efficiency and it makes the IEM a more gentle load for sources to boot (the impedance dip at high frequencies is substantially filled in).

Though arguably the 75 Ohm resistor does an even better job on making this an "easy to drive" IEMand might be easier to fit. But again, I'm interested in hearing what my gear sounds like driving low impedance headphones (this highlights weaknesses) so for me keeping the original 16 Ohm is better.

The predicted response after this is surprisingly close to the Olive/Welti Target (there is a bit of deviation with an overly boosted bass, heck, I don't mind a bit extra bass).

There is a teardown picture of the ZS6 that shows clearly where to add the resistor...


The wires to the BA Treble driver are the pairs of thinner red and copper coloured ones near the top. As said, series wiring might do the same trick without adding a resistor.

Now, the result of adding this single resistor (beware, SMD soldering required) is a very balanced set of IEM's, very listenable, at an unbeatable price which I will consider the best in my current lot, especially when paired with my balanced wired age annealed silver litz headphone cable. Yes, it costs a lot more than either Xiaomi which are stonking for the price, but it to my ears operates on a different level of sound quality.

Running them from the prototype of the iDSD nano black label I feel the sound is excellent for music ranging from Avicii to Allman Brothers "Live at Fillmore East" in MQA via Tidal, sound is exciting, but not fatiguing. Ok, at my desk I still prefer the modified Fostex T-50RP and at home the Kings Audio Electrostat's or speakers and on the go I'd probably still take Meze Model 99, but all these are big, full size headphones. For mobile use, the modded KZ ZS6 rule ok.

Later, KYW

"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." Richard Feynman

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