The Company Name:
According to their own statement, Stoner Acoustics has nothing to do with Marijuana-related slang terms but origins from the calm, steady nature of stones and rocks. And once the company name was registered, they didn't want to change it anymore. Whatever.
This and That:
For a long time, I've used the HiFime 9018d as a small, portable and very convenient desktop source for in-between listening sessions. Its core strengths, to me, are its super low output impedance as well as its precise 256-step volume control (0.5 dB step size) with convenient LCD screen display. The major drawback is its poor hiss performance with sensitive in-ears.
Anyway, this write-up is sort of a comparison of the HiFime and Stoner Acoustics UD125 with a focus on the UD125. Meanwhile though, as a small and portable audio component, the FiiO Q5 (with the AM1 module) has replaced both of them for me.
Anyway - the UD125 comes with a neat storage/transport case and a USB cable. More than the HiFime, however both unfortunately lack rubber feet.
The DAC is surprisingly light. What I really like about it is the internal scheme that's on its top side that also shows what colour belongs to which of the four digital filters.
Instead of the more popular micro USB standard, Stoner Acoustics implemented a mini USB socket on the DAC's back side.
Volume Control:
The UD125 features a joystick-like digital volume rocker that can be pushed in in order to change the digital filter.
Just like the 9018d, the UD125 features 256 attenuation steps with a scaling of 0.5 dB over the entire range wherefore listening at very low volume is easily possible. Very good.
Not so nice is that there is no volume indicator (except for when the lowest or highest attenuation setting is reached). This is where the 9018d is clearly ahead and more convenient.
Fast volume adjustment isn't possible either as pushing the volume rocker to either side and leaving it in this position only gradually changes the volume (so no, there's no acceleration). Pushing the rocker in for at least one second mutes the audio. Repeating this process reverses the muting.
The last volume setting as well as the digital filter setting are by the way remembered when you re-connect the UD125.
Pro: very precise digital volume control; Con: no volume indicator.
Sound:
The UD125 uses the AKM AK4490 DAC chip from AKM in conjunction with the Texas Instruments TPA6120 amplifier chip. Even though both are good components on paper, it is (as always) the final implementation of the chips that determines whether the converter is good or not, which is why I personally don't care much about what chip is used as long as the final result (output impedance, hiss performance, volume control, linearity) is good.
Frequency Response; Output Impedance:
Without any connected load, the headphone output of the UD125 measures with the standard filter as follows:
The frequency response is exactly as linear and flat as it should be (minus the negligible, small 20 kHz "roll-off" that's rather commonly found on many AK4490 implementations). No surprise here.
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That's the output with a critical multi-BA load (my trusty Triple.Fi 10) without a steady impedance response:
The deviation from the measurement without load is only extremely minimal. The calculated output impedance is therefore about tiny 0.1 Ohms, which is as excellent as it gets.
Hiss Performance w/ very/extremely sensitive In-Ears:
There is some hiss with very/extremely sensitive in-ears (e.g. Ostry KC06A, Shure SE846, Pai Audio MR3 - I haven't used the hiss king, my Campfire Audio Andromeda, that is even more sensitive than my other extremely sensitive in-ears, with the UD125 yet though, but you'd expect even some more audible hiss with the hiss king), it's probably in the lower midfield in quantity. So not as hissy as the iBasso DX50, HiFime 9018d, Shanling M2 or iBasso DX80, but also not as quiet as devices such as the iBasso DX200, iBasso DX90, Cowon Plenue 2, Cowon Plenue D or FiiO Q5.
Using still sensitive but less extremely sensitive in-ears, such as my Ultimate Ears Triple.Fi 10 or UERM, hiss becomes very little.
So yeah, the UD125 isn't great but only lower average tier in terms of hiss performance with sensitive in-ears, but at least it's not as hissy as the 9018d.
Subjective Impressions about subjectively perceived "Sound Quality with In-Ears:
Nothing bad to report here.
Probably some minimal softness, but that's it, and likely caused by the hiss.
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Compared to the HiFiMe 9018d:
Both devices' output impedance is basically perfect with ~ 0.1 Ohms.
The same goes for the volume control implementation where both feature 256 steps à 0.5 dB, however the 9018d's screen display is a huge plus whereas the UD125 doesn't really have any indicator.
The 9018d's case is flatter and more rounded, and it also appears more premium. It's also got an optical output that the UD125 doesn't have. Stoner Acoustic's DAC has got a mute feature and four digital filters (HiFime: two), for those who think they need that.
Neither DAC shows ideal hiss performance, but the UD125 is ultimately definitely audibly less hissy with sensitive in-ears.
Subjectively, the UD125 sounds a little soft whereas the 9018d appears a little tighter. Small and hardly relevant stuff though.
Conclusion:
The UD125 is a good and very portable device for its moderate price. While its volume control implementation is great and while its output impedance is about as low as it gets, it's unfortunately not fully convenient as it lacks a volume indicator and while it's less hissy than HiFime's roughly comparably priced product, it is still quite hissy with very sensitive in-ears (that's unfortunately true for many DACs).