Cayin RU6 Discrete R-2R USB-C DAC/Amp

OspreyAndy

500+ Head-Fier
Cayin RU6 - The Analogue Champion
Pros: -
- Analogue neutral crisp timbre
- Mature & polished dynamics
- Reference level technicalities
- Clean background with zero floor noises
- Amazing driving power
- Reasonable battery draw on the host
Cons: -
- Expensive
- Occasionally the volume adjustment may exhibit stutter
- Could use just a little bit more of smoothness
Review Date: 12 January 2022

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SPECIFICATIONS

  • 24-Bit Discrete R-2R Resistor Ladder DAC
  • 1/1000 ultra-high precision TCR25 low temperature coefficient thin film resistors
  • Support up to PCM 384kHz and DSD 64/128/256
  • Low power consumption
  • User selectable NOS/OS DA Mode
  • NOS mode: digital filter-less, low phase distortion, low jitter, no ringing artifact
  • OS mode: increased sampling rate, enhance resolution, reduce noise, improve anti-aliasing
  • 99 steps Multi-segments High Precision Resistor Array Volume Control
  • Hardware Volume +/- button
  • TWO 6-layers PCB, digital and analog circuit on separate boards
  • 3.5mm single-ended phone output delivers 138mW per channel at 32Ω loading
  • 4.4mm balanced phone output delivers 213mW per channel at 32Ω loading
  • Compatible with Android, iOS, iPad OS, macOS, Windows 7/8/8,1/10 and DAP with USB Audio output
  • Type-C USB Audio, Shield USB-C to USB-C cable bundled.
  • High/Low gain control
  • Compact and seamless CNC aluminum chassis with 1” OLED screen at 28g.
  • Optional Leather cases: orange or blue
  • Optional USB-C to lightning cable

Test Equipment:


Headphones:
  • FOSTEX T40RP MK3 (Magnetic Planar, 50 Ohm, 91db Sensitivity)
  • Beyerdynamic DT880 (Dynamic Drivers, 600 Ohm, 96db Sensitivity)
IEMs:
  • Etymotic ER4SR (Single BA, 45 Ohm, 96db Sensitivity)
  • TRN VX Pro (8 BA + 1DD Hybrid, 22 Ohm, 106db Sensitivity)
  • TIN HiFi T3+ (Single DD, 32 Ohm, 105db Sensitivity)
  • Shure KSE1500 (Single Electrostatic 200V, KSA1200 Energizer)
Earbuds:
  • VE Asura 3.0 FE (Single DD, 155 Ohm)
  • VE ZEN 2.0 SLQ (Single DD, 320 Ohm)
Sources:
  • Windows 10, Foobar 2000 (Native USB Drivers)
  • LG V50 ThinQ (UAPP USB Exclusive Mode, Bitperfect)
  • Sony Xperia X Compact (UAPP USB Exclusive Mode, Bitperfect)
Comparative Reference DAC/Amp:
  • iFi ZEN DAC V2 + ZEN CAN Stack (15.1 Vrms)

LISTENING EQUIPMENT USED IMPARTED HUGE INFLUENCE TO SOUND IMPRESSIONS & RATING

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Cayin RU6 is a very special Dongle. It is Cayin's first ever DAC/Amp in Dongle form factor and most importantly, it is the very first non Delta Sigma Dongle ever. Instead of building RU6 on the common DAC chips widely available out in the market now, Cayin opted for the much more complex R-2R design which was adopted from their highly regarded R01 Module for Cayin N6ii. Suffice to say that this has gotten me so pumped up that after completing #donglemadness 2021 with 100 Dongles, I have opted to conclude the adventure and "retire". Alas my morbid curiosity to try out this new innovation outweighs my reservation. And thus #donglemadness came back to life in 2022 simply because with the purchase of my RU6, the floodgates reopened for me to buy every prominent Dongles available out there that I have yet to test :D

Build, Functions, Usability​

RU6 is one hell of elegant device. For the asking price I expect no less from Cayin. Just like my previous DAPs of N6ii E02 and N3 Pro, black theme with glass faceplate implemented here to cement the signature look of Cayin. The unit offers standard accessories needed to run on the fly with any USB-C devices or PC with USB-A (via included adapter). iOS users may need to buy Lightning to C cable separately.

One most prominent feature that I find very interesting with RU6, it offers two switchable modes named OS (Oversampling) and NOS (NON Oversampling). What this does, OS mode toggles the RU6 to resampling of feeds to 384kHz through digital filters. NOS on the other hand, will not do any resampling and will allow the feed to be processed as it is. For my own extensive use, since I am almost exclusively on UAPP bitperfect or HiBy Music Player USB Exclusive Mode, the NOS option proved to be the better choice. It offers 100% source transparency and allowed me to enjoy my music unaltered. The usage of OS on the other hand, may proved useful if UAPP or HiBy Music Player are not used. For example playing Spotify or YouTube Music which does not have USB Exclusive mode, the OS mode will then override stock Android SRC and that to me is a much better option over native Android resampling.

Another great feature of RU6, the volume adjustment. High Precision Resistor Array Volume Control is implemented and I can attest this being one of the most refined volume steps adjustment that I have ever tested so far with Dongles. I must say that the refinement is on par with what I get from my all time favorite the CEntrance DACport HD. To be fair, Luxury & Precision W2, Lotoo PAW S1 and S2 offers similar type of volume adjusters (since all of them has LED screen), but I find RU6 and DACport HD being even more refined. I must note though, that sometimes RU6 will exhibit very brief stutter when changing volume level especially when used with UAPP and HiBy Music Player, so I would not place this as a Cons against RU6, because it did not behave that way on Windows or other non exclusive Android music players.

The other functions worth mentioning is the Hi and Lo Gain selection. This proved to be very useful as I am using my RU6 with anything from as high as 100db to as low as 91db of sensitivity listening devices with resistances up to 600 Ohm. The most powerful output being the 4.4mm BAL port at High Gain.

Other than that, RU6 thankfully is so very spartan as it does not have any EQ functions. And I am perfectly fine with this as I have never used any EQ for any of my gears since forever. For those wanting to alter or color the sound with EQ, then the W2 would be a better choice.

As for battery consumption, I have measured my RU6 on Sony Xperia X Compact (Android 8, 2700 mAH, UAPP bitperfect mode, Low Gain driving TRN VX Pro) and the results yielded 6 hours of continuous play by the time the battery reached 3% (from 100%). Admittedly nothing too impressive here. But assuredly much better than DACport HD which only manage a measly 3.5 hours. On par with Lotoo PAW S2 driving the same load.

Sound Impressions​

This is the most important part. To me, the biggest appeal of RU6, it was designed with a legacy Rail to Rail Array Resistors audio processing which clearly revealed the intent by Cayin on why they choose it - True Analogue sound. Why bother with Analogue sound anyway? you see there was a time audio were all non digital and during those days there's certain appreciation for sound quality that is as real as possible. With Delta Sigma audio processors, what I am hearing today is sound presentation that is very clean, detailed, resolving and analytical - and I can understand that many love it that way. However, they often ended up being too clean sounding that they lacked musicality and some even lacking realism altogether. Real life isn't that clean. Try spending time listening to actual live music or even studio sessions, they are anything but sterile clean. And this is the essence of Cayin RU6 - it is one of the most analogue sounding Dongle I have ever tested so far. Perhaps from critical standpoint, I find Ovidius B1, Apogee Groove and CEntrance DACport HD closer to being organic sounding, but RU6 is definitely there among these three Dongles that I loved so much.

Timbral signature and tonality of RU6 is definitely natural - devoid of any coloration. It is neither warm nor bright - it sounds correct. Analogue presentation with a hint of organic crispness and smoothness. It is wholesome as it is rich in density, no hint of being digital or lean sounding. This is evident when tested with Etymotic ER4SR and Shure KSE1500, my most trusted reference listening devices that will reveal the nature of the source truthfully. ER4SR being Diffused Field Neutral, exhibited the flatness of RU6 sound, which indicated that there's no manipulation of sound frequencies beyond what is normal, no attempt by RU6 to spice things up unnaturally. KSE1500 on the other hand, reveals the natural organic side of RU6. That airy and life like presentation that literally place the listener INTO the music.

Despite being Analogue, RU6 exhibited exceeding competency with technicalities. Tested with my listening gears, what I am hearing is super transparent resolution throughout the dynamic range. Being Analogue sounding does not mean it has to be fuzzy. Details articulation is crisp and convincing, so is the resolution of imaging. The more resolving the pairing partner the better it is. In fact when paired with my ER4SR and Shure KSE1500 I am hearing every micro details that are available in the recordings.

On the other hand, the staging is amazingly spacious as it is wide. It is undoubtedly up there among the very best. For example with my TRN VX Pro which is already natively wide sounding, I am hearing effortless headroom of soundstage that can make me forget I am listening to an IEM. The same can be said for the rest of my gears and even the normally "narrow-ish" Fostex T40RP MK3 sounded sensibly spacious.

However if I am going to be nitpicking, I would say that I would have preferred RU6 to exhibit just a tiny bit more of smoothness. Don't get me wrong, it is already admirably smooth, but comparing it against Lotoo PAW S2, Ovidius B1, Apogee Groove and CEntrance DACport HD, I can hear that RU6 favors crispness slightly more over smoothness. The good thing is, this crispiness approach is still very polished and mature if I am to compare it against the likes of Hidizs S9 Pro, E1DA 9038SG3, TempoTec Sonata E44 etc. - of which these dongles focuses primarily on "modern sound" reproduction. This is not a matter of right or wrong, it's a matter of preferences and I can understand some listener prefer it that way. Audio is still a very subjective passion after all.

As for the presentation of the three major sound frequencies, Mids, Treble and Bass. I would say that being natural RU6 does not color the sound spectrum. A great source should always strive to be transparent and natural. What this means, when I use a specific listening device I expect it to sound as it is. With sound extensions, resolution, textures and details presented in the most truthful way. That being said, I am assuredly happy with the quality of RU6 dynamic range handling. Treble sounded realistic as it is naturally vibrant, Mids as rich as the listening device capable of emitting, Bass having visceral density which is real and engaging. Details and textures present as how they should be.

Driving Power, Gain Control and BAL-SE Differences​

So how does RU6 performs on driving power delivery? To find out I have conducted exhaustive tests with two of my most difficult to drive Headphones. And RU6 does not disappoint. This is where I use my desktop stack of iFi ZEN DAC V2 + ZEN Can (15.1 Vrms) to ascertain how close RU6 able to drive those two monsters. With the ZEN stacks providing 100% experience, this is how RU6 compares: (done on 4.4mm BAL, NOS, High Gain)
  • Fostex T40RP MK3: 90% (Vol 75/100)
  • Beyerdynamic DT880: 80% (Vol 85/100)
The percentage being indication of how close RU6 to that iFi ZEN Stack. Of which the criteria for comparison as follows:
  • Listening loudness (proper level)
  • Dynamic transients and density
  • Headroom spacing and staging
  • Imaging strength
  • Details articulation
The percentage scored by RU6 is truly mind blowing. Comparing this against Luxury & Precision W2, that W2 only managed to score 70% (with Fostex T40RP MK3) at the very best. Clearly RU6 is a much better device for hard to drive partners.

On the other side of the spectrum, RU6 is as amazingly competent driving super sensitive stuffs like multi drivers hybrid IEMs. My TRN VX Pro, having 9 drivers per side and with super sensitivity, does not exhibit any unsavory floor noises nor does it sounded edgy or shouty.

BAL and SE Comparisons:

Etymotic ER4SR (Low Gain)
- SE 3.5mm - Vol 58/100
- BAL 4.4mm - Vol 50/100

TRN VX Pro (Low Gain)
- SE - Vol 35/100
- BAL - Vol 30/100

Marginally crisper dynamics density on BAL, perhaps by 5% of which the SE is no less impressive, in place of pronounced crispiness, less bite with more smoothing. Otherwise from quality perspective I found both to be quite satisfying, there are times I want to have solid crispness and solid imaging and the 4.4mm BAL will do the trick, there are times I enjoy articulated smoothness, switch to 3.5mm, Easy-peasy. In comparison, Luxury & Precision W2 is a disaster with the 3.5mm SE, where everything sounded subdued and outright uninspiring. W2 defacto SQ only to be had from the 4.4mm BAL - and even so, W2 trying too hard on being neutral and clean sounding making it less than realistic (Hi-Fi indeed but lacking soul), of which RU6 has that analogue timbral hint which keeps thing believable.

What I can conclude from this test, RU6 SE will be excellent for sensitive IEMs and the 4.4mm perfect for something a bit more stubborn like my ER4SR.

VERDICT

There's not much I can add to what has already been said. I love this Cayin RU6. Everything that it is, RU6 epitomize near perfect Analogue sound in a very compact form factor. But yes, that's the appeal of RU6, for those seeking the balance between musicality and technical competencies. Not an easy balance to attain. But RU6 did it. However I reckon that being Analogue it would probably mean RU6 will likely score terribly with measurements. So for those prioritizing measurements, RU6 may not be suitable unit.

In the end, finding myself using RU6 regularly alongside my all time favorite DACport HD proved to me that RU6 clicks to my heart. I spent no less than 6 hours daily listening to music. RU6 has found its way to the top segment of #donglemadness.

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⭐⭐⭐⭐⭐ ($250)​


Best Pairing: Anything up to 600 Ohm


#donglemadness
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OspreyAndy
OspreyAndy
@jmwant I have both ER4SR and ER2XR, without a doubt ER2XR is the more efficient one and even less demanding than ER4SR. RU6 as you can see drives even much harder 600 Ohm HP or Magnetic Planar
QuasiSpecies
QuasiSpecies
Great review. I’m really interested to hear an R2R dongle.
Mhog55
Mhog55
This is starting to sound like a must have device. Nice review 👍

NewEve

1000+ Head-Fier
Portable R-2R DAC/Amp
Pros: R-2R
NOS & OS
3.5 & 4.4mm
Flexbility of a DAC/Amp dongle
Stunning level of detail and musicality
A Return on Investment (ROI) difficult to beat
Cons: Can be sensitive to EMIs when streaming but nothing major


Short of raw power and holding your music on a dedicated device, the gap in-between DAPs and DAC/Amp dongles has never been so small since the introduction of Lotoo's PAW S1/2, Luxury & Precision's W1/2 and, more recently, Cayin's RU6.

The RU6 exhibits a level of detail and musicality which is absolutely stunning. I personally prefer it in NOS as I find the OS mode to sound more like a delta-sigma DAC but that's just me — there's no wrong choices here.

I bought and received Cayin's RU6 shortly after I got the FiiO M11 Plus LTD. Compared to the M11 Plus LTD, the RU6 set in NOS breathes a bit more, has a bigger soundstage and is overall more organic and effortless.

You'll have a small interruption/pop every 10 volume steps but it isn't much of an annoyance and is normal. One can hear EMIs, especially while streaming, but it depends on the wireless equipment around you.

With sensitive IEMs such as the Campfire Audio Solaris 2020, background noise wasn't an issue for me but I'm not the pickiest about this.

Power draw is similar than via L&P's W2 which means there's consequent drain on the battery of an iPhone XR which will impact you if you genuinely need to use your phone during the day.

Unless one specifically seeks a DAP, there's hardly anything at fault here. Cayin's RU6 is a safe bet for a high-end DAC/Amp dongle with all the flexibility DAC/Amp dongles bring, an excellent foray into R-2R technology as well as into genuinely portable Hi-Fi — without breaking the bank.

IEMs: 64 Audio U6t (MX and M15 module), stock cable, medium silicone tips | 7Hz Timeless, KBEAR 4.4 8 Core Single Crystal Copper UPOCC cable, Final E tips | Campfire Audio Solaris 2020, stock cable, medium silicone tips

Headphones: Grado RS2e with G-Cush pads
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Sharppain

New Head-Fier
Cayin RU-6 - the Portable Bliss to Enjoy an R2R Replay!
Pros: - Solid and elegant build
- Combines natural replay (especially vocals) and very good resolution
- Wide soundstage (especially depth)
- Good imaging and instrument separation
- NOS and OS like two different sound signatures
Cons: - Might be prone to scratching
- Delay when volume increase (especially when not warm)
In the portable music world, Cayin is well known brand that offers product range from the budget N3 Pro up to the flagship N8 with the N6ii sitting in the middle. Now Cayin offered portable R2R dac/amp at affordable price that, according to me, will win many fans. For me it is a bit a game-changer since the price you pay for getting into the R2R replay is peanuts.
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Cayin took a bold step and built the RU6 with thin-film resistors with 0.1% precision and a 25ppm temperature coefficient and brought the price tag inline with some leading MQA dongles thus setting a challenge to other premium brands. And according to me it achieved building a very solid product and R2R replay.

The RU6 offers two gain levels and 3.5mm single-ended and 4.4mm 'balanced' outputs (the 4.4 is not a real balanced, it has 4.4 balanced power). The RU6 also offers oversampling as an option that increases the bit-rate of the input sample to 384kHz - it decreases noise and increases detail level; according to me, both NOS and OS have very solid replays.

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Power
One of the achievements that surprised me most is that RU-6 drives successfully not only demanding iems but also demanding headphones - it drivers perfectly the Z1R iem but also the Z7M2 headphones, it also drives very well the P1 but also even the T60rp (for the headphones I used the 4.4 output). Meanwhile, I do not hear any hiss even with sensitive iems like Thieaudio Voyager 3 or Andromeda.

Technicalities, Replay and Timber
The RU-6 has a very natural tonality (especially for vocals) with good detail retrieval - I find it very natural R2R sounding and do not miss resolution:
- Bass is tight, sub-bass goes deep, there is good detail and separation between sub-bass and bass.
- Mids and vocals are naturally warm, naturalistic and have very good details. They are a little bit forward but in a very good way that doesn't overpower the bass and treble. These are mids that give you the entry level to R2R signature - do not expect Ares II performance but the level produced is far different and higher in naturalism than leading MQA dongles.
- Treble is done perfectly well for me, no roll-off, good resolution and airiness, details are not attacking your years but are well articulated. They fit the whole signature very well and play good role for the sound staging.

The sound stage is wide, especially in depth; the layering is impressive; the separation between instruments is solid and I even find that RU-6 has 3D presentation. Attack and decay on/of tones is fully Ok.

Pairing
I cannot give description how the RU-6 matches the 100s of iems from my collection but I will do short descriptions with some favorite of mine iems from the last 15 years:
- Sennheiser IE80 - a great pair, RU-6 opens the big sound stage of the IE80 more and brings forward the vocals;
- Sony XBA-A2 - a great pair, RU-6 controls the XBA-A2's bass very well, increases sound staging and vocals enjoyment;
- Sennheiser IE800 - RU-6 makes the vocals come forward and makes the highs more natural and airy;
- Blon-03 - RU-6 enhances the natural replay of the iem and shows that it deserves a better iem :)wink:
- Tin T2 - a very good pair, RU-6 makes the T2 even more unjoyful and shows the value of the T2;
- 7hz Timeless - OMG this combo is as impressive as the that with the Drop JVC HA-FDX1;
- Thieaudio Voyager 3 - O-o-o-o musicality bliss, mid-price perfection, endless enjoyment;
- Moondrop B2 Dusk - finally the sometimes dull Dusk gets some emotion and bigger sound stage without loosing technicalities; the pair shows the beauty of Harman tuning;
- My two top pairs - IER-Z1R and UM Mest - so perfect: well controlled bass, improved vocals , enlarged sound stage and 3D.

I will not fall in so many details on headphones since I think the point is to explain how RU-6 enhances different replays but I will outline that RU-6 pairs well with Sony MDR-7510, CAL! SE, HB560s, Sundara, Focal Listen Pro, Focal Elegia, T60rp, Z7M2, HD800s. And I really enjoyed it with the CAL! SE, the Z7M2, the Austrian Audio Xi-55 and Denon 5200 and the Sundara.

The RU-6 also pairs very well with the Burson Soloist 3X and the TEAC HA-501 enhancing the natural replay of the first and the musicality of the second and paired with these two gives me same but also a bit different enjoyment vs the Mytek Liberty DAC.

Comparison
I will cover only dongles that I know very well and are good benchmarks and will make it short but you will get the point:
- vs HiBy FC3 - the RU-6 is far more naturalistic and smooth and has far better staging and layering;
- vs THX Onyx - well think that the Onyx has blacker background but in A-B test sometimes it sounds very analytical, close to clinical, and cannot compete in naturalism and sound stage wideness; I would always choose the RU-6 vs the Onyx;
- vs Hidizs DH80s - the DH80s is having good musicality but favors analytics vs naturalism of RU-6; these two are really a good pair if you like musicality, power and want a good value performance.
Also what is worth noticing is that streaming MQAs from Tidal via the RU-6 works highly satisfactory and unjoyfully for me.

Conclusion
I think Cayin did a great product - RU-6 is the 2021 dongle for me, a dongle that will make premium brands sweat. RU-6 value is its naturalism of replay and presenting the flavor of R2R w/o dropping technicalities. NOS and OS are giving different replay and the power is enough to have enjoyment with any iem and most of the headphones up to 300 ohms.
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QuasiSpecies
QuasiSpecies
Great review!
R
Rizzoli
Hello, sorry for my ignorance (I'm kind of new to the hobby) I own MEST MKI coupled with a FiiO BTR5 - Does the Cayin RU6 offer bluetooth capabilities? Or is it exclusively an inline DAC/AMP.

And how is the quality of the overall system compared with the FiiO BTR5?

Thanks,
Rizzoli
R
Rizzoli
Hello, sorry for my ignorance (I'm kind of new to the hobby) I own MEST MKI coupled with a FiiO BTR5 - Does the Cayin RU6 offer bluetooth capabilities? Or is it exclusively an inline DAC/AMP.

And how is the quality of the overall system compared with the FiiO BTR5?

What options are there better than the FiiO BTR5 - I am looking for a small form factor with bluetooth capabilities - I've heard great stuff about the LP W2. Any recommendations?

Thanks,
Rizzoli
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