Resistive and Addictive
Pros: + Extremely musical and analogue sounding
+ Full bodied and visceral presentation
+ Lifelike mids – smooth, non fatiguing treble
+ Great technicalities
+ Holographic soundstage
+ NOS and OS modes
+ 4.4mm and 3.5mm headphone plugs
+ Very powerful can drive full sized headphones
+ Neat and simple operation
+ Hardware buttons
+ Not power hungry
+ Good build quality
+ OLED screen
Cons: – Technicalities not on par with the best delta-sigma rivals
– Could do with more extension and resolution
– A little bulkier than the competition
– More expensive than the competition
– Accessories pack is poor considering the price
– EMI interference can cause hissing issues
The RU6 was kindly provided by Cayin in exchange for my honest and subjective evaluation.
The selling price is $249.99 and you can order it from all authorized dealers across the world.
Full product specifications are available here.
The Cayin RU6 is a 24-bit USB dongle DAC/amp featuring a discreet R-2R ladder network.
The recent chip shortage, led many companies to revert back to the roots, the R-2R era where digital conversion was done with this particular method before the invention of the modern D/S DAC chip.
(The text includes comparison against the FiiO KA3)
Back to the basics
The Cayin RU6 features a self-developed 24bit discrete R-2R resistor ladder circuit that can decode up to 384kHz PCM audio.
The basic idea of the R-2R ladder is a matched pair of resistors, the first is “R” and the other is “2R” which has twice the value of R.
To achieve 24 bit decoding, RU6 equips 48 units of resistors (23 x R and 25 x 2R), and that’s for one channel.
So for a stereo 24 Bit R-2R decoder, RU6 features 96 units of resistors for life-like audio reproduction.
With the advent of the R-2R circuit, Cayin RU6 establishes itself as master of the fully-discrete resistor DAC topology in portable audio.
Cayin is using only resistors that can deliver extreme precision and remain stable during temperature changes.
0.1% Ultra precision low TCR thin film resistors are used inside the RU6.
Oversampling and NON-Oversampling modes
Cayin offers both Oversampling and Non-oversampling DA modes in RU6. For Oversampling mode, the Digital Audio Bridge will upsample the digital audio data to 384kHz through digital filters.
This will enhance the resolution, reduce noise and improve the anti-aliasing of the digital signal.
The Oversampling DA mode will offer a noticeable improvement on details and frequency extension.
The playback is clean and sharp with a darker background.
On the other hand, NOS DA mode maintains the sampling rate of the original bit-stream and playback will become very musical with a natural and organic presentation.
This deactivates the digital filters and the signal is retained in perfect timing. That is, the phase distortion and jitter remain at a very low level with no artifacts in the output signal.
Analog resistor volume control
Most USB DAC/Amps will rely on the volume control of your mobile phone to control the volume of the headphone output.
Technically this was not a viable solution to Cayin RU6.
Understanding this issue, Cayin developed a fully analogue resistor array volume control circuit that provides 99 steps volume control through 9 segments of resistors and switching relays.
Resistor Array volume control features a very high-quality volume control design, extremely transparent when implemented correctly and it is found in a lot of high-end preamps and integrated amplifiers.
The resistance volume control includes a switching relay that mutes the output for a short moment in order to avoid crackling noises during level adjustment. As a result, however, a small delay in the volume adjustment has to be accepted.
Cayin therefore recommends a music player app for the smart device that supports Bit Perfect USB and thus bypasses the level control of the source device.
To minimise audio signal interference, Cayin has split the RU6’s circuit into two 6-layers PCB, with digital and analogue circuits installed on separate boards.
This safeguards the audio signal integrity and ensures the noise from the reference clock and DSP do not bleed into the amplification circuit. This architecture in turn delivers artifact free audio every single time. This circuit design combined with the R-2R architecture enables the RU6 to support hi-resolution PCM and DSD audio formats with support for 384kHZ PCM and DSD 64/128/256 natively.
Discreet R-2R circuitry is a low power DAC circuit when compared to other highly integrated DAC chipsets.
This architecture minimises power drain from the mobile source allowing you to enjoy music without the need to worry about battery drain.
USB-C connection offers compatibility with a wide range of phones, consumer electronics and computers. The hardware has been designed in an intuitive manner, where the volume buttons double as menu selectors and the OLED screen displays playback information accurately.
The Cayin RU6 also offers two gain settings to enhance performance with various IEMs and headphones.
I measured the power draw with an inexpensive USB power meter and I found it was about 0.12A / 0.55W while playing music with the FiiO FD7.
Build quality and appearance
The RU6 has a rectangular shaped body which is bulkier and heavier than the usual D/S USB DAC/Amps like the EarMen Sparrow.
The net weight is 28g and the dimensions are 65×25.4×13.7mm.
Still for a device featuring an R-2R network and resistor array volume control, it is quite compact and lightweight, not that larger than the FiiO KA3.
The chassis is made from CNC aluminium while the side with the small OLED screen is protected by glossy glass.
The appearance is quite modern and minimalist while finish and build quality are excellent with the only negative being that the glass surface is a fingerprint magnet but if you ask me, I wouldn’t care a lot.
The three physical buttons that are located to the one side of the chassis are of good quality and offer tactile feedback.
At the top of the device there are the two headphone outputs and at the bottom lies the USB type C socket.
The Cayin RU6 comes bundled with a short USB-C to USB-C cable and a USB-A adaptor.
The leather case for extra protection and a lightning cable for iOS users are sold separately.
The RU6 is plug’n play compatible with all USB-OTG devices (iOS and Android), Mac OS, Windows PC through the available driver and it can also be connected to Digital audio players that support USB out.
The DAC itself is power efficient so you don’t have to worry about draining the host device battery.
The OLED screen will display sampling rate, volume level, gain H/L and whether you are using NOS or OS mode.
You can use the mode button to enter the menu and configure gain settings, NOS/OS mode and screen timeout.
While adjusting the volume, sometimes you can hear a clicking noise, that is instant and doesn’t affect the performance.
Neat and simple it has the advantage of the hardware buttons and the OLED screen but it doesn’t offer the same kind of customization as the FiiO KA3 does through the supporting application.
The RU6 is prone to picking EMI interference from a phone that causes background noise which can range from a faint hiss to a louder humming especially when using more sensitive earphones.
The effect is more pronounced when streaming and not as severe when playing music from local files but still there.
You can try using a longer USB cable or other DIY solutions, like wrapping the cable with aluminium foil in order to shield it but in the end I didn’t manage to completely get rid of the noise.
This hissing noise is present when cellular reception is active.
In contrast the FiiO KA3 was dead silent with all mobile phones that I have tested.
I was asked to burn the RU6 for 100 hours minimum before listening and thus I did.
I used various earphones and headphones ranging from budget to more premium to find out that the RU6 not only scales very well but the use of higher quality partners is quite mandatory in order to get the most of it.
The RU6 with its full 4V swing and 213mW/32Ω of power output, is powerful and can drive a wide range of headphones with good grip and plenty of headroom.
This is one of the most powerful USB DAC dongles together with the FiiO KA3 and a few others.
The performance was very satisfying with headphones like the Sennheiser HD660S and the Focal Clear Mg while the gain setting proved very useful for driving more sensitive earphones like the FiiO FA7S.
Sometimes the faintest hissing was still present even with host devices without cellular antenna but it must be related to the host device power supply.
With my laptop was completely silent.
In contrast the KA3 which has a discreet power rectifying circuit remained dead silent under all circumstances.
The RU6 provided a great listening experience with the Sennheiser IE600.
The sound performance of the Cayin RU6 is stellar with a touch of a pleasant, tube-like, euphonic warmth without lacking in transparency.
With the mode set to NOS, it sounds exceptionally musical with a very natural timbre, smooth texture and the presentation is realistic but still precise and true to the source.
Voices and acoustic instruments are heard organic with natural overtones and plenty of harmonic wealth, a wonderful combination which leads to an engaging and authentic listening experience.
At the same time, technicalities are good and close enough to D/S performance but not in the same ballpark as top D/S performers like the FiiO KA3.
The bass is visceral, weighty but still tight and well controlled, fast, clearly defined and multi-layered.
Attack and decay are very convincing as is the dynamic contrast but then it doesn’t have the lighting speed or the boldness and ruthless impact of the KA3.
Nonetheless, with all my music the RU6 was punchy, full sounding with great macro dynamics from electronic music to classical.
The mid range is the star of the show, not because of the lack of linearity but due to some kind of a pleasant effect that seems to be imbued from the R-2R design.
It has a well sculpted texture with great liquidity, voices and solo instruments sound very engaging, fluid, articulated, large and three dimensional.
Timbre is utterly natural and so close to reality that in the end you forget about the technicalities as you are immensely drawn to the music, song after song as the time passes by without noticing.
One of my favorite albums is entitled “Hush”, a fuse with Nora Fischer singing baroque arias accompanied by the electric guitar of Marnix Dorrestein.
With the RU6 her amazing voice perfectly blended together with the distorted guitar in the most charming way and I literally forgot how many times I listened to the track named “Cum Dederit”.
Treble is silky smooth with the impression of being laid back, not too extended but very well behaved without a hint of brightness.
Natural is the word to use again, not only to define the timbre but also for expressing the decay, the notes are born and then fade away with the perfect timing.
High pitched percussion instruments sound full blooded with a convincingly natural decay, not splashy or thin.
In contrast the KA3 is more extended, faster, with increased clarity and more detailed presentation but it sounds somewhat leaner while it can’t touch the naturalness and physicality of the RU6.
The tuning is very easy to the ear but it cannot be said that the RU6 is not resolving, missing in clarity and agility or lacking in energy as to sound dull or slow.
Tonality is very cohesive from top to bottom and there is great consistency of note intensity throughout the whole frequency range where the KA3 tends to lose some of its weight while reaching the higher frequencies.
A good example of the even treble performance is in the following album where the solo recorder can become very strident with some USB DAC dongles that I have tested.
R-2R DACs usually have the edge when it comes to soundstage and the RU6 certainly delivers.
The soundstage is not as extended and wide with the same laser sharp positioning of the FiiO KA3 but it has far better holography, a more dimensional shape and it is further convincing in communicating the ambience of the recording venue.
The FiiO KA3 might have the technicalities edge for electronic music and such genres or be better suited for ultimate detail and clarity lovers but the RU6 with its more natural timbre and grander soundstage is the obvious choice for classical and acoustic music.
Except for the timbre and texture qualities of the RU6, the next more notable sound characteristic is the complete absence of the so-called digital glare that is plugging a lot of D/S implementations.
No artifacts heard, no metallic harshness, no edginess to the sound which in contrast gets close enough to the analogue ideal at least as this is possible for such a USB powered device.
The FiiO KA3 is one of the best examples of a well implemented D/S USB dongle with the least possible digital imprint but the RU6 is a step above with a more organic and natural texture.
NOS or OS?
Switching to the OS mode doesn’t induce extra digitus but the sound somewhat loses some of its analogue, organic magic.
To be honest, it is quite difficult to tell the two settings apart and it requires attentive listening and good headphones.
With the OS setting you give up some timbre, fullness and staging abilities in order to gain a little extra clarity, better definition between the lines, a leaner, tighter presentation and a touch of treble extension.
As said, differences are not night and day while the overall sound characteristics remain the same.
In the end
Thanks to the Cayin RU6, it is the first time that the user can experience the analogue like sound characteristics of the well designed R-2R technology from a USB powered DAC dongle.
In a market already flooded with delta-sigma USB dongles that sound more or less the same, the RU6 with its natural timbre and organic character is a welcomed addition.
Sound-wise the RU6 is an absolutely stellar performer and more than highly recommended especially if you are already a fan of the R-2R technology and you would like to experience it from a portable device or if you seek to discover the R-2R sound without breaking the bank.
Judging from sound performance alone, the RU6 is a five star rating without the slightest hesitation.
Unfortunately the possible hissing due to EMI interference would not allow for the full rating and is forcing a half star deduction.
But in the end, the RU6 is so good that it is fully enjoyed at home and not from your phone.
Without hesitation is one of the best sounding and most powerful USB DAC dongles available right now.
Copyright – Laskis Petros 2022.