Cayin RU-6 R-2R DAC/Amp Dongle Review

Cayin is riding the wave of R-2R devices that have seen a recent revival with a first (as far as I know) in the RU6. This is a tiny USB-C dongle that contains a totally discrete R-2R DAC circuit and is powered by the USB source in a rectangular box shaped device that'll fit easily in any pocket. 

Its not the first portable R-2R device, as Luxury & Precision have done this with DAPs and even Cayin has released their own R-2R DAC/Amp module for their DAP recently as well, but this is still going to raise some eyebrows with its small form-factor that will work with any USB-C device, and iPhone devices with optional lightning cable, and a relatively low price point of just $249.

I was trying to resist the temptation to try this, but curiosity got the better of me, and I had to see how this little engineering and packaging wonder sounded. I ended up buying a set from MusicTeck along with the blue leather case.

Device Build and Features

The device is all-black with a metal and glass exterior. On the top surface, half the space is occupied by a small OLED screen. On the screen, displays the current volume number, gain level, sampling type, and the bitrate of the current audio playback. 

On the side is three buttons. Two control the volume up and down, and also work as menu controls for the third button, which when held, activates the menu. When just tapped, it'll turn on and off the display.

The menu features a few settings such as gain level (high or low), sampling mode (oversampling or no-oversampling), and backlight display (on/off).

The RU6 has two output jacks. One is the normal 3.5mm headphone jack, while the neighboring jack is a 4.4mm balanced type. This dongle is not a true balanced circuit, but 4.4mm does benefit from an additional amp and additional power rating for harder to drive headphones.

The opposite side of the output jacks is the USB-C input port. The Cayin comes standard with a USB-C to USB-C cable, along with a USB-A adapter, but for iPhone users, a lightning cable can be purchased separately.

I used the RU6 with a salvo of different transport devices: My Samsung Galaxy S21 phone, my iPad Mini 6th Gen (USB-C model), my iBasso DX240 Digital Audio Player, my Asus Chromebook, my Dell Precision laptop, and my Intel Canyon Hades NUC desktop PC. I did not have any issues with getting audio working with any of the devices, however with the Chromebook, I did get the occasional stutter and pop when I was loading a webpage or doing other background activity that was may have interfered with the audio stream. I felt like an audio buffer setting would have fixed this but the Roon Remote app on ChromeOS did not have a feature for this.

Sound Impressions

I spent a short period of time going through Oversampling mode and No-Oversampling mode and decided to just stick with the NOS mode for my music listening, as this provided the most pure form of listening to the R-2R DAC. The Oversampling mode made the sound closer to a Delta Sigma DAC with less treble roll-off, and more precise edges and a slightly quicker transient response. 

Straight out of the box, I grabbed my Unique Melody MEST Custom IEMs and plugged them into the 4.4mm balanced jack of the RU6, switched on NOS mode, and was playing music out of Roon from my desktop PC. With my previous experience with R-2R DACs, I was expecting a little bit of warm-up period, and with the RU6, this seemed true as well.

The initial listen was just a tad harsh and bright in the upper midrange, while the low end was warm and thick, with a smooth and relaxed treble. Resolution wasn't on par with a well implemented delta-sigma DAC, or my desktop Holo Spring 3 KTE R-2R unit, but it still had sparkles of clarity. After about 30 minutes, the harshness subsided, and I believe the quick and initial "burn-in" was complete. I haven't had that same level of uppermids harshness since then, and initial warm-up period was only a minute or two as this unit is quite small and getting it up to operating temperature should be quick.

Now that aside, my general thoughts through my 2 weeks of listening to this unit so far have been both positive and also filled with some negatives.

I'll start with the negatives I guess.

Due to the way the R-2R circuit is designed, there are moments where the volume with quickly pause and pop. This can happen between tracks if the bitrate changes, or switching volume quickly, or at every 10th volume step (i.e. 10, 20, 30 etc.). It's not a deal breaker for me, but it is a little annoying from time to time to hear this.

The other negative is that Oversampling mode sounds plain. Not in a bad way, but also not in a special way either. I prefer non-oversampling mode to give it the special sauce that makes this DAC a little unique compared to the competition.

And finally, if anything, in NOS mode, the treble is slight rolled-off, and also the presentation can come across as overly smooth and perhaps too rounded for some who like more precision details and edges, and fast transients. That's not what it sounds here.

The RU6 has a lot of the traditional R-2R sound that I do enjoy, but it's not quite hitting it home with the big desktop setups like my Holo Audio Spring 3, or the Musician Pegasus. But, for this tiny little dongle thing that cost $249, this thing is quite decent and enjoyable.

I find the tonality very much similar to the Chord Mojo, with a warm and intimate presentation, and overall smoothness that sounds mature and inviting. The Mojo has a little better depth and detail, but the RU6 isn't too far behind it either. 

If anything, the RU6 has more treble extension than some may believe, given its warmer low end. It doesn't lose any detail overall, but I do prefer this with more neutral or brighter IEMs. With warmer or bassier IEMs, the RU6's charm is not as balanced, and with things like the CCA CRA, I find it too bottom heavy. 

With the Empire Ears Odin, which does have a big bass boost, I love this combination and have been enjoying this and the Hidition Viento combination quite a bit, that I've put down my over-ears for the past couple weeks and focused on just this dongle. I've been gravitating towards this dongle over the iBasso DX240 lately too, but part of it is due to a new experience, but it's also a nice intoxicating blend of soulfulness and low-fatigue that provides a long lasting listen.