Introduction

I think I can say that everyone who loves tube amplifiers knows Cayin. They have many awesome products in their offer, like Cayin N3 Pro which won the 2020 award for the best DAP and product at Ear Fidelity. Well, I’m not going to hide that I love many of their products, like HA-6A, HA-1A Mk2, N6II, and many others. This time they made a really good job with RU6, a portable R-2R DAC. It keeps Cayin’s sound style, but it isn’t exactly like many of its products. I was waiting for some interesting, fresh DAC to replace Cayin N6ii R01, and here it came.

The market for tiny, portable DAC/AMPs that plug right into your phone via USB has been growing for the past few years, and right now it is full of good offerings from many brands. All of them use Delta-Sigma DACs though, and the RU6 is the first r2r based dongle on the current market. If you’ve read our review of the Hifiman Deva Pro then you know that we’re really into fresh ideas. R2R DACs have been on the market for ages, but it is quite recent to have them implemented into small packages like the RU6. Having in mind how many people really like their sound signature, it is a much-welcomed addition to the world of current DACs. 

At the end of the day though, a revolutionary approach to engineering doesn’t mean anything if the sound is bad. Cayin never disappoints though, so this is going to be interesting.

Build Quality & Packaging

All Cayin products are made perfectly, and it’s no different this time. RU6 is a little bulky and heavy, but it’s all because of the materials used and R-2R DAC inside. There are CNC chassis and a sheet of glass on the top. Under the glass, you’ll see an OLED, 1-inch screen that shows the mode, volume, and gain. You can connect with the RU6 using a USB-C cable, and plug the headphones with a 3,5mm jack, or balanced 4,4mm. The USB-C cable that comes with the RU6 is very soft and thin, so it’s not bothering me when I’m using this on a walk, but I’d be careful during a longer period. On the top of RU6, you’ll find three buttons. The first and second ones are there to control the volume. The third one goes into the menu, so you can change the gain, turn the oversampling off or change the time before the screen will fall dark again.

Along with RU6, we received a leather case, but it’s a little disappointing. It’s well-made, but it’s too loose. It slips off the RU6, so it’s a little problematic to change the volume sometimes. If I won’t grab it strong enough, it’s going away.
In a small box with a render of RU6 on the top, we will find a modest set of accessories. There’s a 10cm USB-C to USB-C cable and an adapter for USB-A.

Having all that in mind, I’d call the overall physical aspect of the RU6 very good. While it doesn’t look too different than the rest of the market, its build quality is more than solid. The packaging contains basically all the essentials, so you’ll be good to go from the moment you’ll unpack this little guy. 

Tech

Cayin RU6 is the cheapest R-2R DAC on the market. A resistor ladder is an electrical circuit made from repeating units of resistors. It performs a Digital to Analog conversion by using repetitive arrangements of precise resistor networks in a ladder-like configuration  Another interesting thing inside the RU6 is the resistor-based volume control. It’s a new thing in the world of dongles. It’s recommended to maximalize the volume everywhere before the RU6 because it’s maximizing the sound itself and dragging it down using these resistors. There’s a 99 step volume control, and sometimes you can hear a short break in the sound. Don’t worry, your RU6 isn’t broken, it’s just how it’s made. It’s not a bug, it’s a feature.

Another thing implemented in this dongle is a simple oversampling. There’s no DSP like in Luxury And Precision W2 or Lotoo PAW S2, but I’m this type of person who never uses them. Unless they’re really well-made, I’ll always feel that’s something isn’t right. So, the provided oversampling is getting everything up to 384kHz. There’s a slight difference with it, but I’d say that it doesn’t boost the details. It’s giving a little more life to the sound, which’s a good thing for some.

Sound

At first, I must say that RU6 is really affecting the original sound signature of the IEMs. It’s a very natural sounding DAC/Amp, and some may say that it’s too kind and boring. Well, yes and no. It is providing more life and gives this missing claw to all very kind, neutral IEMs, like Fir VxV. And it can take away this claw along with the fun of other, live-sounding earphones, like Campfire Audio Honeydew. 

The bass is rather smooth but can be really powerful. For example, it goes well with Dunu EST112 and others that have a dynamic driver for low frequencies. Even if RU6 has a fair amount of power, it’s better to get an IEM that isn’t really power-requiring. With a DD, the bass is a little boomy, but provides a nice tempo and isn’t going over the rest of the sound. When I’m plugging some BA earphones, it’s lacking a beat, goes a little further. On the other hand, it gains a lot of speed. When I’m listening to rave, it’s the best with a single DD IEMs, like Honeydew. It’s full of joy and lets me get the fun from the music. Things are changing when I’m plugging in the Craft Ears 4 with Cross Lambda Ignite Pro. The lows are calmer, especially the subbass and kickbass. The midbass is going totally wild with this setup, it’s rich, full of meat, with a pleasant texture. It’s satisfying me with some metal, like “Marysia” by Percival Schuttenbach. This song with RU6 and CE4 sounds unique, and I think that’s where this combo gets really impressive. It’s also lovely with modern pop, for example in “Boys Will Be Boys” by Dua Lipa. This song isn’t bass-heavy, but there’s a great midbass, and it fulfills the song perfectly. 

The midrange is offering a lot of details, combined with bold, strong male voices. It goes great with very different singers, starting with Ray Charles, going through Michael Kiwanuka, and ending with Kendrick Lamar. They all have absolutely various voices, but each of them is like the icing on the cake. Joyful, pretty sweet, and really enjoyable. Let’s talk about Mac DeMarco and his “Chamber of Reflection”, it’s a slow song with a variety of instruments, with a little recessed voice. Some gamers probably know how they’re moving their head when they’re playing racing games like it could help the car to turn. Well, it’s similar here, because I’m always dragging my head towards the singer’s voice. Female voices are shown a little different, they’re still bold and strong, but they’re rather becoming a little tougher, like your girlfriend when you had done something wrong. For me, it sounds really great with Billie Eilish’s voice, especially on her older tracks, from “When We All Fall Asleep Where Do We Go”. It doesn’t happen when there’s a singer with a sweeter voice, like Taylor Swift (don’t judge me). They’re a little nasal with BA at mids, but they still keep this sweetness. With full-range DD drivers, the nasality disappears. 

So, I mentioned the details at the beginning, and I must admit they’re awesome. Obviously, it’s not a DAC for those who are all about the technicality, as those details are rounded and smooth. Anyhow, the RU6 is helping the recordings which weren’t well-made, but for some, it can lack the sharp-cut finish with TOTL recordings. 

The treble is rather calm and charming. You won’t find any additional sparks there, but also there are no sibilances and razors (I’m sorry electronics fans). It’s just a pleasant, detailed yet smooth treble. I played my favorite song for highs, the “Pristine” by Snail Mail, and for me, it’s lacking those sparks and joy. On the other hand, every other artist I’ve listened to sounds great. It’s wonderful with the whole “The Witcher 3: Wild Hunt” soundtrack. There’s no feeling of this boredom from the “Pristine”. Overall, I’d say that the treble of RU6 is perfect for the most soundtracks I listen to. Let’s take “Cyberninja” from Cyberpunk 2077, or “Troopers” from “The Mandalorian”. These songs got a nice treble, and it’s great marked, even if the RU6 is trying to calm them down. Even the slow, sad soundtrack from “The Pianist” is great, as the R-2R DAC is giving further those sad emotions from the movie, and it makes them sound so natural. With a Dunu Falcon Pro using the “Reference” filter, it’s almost feeling like a live instrument. 

The soundstage is the part where Cayin RU6 is ripping the skin off its opponents. There’s no dongle with such a great soundstage. It’s wide, deep, and exact. It’s similar to the Earmen TR-Amp, which is nasty at its size and price point. I can easily distinguish all directions, and they are shown in a very natural manner. In “Come With Me” from Mandalorian, I can hear all the violins in front of me, making a wall from instruments. The next song is a “FREEPORT” from Tenet, and this rumbling bass is very impressive, it’s going all along through the space. The imaging is also wonderful, it’s not about raw pinpoints, it’s showing the sizes and shapes of the sound source, using all space. It’s only different with songs like “Sleeping on a Blacktop” by The Dead Tongues. In this case, the soundstage is separated into three places by side, and there’s a fair amount of space between them. 

VS. Lotoo PAW S2

I’d like to thank the Polish distributor, Audeos.pl for providing me the sample of Lotoo PAW S2 for this comparison.

Lotoo PAW S2 is a very good portable DAC, it’s more universal thanks to built-in DSP, but it offers a delicately different sound. Both dongles sound natural, but Cayin RU6 is cleaner, especially in the midrange. Lotoo PAW S2 has a little more bass and life, but even if the RU6 has a smooth treble, the S2 is going a little further in this case. The biggest difference between those two is the soundstage, where RU6 wins without any doubt. It’s higher, more profound, and more exact. S2 provides a little thicker sound sources, but it gets lost with bigger squads and single DD IEMs like Dunu Falcon Pro. 

VS. Luxury&Precision W2

L&P W2 offers a denser and a little warmer sound signature. It has more subbass and delicately more treble, but it has a smoother sound to the midrange. By saying dense, I mean that everything is a little bigger and closer to the listener, even more than with S2. That also means that RU6 is again winning in terms of soundstage, as it’s bigger, more exact, and more spacious. Of course, we can’t forget about the DSP in L&P, which is great, but as I said before, I’m not the one who’s going to use it for anything else than testing personally. 

Summary

Cayin RU6, the best DAC of 2021 at Ear Fidelity. Well, this title wasn’t granted by accident, and we’re sure of it. This is the cheapest R-2R DAC, with a fantastic soundstage, very good technical capabilities, and a lovely timbre. It’s may not be for everybody, especially as it’s changing the sound signature drastically, so if you own several pairs of IEMs, all of them will be somewhat brought together in terms of tonality. However, it is a lovely, R2R-ish type of tonality, so there’s much to love. 

Definitely recommended. 

Gear used during this review for the sake of comparison and as an accompanying equipment:

Headphones – Hifiman HE-5XX, Dunu EST112, Dunu Falcon Pro, Craft Ears Aurum, Craft Ears Four CIEM, Fir VxV, Campfire Audio Honeydew
Sources– Lotoo PAW S2, EarMen Eagle, EarMen TR-Amp, Luxury & Precision W2 (for a short moment)