This thing is awesome. For years now, I've been on the hunt for a portable source that drives headphones well enough that I can enjoy them as much as more powerful desktop gear.

There have been many candidates (Fiio KA17, Hiby R4, etc.) but none of them have satisfied me.

Until I got the Cayin RU9. While it has its limits (I don't enjoy the Sennheiser HD 800S on it, for example, though that's considered a notoriously amp picky headphone by those who believe strongly in synergy), I have thoroughly enjoyed every other headphone I've used with the RU9 - including planars!

My experience has been that dynamic drivers can often sound pretty okay on portable gear, but I hadn't used a single portable device that I enjoyed something like an Arya Stealth or a Moondrop Para 2 on until the RU9.

The Modern and Classic tube modes are both great (I don't really touch the solid state mode, which sounds fine, but the tube modes are just better). I lean toward the Classic tube mode for more neutral/brighter headphones and older/rougher recordings, and the Modern tube mode for well recorded music on warmer headphones (like the Z1R).

The ability to quickly switch between the modes is awesome.

There are only a couple small downsides I've encountered: the 3.5mm connection is not as good as balanced (not unexpected for a device that's built to be balanced) and I have encountered some noise when using it.

The battery life isn't great, but I love that there's a second USB input so you can hook it up to a battery bank and not worry about that. (HyperMode, which gives you over 1W of power, is also awesome to actually get good performance from harder to drive headphones.)

The Bluetooth functionality is awesome and I've used it frequently when I need to charge my phone. LDAC stability and sound quality is great as long as your phone and the RU9 are close together.

This isn't the kind of device you can carry around the house while your phone is in another room and still get good performance. But if you think of the Bluetooth mode as an alternative to USB-C for when you want more freedom (because you're actively on your phone or need to charge your phone) then it's great, and I sort of suspect it may force the highest quality LDAC offers, which would explain why it works awesome close to the source device and not so awesome when you walk away from it.

Overall, this thing is incredible. $500 seemed like a lot for a "dongle" DAC (technically) but it's totally justified by the performance, and I've probably spent as much time on my couch enjoying music with the RU9 in the last month as I spent in the last year doing so on other devices.

I’ll try and keep this short and only talk about the important things in an abbreviated form (as best as I could given the word count I ended at) but the RU9 is a product that I both really like and dislike in certain ways. The overall package with this device at $499 is fantastic and I am satisfied with this purchase but there are issues that hold it back and is why I gave it the score I did even though I really love the sound coming from it.

I wont talk too much on the sound but it was incredible on all three available modes and was immediately an improvement over the Qudelix 5k I use at work. Testing was done with Hexa’s, Monarch MKIV, and Focal Clears. Classic tube mode ended up being my favorite but found myself switching between Modern and Classic depending on my mood. The solid state mode was nice and neutral sounding but I don’t really go for it as the main point of this thing is to listen to one of the tube modes in my opinion and is really what gives this thing the value it has and was pretty stable with no tube ring unless I was really trying to get it to come out.

Battery life on the RU9 in my experience sits somewhere at a bit over 3 hours with Balanced input and closer to 3.5 hours on the 3.5 mm all done on Tubes. I was able to get closer to 4 and 4.5 respectively on the solid state mode. This is definitely a bit shorter than most would like but with my case I’m lucky to usually be not too far away from an outlet. Using the usb + power mode available in the setting gives really great battery life at the cost of your phone battery. Another option is to use a magsafe battery such as the INIU which also gives access to hyper mode which is probably the best option and is what I do.

The magsafe feature really helps with the transportability of it and paired with a relatively thin phone gives the RU9 a great pocket feel when heading out on a walk. However, on extended listening experiences I found myself using the magsafe to attach to a portable battery bank rather than the phone. Adding this all together in a pocket does seem like much even with looser clothing so after about a week or two I found I’d much rather reach for a TWS in these cases if I needed an extended listening experience on the go.

Bluetooth in my opinion could have probably just been left out if it cut the cost. The range is pretty terrible and just walking from one end of the room to the other end was enough for the RU9 to drop the connection. The only time I use the bluetooth mode is for resolving issues I had with the RU9 which the rest of this review talks on.

Now the problems I have with this thing come from absolutely terrible crackling I experienced taking this thing to work (The reason I purchased this in the first place). The RU9 seems to be extremely susceptible to EMI so I had to go through multiple cables and leave it at a specific spot on my desk to get the crackling down to tolerable levels. This makes it pretty much unusable at work unless I put the RU9 on bluetooth to keep it away from both my phone and desk which eliminates it entirely. Using this thing at home though is great and I don't have any of the major crackling problems that I do at work but was not really why I purchased this in the first place since I already have an at home setup.

When in my pocket I also occasionally notice crackling but within tolerable ranges and suspect it has to do with the antenna position in my Zfold7 but I’ve learned to deal with it. It’s honestly disappointing but I assume people using a DAP or phones with better antenna positioning wont have this problem at all.

Cayin RU9 | Vacuum Tube Portable DAC and Amp

This product is absolutely spectacular and as far as portable amps go it does a phenomenal job with my headphones. There was a factory defect with my first pair and Bloom did an exceptional job of replacing it, and I have never been happier with an audio product..

Coming from the orginal ifi go blu paired with my hd6xx this is a game changer.
I listen almost exclusively in bluetooth mode, as LDAC sounds as good as a wired connection to me and I like feeling untethered.
I always had a feeling I am not getting the most dynamics out of my hd6xx with the ifi go blu although it does reach satisfying SPL levels.
By contrast, the Cayin RU9 can push 4 times as much power through the hd6xx drivers in hyper mode (minimum 20watt usb port). I was pleasantly surprised with the greater flexibility and ease I was granted in eq'ing the hd6xx to my preference without reaching the limit of distortion. Having greater flexibility in tonal control with the classic, modern, and class A modes is nice, though I tend to leave it in classic mode.
I recall tube rolling genelex gold lions into an old little dot amp, and the gooey tube character was very apparent and charming, if syrupy. The Cayin RU9 does not reach the full-fledged gooey tube nature like that amp. But it does introduce just enough tube character to keep things interesting, without generating significant heat like traditional tubes.
If I had to detract anything from this device, it's that I am not finding great use for the magsafe compatibility, bluetooth range is not as good as the ifi go blu, and that the battery life could be better. In terms of battery life, best case is I can get 4 hours in medium gain listening at medium to high levels. At the same time, I don't leave the house with this dac so I am always near a power source. To me, the Cayin RU9 is portable in the sense that I can take it from the living room to the dining room.
My suggestion for those considering this is you have two options if sleekness, power, and bluetooth are your priority, this or the ifi gryphon which has equivalent power output into 300ohm, without necessitating auxillary power. I wanted a departure from ifi, and I was curious about how Cayin implemented the nutubes into this hybrid dac. For my use case, 5/5