Today, Marcus reviews the Cayin C9ii, a dual solid-state and Gen 5 Nutube circuit portable headphone amplifier with up to 4.1W output power. It is priced at $2399.
Disclaimer: This sample was sent to me for my honest opinion. Headfonics is an independent website with no affiliate links. I thank Cayin for their support.
You can click here to learn more about Cayin products I have previously reviewed on Headfonics.
This article follows our current scoring guidelines, which you can read in more detail here.
It has been just over 4 years since Cayin released the award-winning C9 portable dual NuTube and solid-state headphone amplifier, so, like the recently reviewed N6iii, it feels like now is the time for a refresh.
The new $2399 C9ii is that refresh and then some. It’s physically bigger, and aesthetically bolder, with even more operation and timbre modes than its predecessor.
With the same excellent output power rating as the original, the C9ii should be able to drive a wide range of demanding headphones currently available.
For many, the C9 was the ultimate portable NuTube headphone and IEM amplifier. With the notable exception of the recently released iBasso’s PB5 Osprey, there is still very little like it in the market today, which was pretty much the case in 2021.
Does the new C9ii still stand alone on a lofty perch in 2025? I found out in my full review below.
Features
Modes Galore!
The Cayin C9ii is a portable/transportable discrete-engineered headphone and IEM amplifier. Like its predecessor, the C9, it offers a choice of amplification and lineout/PRE, including a Gen5 Nutube 6P1 tube setting and a quad Toshiba 2SK209 JFET transistor-based or solid-state option.
Each topology also has the familiar DAO or Dual Amplification Operation capability, meaning you can switch between Class A and Class AB with significant sound changes.
However, the C9ii goes further with an additional Hyper Mode, a setting I recently tested in my N6iii review. A short analogy of Hyper Mode would be a hybrid of Class A and Class AB, delivering a very dynamic and technically capable sound signature at the cost of a shorter battery life.
The C9ii also retains a timbre mode option. However, this time, it has been enhanced from 2 to 3 choices: a cleaner modern tube sound, a warmer and richer classic mode, and an energetic Class A solid-state option, the genesis of which I had already encountered in my review of the N3Ultra last year.
But there is more. In either tube timbre mode, you can further tweak the tuning with a new high and low Anode mode switch.
This switch adjusts the operating voltages of the NuTube plates to create two different performances: one that is heavier and denser sounding, and the other lighter and airier in delivery.
We are not yet done with new modes. The C9ii also includes a new NFB or Negative Feedback switch on the side, which, when turned on, further tweaks the amplification circuit via local negative feedback, producing an even fuller sound compared to the stock mode performance.
Performance Numbers
Technically, the output power of the C9ii is unchanged from the C9 at a maximum of 4.1W balanced (16Ω), suggesting Cayin felt the available output power numbers were still competitive in today’s headphone and IEM market.
While that may surprise those chasing after the “newer, more powerful angle,” one only has to dig into the underlying numbers of the C9ii amplification to see that Cayin has focused on the quality of that power rather than just upping the output.
For example, in a high gain setting with a 32Ω, the C9ii’s dynamic range has improved between 5 dB and 9 dB (A-weighted) depending on the amplification mode and whether you opt for balanced or single-ended pairings. You get similar dB increases for SNR on the same load.
So, while the Cayin C9ii output baseline numbers are unchanged, the quality of its output power has significantly improved on paper, meaning a blacker background and less background hiss for sensitive gear users.
Design
The C9ii is an absolute beast. I knew it had to be bigger than the already large C9, but placed alongside the original, the C9ii is heavier by 100g, wider and taller, and with the length of each being the only dimension that comes close to matching.
However, there are benefits to that enhanced size. The C9ii now has a welcome set of guard rails on the front panel to protect all the controls from accidental drops. It also has improved heat dissipation with ribbed heatsinks built into a curvier aluminum alloy side panel design.
Size also means a larger, grippier volume potentiometer dial with improved space on either side for easier adjusting. The wider top panel also means Cayin has been able to integrate larger glass panels, so the tube’s green lighting effect is more striking than the original.
Perhaps the most useful change in the new design is the new quick-release battery tray system at the rear of the unit. Ditching the screwdriver system of the original mirrors the company’s change in approach to their N6ii and N6iii DAP and motherboard system.
That means no more screwdrivers and tiny screws with a quick press unlocking the tray and sliding out in seconds to reveal 4 high-drain Samsung 35E 3400mAh 18650 lithium batteries that deliver between 8 and 17.5 hours depending on the output connection and mode/timbre setting.
These batteries are user-replaceable, can be charged outside of the unit if so required, and can invite ‘battery rolling’ for other solid options such as the LG MJ1 (green) INR or the Panasonic NCR18650GA.
I/O
The back of the C9ii is quite similar to the last generation with a single USB-C port for charging and a 4-stage LED light system denoting the level of charge available.
You can opt to just plug in the supplied USB cable to charge the C9ii, or like me, use a dedicated 18650 charger such as the Nitecore i4.
It is the front of the device where all of the sound-related I/O are. As you can see from the picture above, the C9ii’s larger size has an advantage with clearer labeling, thicker and easier-to-manipulate switches, and a much bigger ALPS potentiometer (volume dial).
Everything now feels a lot easier to read and operate than before. And it had to be because you have a lot more mode and timbre options to choose from that I think would be impossible to arrange neatly on the older, smaller unit.
Technically, the zoning of the functions has not changed, save for the PRE and new NFB features, which are on the side panel. These two features are now easier to see and operate by pressing rather than sliding the PRE button on the C9.
The PRE operates similarly as before, with a long press to activate. Just ensure your source volume is low for PRE operability and the C9ii volume dial is at zero when you start a regular lineout signal to the amp.
As before, line-in ports are to the left, and headphone outputs are on the right, with both offering single-ended and balanced options.
Controls
The change to front panel toggle switches from the older sliding switch system on the C9ii feels more robust and makes the amplifier much easier to control from your fingertips.
It is also the reason why we have the new guard rails on either side of the C9ii front panel to protect the toggle switches, something that was less necessary on the older C9 flatter front panel control system.
The larger ALPS potentiometer dial also has a wider diamond cut or grippy region with wider spacing on either side, which gives the volume control of the C9ii a nicer feel than the original. This is a smoother dial, much better for micro-adjustments than the comparatively more resistive and choppier C9 version.
I must also mention the bolder and whiter labeling on the C9ii’s front panel, which soothes my aging eyes more than the diminutive equivalent on the C9. In short, it’s more legible to read what each switch does.
Packaging & Accessories
Because of the new quick-release battery tray, Cayin no longer needs to ship the packaging with a screwdriver and spare screws inside.
So, the C9ii accessory lineup is slightly more frugal than the C9 in terms of individual elements, but in this case, it’s a bonus rather than a negative.
The packaging itself is dimensionally similar to the C9 version but has a snazzier external printed finish.
Instead of a layered tray box, you now have an expandable jewelry-type slider tray system with the accessories and manuals tucked into the lower tray and the player deep set into a very protective upper tray.
The most important accessories are the USB-C to USB-A charging cable and the stock, short, balanced, and single-ended IC cables, which are the same ones that came with the C9.
The exact code name for each cable is CS-44C44II for the balanced version and CS-35C35II for the single-ended option.
The wiring for both is a mix of 8-wire crystallized copper and 24-core 6N OFC copper geometry packed in a translucent PVC jacket with supple braiding and a solid black stiff rubber strain relief at the base of each connector.
Click on page 2 below for my sound impressions and recommended pairings.
