Sorry for the delay! Having spent many hours now listening to this thing, here are my (very subjective) thoughts on this device.
Build
TL;DR: 8.5/10 Fantastically built unit with points taken for easy fingerprint/oil collection and for the easily-scratched glass bottom.
The device itself is extremely well-built. The volume knob has a decent amount of resistance but still rotates smoothly. The power button has a satisfying tactility and everything feels very nicely put together.
The addition of a battery module with replaceable, commonly-available batteries is a very nice design consideration that means this device will be useable for a long time to come.
However, I do have two gripes with the build.
Firstly, the flat glass bottom. Even though it comes with a protector pre-installed on it, I still feel the need to be careful to clean off every surface I set it on so I don’t scratch the bottom.
Secondly, the matte-black paint is a bit of an oil magnet which keeps the device from looking like the $2,000 device it is at all times.
Connectivity
This device is fairly simple, with exactly 5 ports total:
Inputs
- 1x USB Type C for charging
- 1x 4.4mm pentaconn balanced input
- 1x 3.5mm TRS single-ended input
Outputs
- 1x 4.4mm pentaconn balanced output
- 1x 3.5mm TRS single-ended output
Accessories
This device also comes with a nice suite of accessories:
- 3.5mm to 3.5mm interconnect cable
- 4.4mm to 4.4mm interconnect cable
- USB Type-C charging cable
- T6 screwdriver
- User Manual
- Extra battery module screws
A Note on Power
Shortly after ordering the C9, I received an email with very explicit instructions around the way the battery and power circuitry in the device works. In short:
- The charging circuit and audio circuit have separate grounds. If you simultaneously charge the C9 and whatever source device you’re using from the same source, you can short the grounds.
- Unlike cellphone batteries, the batteries in this device can over-charged and over-discharged, which can permanently damage them.
- Listening to the C9 while charging can cause the batteries to over-charge if the charger provides more power than is being consumed or under-charge if the charger provides less power. Make sure to unplug the device when the batteries are full or to stop using it when they get too low.
- Do not charge with Huawei chargers, as they use a different quick charging technology that is incompatible with the C9.
- If any of these cases occur, the C9 will enter a power protection mode. To get out of this mode, remove all connected devices for at least 10 seconds.
In my usage thus far, I’ve only had it enter power protection mode once from touching it and getting a static electric shock.
However, given all of these potential issues, it really feels like the battery status indicator should be on the top of the device at the very least or, better yet, they should have resolved these issues prior to release.
You can read the full, exact C9 circuitry warning document here.
Instructions on Charging and Protecting Circuit for C9
The 18650 is very different from the 3.7V Lithium battery that are commonly find in mobile
phone. Overcharge and discharge of lithium-ion batteries will cause permanent damage to the
positive and negative electrodes. Excessive discharge leads to the collapse of the negative
carbon layer structure, and the collapse will cause the insertion of lithium ions in the charging
process; overcharging will cause too much lithium ions to be inserted into the negative carbon
structure, causing some of the lithium ions to no longer be released. (Extracted from HERE)
A fully charged 18650 is around 4.2V and a discharged battery should stay at around 3.0V. A
deeply discharged cell has fallen below 2.75V, it goes into a sleep mode and the battery is
essentially useless.
- The Battery module and the C9 audio circuit did not share a common ground, so it is likely
that there will be potential different between the two grounds. Therefore it is very important to
charge the battery with a separate USB charger. Using the USB from connected audio source
equipment (such as computer or media player) to charge up the C9 batteries will trigger the
short circuit protection, and might even damage the device and/or connected source equipment.
- In case your C9 (or the battery module) entered protection mode, you need to remove
everything and connect C9 or the battery module to a charger, charge it for at least 10 seconds
and the device or battery module will revoke from protection mode and resume normal
operation.
- After replacing or dismounting the battery, the battery module enters protective mode, you
need to connect the battery module to a charger, charge it for at least 10 seconds, and the
battery module will then revoke from protection mode and resume normal operation.
-
- Do not charge and playback both C9 and the connected source (let’s assume that is a
DAP) at the same time.
The four pieces of 18650 batteries are connected as a +8.4V and -8.4V power supply for the
amplification circuit, and there is a set of ground line in between and that is connected to the C9
chassis. The charging circuit via USB-C has its own reference Ground connected to the USB
socket. Be reminded that these two Ground are completely independent. If C9 and the DAP are
charge and play at the same time, the power supply Ground (C9 Chassis) and charging Ground
(USB socket) will short circuit by the charging cable and interconnect between C9 and DAP. C9
will enter protected mode. You can revoke C9 from protection mode by disconnecting
everything and charge it for 10 second.
-
- C9 will enter into the protection mode if C9 is overcharged.
You can charge and playback C9 at the same time before the batteries is fully charged. 18650
Lithium battery charge control is divided into two phases. The first phase is constant current
charging. When the battery voltage is lower than 4.2 V, the charger will charge with a constant
current. The second stage is the constant voltage charging stage, when the battery voltage
reaches 4.2 V, due to the characteristics of the lithium battery, if the voltage is high, it will be
damaged, the charger will fix the voltage at 4.2 V, and the charging current will gradually
decrease. Therefore, if you charge and playback at the same time and your charging rate is
faster than discharge rate, the 18650 batteries will reach 4.2V eventually. The problem is, when
you continue to charge and playback at this moment, C9 power management cannot enter the
constant voltage charge stage, this will put C9 into protection mode. You can revoke C9 from
protection mode by disconnecting everything and charge it for 10 second. You are advised to
disconnect C9 from charging when the fourth battery LED is flashing in these circumstances.
- C9 will enter into the protection mode if C9 is over-discharged.
If you charge and playback C9 at the same time and your charging rate is slower than discharge
rate, your battery will consume slowly. At certain point, the batteries will drop below 3V and
triggered the protection circuit to power off the C9. . You can revoke C9 from protection mode
by disconnecting everything and charge it for 10 second and charge up the 18650 batteries.
Since deep discharge might damage your battery, you are advised to stop your playback and
start to charge up C9 when only 1 charging LED remains flashing.
- Please do not use USB charger from Huawei mobile device to charge up C9. Huawei has it
own quick charge protocol that is not compatible with C9.
Sound
My setup & Testing Methodology
To test the Cayin C9, I used Roon 1.8 as the source via USB into a FiiO Q5S Type-C (in Exclusive mode), then the Q5S line out into the C9’s unbalanced input. I then tested it with my HD6XX headphones and with my 64Audio Tia Fourte Noir (the most sensitive IEMs I have).
For this testing, I used the following Qobuz playlist, purchased and downloaded into Roon (which I have a copy of in this Tidal playlist):
- “The Uruk-Hai” from The Lord of the Rings: The Two Towers Soundtrack by Howard Shore
- “HIM” from The Thrill Of It All by Sam Smith
- “Planet Earth II Suite: Main Theme” from the Planet Earth II Soundtrack by Hans Zimmer
- “Hello” from 25 by Adele
- “Take It, It’s Yours” from The Queen’s Gambit Soundtrack, by Carlos Rafael Rivera
- “Little Girl Blues” from The Devils’ Music by Naomi & Her Handsome Devils
I’ve also been listening to it with a pair of Focal Radiance (which take surprisingly well to tube amps given their relatively low impedance) but I have not done any critical listening with them. That being said, if you have any questions about this pairing or pairing this device with any of the other equipment listed in my profile, please do not hesitate to ask!
Noise Floor
TL;DR: Don’t use low-impedance headphones/IEMs with this device. In fact the higher impedance, the better.
With my most sensitive IEMs (64Audio Tia Fourte Noir), the noise floor varies depending on Gain setting, Timbre setting, and which output you use.
With the lower-powered, single-ended output, there was only a very slight hiss in Low-Gain/Solid-State configuration with a more significant hiss in Low-Gain/Tube configuration. For me, the noise floor in these two configurations is perfectly acceptable. However, High-Gain/Solid-State had slightly more hiss, while the High-Gain/Tube configuration had the most significant noise floor for the single-ended output and I would consider neither to be acceptable with these IEMs.
With balanced output, there was moderate to severe hiss, depending on whether Gain was set to Low or High, respectively, but it was enough that I would not recommend running any highly sensitive equipment off of the balanced output.
With these IEMs, and in spite of the built-in suspension system, microphonics are easily heard any time the unit is tapped, any time a configuration switch is flipped, or the volume knob is turned while the Timbre mode is set to “Tube”.
However, with less sensitive devices such as the Focal Radiance or the HD6XX (the highest impedance headphones I have), none of these issues were present. The noise floor was absolutely silent in all configuration combinations and there were no microphonics to be heard, even when tapping the unit fairly hard.
Overall, I’d recommend using the C9 with higher impedance devices as you’ll get the best experience out of them.
Impressions
TL;DR: Want a warm, clean, and clear configuration? Solid-State / Class A is for you. If you’re looking for a clinical and analytical sound, try the Solid-State / Class AB. For a more ethereal, idealistic timbre, the Tube / Class AB configuration has you covered. Finally, for that rich, tubey gooiness, the Tube / Class A configuration will likely meet your needs nicely.
Timbre: Solid-State / Class: A
In this configuration, this amp offers a slightly warm, clean, and very clear listening experience. While not the most detailed configuration, this is extremely enjoyable for pure pleasure listening. Great instrument separation, layering, and texture made orchestral pieces such as “The Uruk-hai” and “Planet Earth II Suite; Main Theme” really come alive. Vocals are lush and full but not overly intimate - that comes later. My favorite track in this configuration though has to be “Little Girl Blues”. This configuration added just the right amount of warmth and liveliness to this track and literally made me halt my listening session just to dance.
Timbre: Solid-State / Class: AB
This configuration is the tightest and most analytical of the various configurations I tried. It also has the longest-lasting battery of these configurations, lasting a solid 14 hours before draining to 1/4 battery. Complex pieces like “The Uruk-Hai” or “Planet Earth II Suite” were cool and crisply presented with details brought front and center, but lacking the musical warmth of the Solid-State / Class A mode. It also had a shallower soundstage than the Solid-State / Class A configuration, even though the soundstage was just as wide. This was particularly noticeable with “Him” and “Hello” where the lead vocals were very up-front with the backup vocals only slightly behind them. With “Little Girl Blues”, the clinical nature of this configuration was clearly felt, with it pointing out all of the flaws in this recording extremely clearly.
Timbre: Tube / Class: AB
This was an extremely interesting configuration to listen to, with smooth, dynamic, and detailed (but not clinically so) characteristics. It’s definitely a less warm configuration than Solid-State / Class A, but warmer than Solid-state / Class AB. It brought an intimacy to “Him” that was missing from the Solid-State configurations while bringing a dreamy/ethereal quality to more complex scores like “Planet Earth II Suite”. For simpler instrumental scores like “Take It, It’s Yours”, it brought a spacious sense of idealism (rather than realism) to the music. “Little Girl Blues” with this configuration made me feel like I was there in the recording room listening to them play with Naomi right in front of me - a truly fantastic experience.
Timbre: Tube / Class: A
This was my favorite mode, as it brought a fuller, deeper soundstage along with much more intimate vocal presentation to every song, without losing much clarity (as I find tubes can often do). With “Hello”, I was forced to ponder why Adele had appeared in my house when we’ve never dated. “Him” blew me away with intimate vocals and an incredible soundstage. With “The Uruk-Hai”, I felt that I was seated in the middle of the concert hall, surrounded by the orchestra. The same happened with “Planet Earth II Suite” and with both, the sound became somewhat overwhelming, leading the instrument separation to suffer. Sipler compositions like “Take It, It’s Yours”, on the other hand, were presented with a fantastically lush and encompassing sound and were extremely enjoyable. “Little Girl Blues” was really brought to life in this recording. Instead of being in a recording studio, I felt like I was front-and-center-stage in a concert hall. This configuration immediately brought me back to the pre-COVID-19 times where I actually danced to this song in-person - something that only my Manley Absolute has been able to do with the same sense of clarity. This configuration also hid a lot of the flaws in this recording, which I really appreciated.
Final Thoughts
This device is easily my favorite portable amp that I’ve tried to date. It gives a wide range of musical qualities in a relatively small package, allowing you to listen to your music the way you want to. As with all devices, it has its down-sides, but for me those down-sides are far outweighed by its up-sides. For me, this device is well-worth the $2,000 USD price tag, as it is effectively four devices in one, each of which can easily cost >$500 USD.
And now, time for some more music!
Edit 1: Update the section on power delivery.
