Cayin C9 - Portable Amp

The C9 is a portable, fully-balanced, dual timbre (KORG Nutube Vacuum Tube/Solid State) headphone amplifier from Cayin. It has the following features, according to its entry on Cayin’s website:

  • Fully balanced, fully discrete, 4-channel headphone amplifier
  • Delivers up to 4,100mW (at 16Ω) or 2600mW (at 32Ω) per channel.
  • Select between Vacuum Tube and Solid State timber on both balanced and single-end inputs and outputs.
  • The tube timber circuit is designed around a pair of KORG Nutube vacuum tubes.
  • Switch between Pure Class A and Class AB amplification modes.
  • Dual input mode: regular LINE input mode and PRE-amp input mode (or known as pure power amplifier mode).
  • Supports 3.5mm SE and 4.4mm BAL for both input and output, the amplifier will also optimize BAL to SE or vice versa.
  • 4-channels ALPS potentiometer with a pair of stereo electronic volume.
  • Removable battery module with 4x user replaceable Sony18650 rechargeable lithium batteries.

This is the place to discuss the C9, which is available for $1,999 USD.

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I got mine in today, so I’ll post a review of it here in the next couple days once I’ve had some time to listen to it.

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I wish they would make an e-stat amp…

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Huh, that’s an interesting thought. When you say “they”, are you referring to Cayin specifically making any e-stat amp or just someone making a portable e-stat amp?

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Well, Cayin seems to have more experience with the little triodes. I have also tried to put this idea into Sajeev’s head @nectarsoundnet, and would be pleased to see it picked up by iFi. Listening, @SebastienChiu?

According to Birgir at Mjölnir Audio, there is no existing high quality mobile e-stat amp, and he doesn’t have any interest in making one. I’d be OK with any good one, but the tube option would make it special.

Serious thought would need to go into battery capacity, possible removable batteries, and/or option to use an external battery pack. Removable might be pretty easy - I’m thinking of my GoDox Li-Ion flash unit.

Always listening ;), we’re watching and keeping track!

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Initial Impressions

First off, I need to say that there hasn’t been a set of configurations that I haven’t enjoyed yet. This amp so far is incredibly enjoyable to listen to. So much so that my more critical review has been delayed multiple times now because I’ve been enjoying listening to it so much.

The quality of music produced by this device comes close to my Manley Absolute Headphone Amplifier - a $4,500 desktop tube amp, and definitely surpasses my A90 for musical enjoyment, if not necessarily for extreme detail. It also easily surpasses the amp in the Q5S (shocking, I know) that I’ve primarily been using as the source, given it’s the best portable DAC I have. That being said, I’ve also fed it with the RCA out from the Topping D90 and from the RME ADI-2 FS R Black Edition. I’d say the D90+C9 combo can easily match the technical performance of the D90+A90 stack, apart from extreme detail (I’d say the A90 is just slightly more detailed, but not always in a good way). The RME+C9 easily out-performs the RME’s built-in headphone amplifier.

I still have a more complete and detailed review coming, I just haven’t been able to finish it yet. Hopefully I’ll be able to do so this weekend.

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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.

  1. 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.
  2. 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.
  3. 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.
    1. 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.
    1. 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.
  4. 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.
  5. 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.

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Nice write-up.

I’m really glad you called out the charging/power nonsense.

I was going to order one of these on Monday, as a transportable “tube”/solid-state amplifier (long story) for my travels.

Though I guess that isn’t going to happen now … unless their section “Instructions on Charging and Protecting Circuit for C9” is in error (hard to tell; it’s one of the most poorly written sets of instructions/cautions I’ve seen). Either points 1 and 4 mean the same thing from a different perspective, or they’re different issue with a common outcome. Either way, the take aways from this are:

  • You can’t charge the unit and the source while listening!

  • You can’t use a laptop as source directly nor into a USB DAC (or DAP acting as such), while charging the C9 from the laptop, even if the laptop isn’t being charged.

Those are pretty big issues from a transportable use-case perspective, and a long way from ideal when it comes to preserving the battery longevity of the source device (you don’t want to run lithium batteries down and then fully recharge them over and over if you want them to last).

Notwithstanding the potential for damage if you forget to disconnect one device from charging before connecting the other.

You could work around that to a certain degree by using more than one set of cells/battery case with the C9, but that requires a screwdriver to change. Having screws as an option, with a simple metal locking slider so they’re not necessary would make that a lot less hassle.

Pity.

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Have yiu had hands on a bx2plus to be able to compair the technicalities of the two?

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Yeah, when I got that email I was slightly perplexed, and somewhat worried even. Having had it for a little while, I haven’t had any major issues with it.

So, the way I interpreted it, you can charge it and listen to music at the same time, so long as the music source’s ground isn’t the same as the ground from which you’re charging it, but maybe I misinterpreted it? Based on that understanding, I’ve been using it while charging about half the time and haven’t had any issues. Luckily when it does go into power protect mode it’s pretty easy to get it out of it and the protection mode seems robust, but I’m not an electrical engineer so I can’t say for sure. I may send it to a friend of mine who is an EE and get his thoughts on it.

I absolutely agree about the battery compartment, and I’m considering getting a few little thumbscrews instead of the T6 screws that it comes with by default when I go back to working from the office where I plan to use it.

For me, these aren’t dealbreakers (as I understand them), but rather they’re just things to be aware of when it does enter power protection mode. That being said, I can totally understand how they’d be issues for other folks.

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I haven’t heard the BX2 Plus, but from what I’m seeing in the spec sheets, the BX2 Plus is a DAC+Amp, whereas the C9 is just an Amp (no DAC). It also looks (from what I can tell) like the BX2 is Solid-State / Class A only, whereas the C9 is Tube or Solid-State / Class A or Class AB, so the C9 provides more flexibility for the amplification circuit.

Nah. Bx2+ is amp only. They say pure class A but I’m slightly skeptical at 5.9w

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This is what confuses me. Most type C chargers don’t even use a ground prong, so how do you get a shorted ground? Is the issue that you don’t have a ground? Cause if so thats an easy fix ss well

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Yeah, I’m honestly not sure. Electrical engineering classes taught me one thing: I’d make a terrible electrical engineer. Hence why I stuck with Maths & Computer Science :smiley:

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Point 4 simply states not to charge the amp and the (connected) source at the same time. Point 1 talks about using different USB chargers. It’s not clear if point 4 doesn’t apply in the case that you’re using different chargers to observe point 1.

Protection circuits aren’t infallible. I’ve had protected devices die (sometimes taking out what they were connected to as well) in the past. The chances of me screwing up when I’m tired, traveling and probably distracted are too high/frequent to rely on that.

Notwithstanding that I don’t want to have to carry multiple chargers.

The thumbscrew option for the battery box is a good idea.

Though fundamentally, for my use case, the ability to play and charge both source and amp while listening are not really optional. And not just because that’s really hard on the battery in the source (which, unlike the C9, can’t be swapped easily if at all).

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Damn, you’re right. I dunno how I misinterpreted point 4 so badly, and I’ve definitely been doing that wrong. I’m definitely going to send it to my friend to see if he can figure out a solution for that because that’s a pretty big design oversight.

Both totally valid points. I’ve been lucky enough not to have encountered such problems before, I guess, but you’re absolutely right.

Out of curiosity, is there anything else you’re looking at instead? I was originally looking at the WA8 for portable tubeyness but opted for the C9 instead due to its dual-timbre/dual-class functionality.

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Not really.

I looked at the W8 (for somewhat different purposes) when it came out. While the amplifier section sounds great, the DAC I don’t care for, and the unit it is too big (almost double the volume of the C9), needs a dedicated charger and the battery life is too short.

On a practical level, I’d never take it anywhere.

The C9 was appealing as a potential way to add a USB-chargeable transportable “tube” output option to either my SP2000 and/or Hugo2. Then I’d sell my N8 as I really only have it for its tube output at this point, as the N8’s lack of on-device streaming is ever more irritating and limiting.

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This was my reasoning, too. The only other portable tube amp I’ve looked at is the LittleBear B4-X, but I opted for the C9 instead.

Yeah, I looked at the N8 too, but I opted against it for the exact same reason: streaming. I’m hoping that portable tube devices become more common since I feel it’s a really neglected market segment.

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Any reason y ou toyed with the idea of a bx4 and not the BA300S?

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