Cayin C9II Portable Headphone Amplifier

Description

Cayin C9ii – Reaching New Heights

 

When Cayin released the first generation, TOTL C9 portable amplifier back in 2021, it quickly gained recognition among audiophiles for its Tube/Solid-state dual timbre, direct-drive battery-powered circuit, discrete Class A/AB  DAO (Dual Amp. Operation), Pre-Amp input (pure power amplifier) mode, and many other unique design and amplification techniques.

The C9 was a niche, highly specialized, and distinctive product within a very niche market, it unexpectedly brought Cayin market success and a strong reputation, showing that the market for audiophiles in pursuit of audio perfection is out there waiting for the right product.

Three and a half years ago, we owed C9 a live stream. Three and a half years later, the C9ii has been relaunched.

The fundamental purpose of a portable headphone amplifier lies in its core function: enhancing sound quality and broadening its ability to drive a wide range of loads. Now that many DAPs have embraced significant improvements in circuit performance, can the C9ii still achieve the same level of market recognition as the C9 did three and a half years ago? This is a question on the minds of many audiophiles. Some even ask us which offers better sound quality, the N30LE or the C9ii?

The C9 portable amplifier became a phenomenon three and a half years ago, primarily due to its innovative design and its unique and powerful features. Today, let’s take a closer look at the new design, features, and updates of the newly announced C9ii.

Gen5 Nutube 6P1 Vacuum Tube Audio Circuit

Cayin has applied this vacuum tube across multiple products, from the N8, C9, N8ii, and N30LE, and now the C9ii. After years of continuous circuit experimentation and technical refinement, Cayin has equipped the C9ii with an adjustable anode (plate) voltage feature, in addition to the highly praised Classic and Modern dual tube timbre. This adjustable plate voltage feature offers two levels: high and low.

Dual Tube Timbre Circuit

The Nutube Timbre circuit in the C9ii utilizes a three-stage amplification design, with the core components consisting of a matched pair Nutube 6P1 dual directly-heated triode (DHT) vacuum tube and 16 pieces of Toshiba 2SK209 audio grade JFETs.

• The first stage is a discrete buffer amplifier composed of four pairs of selected Toshiba 2SK209 low-noise JFETs, one pair per differential amplification channel.
• The second stage is the tube voltage amplification stage, which provides the primary voltage gain and determines the overall tonal character of the timbre circuit. This is the heart of the C9ii timbre circuit and consists of a matched pair of Nutube 6P1 vacuum tubes.
• The third stage is a buffer driver stage and in this implementation, we adopted a source follower circuit with a pair of Toshiba 2SK209 JFET transistors. Instead of using conventional power resistors, the source-load circuit employs 4 pairs of selected2SK209 to create a high-performance constant current source. This will significantly enhance the performance of the source follower and improve the current driving capability of the tube timbre circuit.

Additionally, Cayin engineers carefully designed two feedback and fine-tuning circuits to process the output signal of the tube sound circuit, resulting in two distinct sound characteristics:

• Modern Sound: Introduces a large-loop negative feedback system, with total gain determined by the large-loop negative feedback circuit. By incorporating negative feedback over multiple amplification stages, the total harmonic distortion (THD) will be reduced significantly. The harmonic distortion characteristics and sound signature lean more towards a modern style, with greatly reduced thermal noise and a slightly cleaner tube sound. The original C9 was designed with a similar NFB circuit.
• Classic Sound: No large-loop negative feedback; each stage operates independently through local feedback, with the overall gain determined by the cumulative gain of each stage. The distortion characteristics of the tube amplification circuit dominate the sound signature, resulting in a higher THD value. The distribution of harmonic distortion and the sound signature exhibit a classic vacuum tube amplification circuit, with relatively more thermal noise and a richer vacuum tube presentation. The N8 DAP was designed with a similar NFB circuit.

Anode (High/Low)

Cayin introduces a new feature “Anode” to C9ii. The High/Low adjustable anode (plate) voltage further enriches and refines the range of vacuum tube timbre. The working principle is that applying different operating voltages to the plate of the vacuum tube alters its operating state, ultimately affecting the sound signature of the timbre circuit. This mainly manifests in changes to audio linearity, and it will adjust the overlay, spectrum, amplitude, and ratio of second-order harmonic, resulting in perceptible audible differences.

Here’s a practical scenario: For example, when using the Classic tube timbre to listen to extremely detailed vocals, some audiophiles might find the sound overly dense, with too much focus on vocal imaging. By switching the plate voltage to the L (low) setting, the sound characteristics might lean to laid back relatively and better suit the listener’s preferences.

On top of being a high-quality portable amplifier, the core competence of the C9ii also involves providing numerous features, amplification status, and timbre adjustments to satisfy the subjective preferences of our users, and this will include but not limited to exploring their music library at different time/mood, different IEM/headphones, and different music genres.

The C9ii also features a solid-state (SS) timbre.  We have developed a discrete timbre circuit with four 2SK209 transistors in a single-ended Class A source follower circuit. A high-performance constant current source is implemented using four discrete dual-triode transistors, finely tuned to deliver a clear, smooth, and fatigue-free audio output with no harshness.

It is important to note that the plate voltage adjustment does not affect the solid-state timbre. Thus, in terms of choice of timbre, the C9ii offers four vacuum tube timbre options and one solid-state timbre, for a total of five variations, whereas the original C9 only offers two timbre options.

To put the timbre options of our Gen5 Nutube 6P1 Vacuum Tube Audio Circuit into context, we shall summarize these options with a “generalized”

 

• Classic + Anode (H): The richest tube sound, with intense emotional depth, suitable for vocal music with a lot of details.
• Classic + Anode (L): Slightly lighter tube sound, suitable for vocals with thicker tones.
• Modern + Anode (H): Less tube character, with moderate smoothing in the midrange and a slight sweetness added.
• Modern + Anode (L): Minimal tube coloration, without affecting resolution and definition.
• Solid State sound: Balanced across the entire frequency range, musical without tube coloration.

Fully Discrete, Fully Symmetric Differential Headphone Amplifier

 The headphone amplification circuit of the C9ii is significantly different from the first-generation C9. While both amplifiers employ a fully discrete, fully differential four-channel headphone amplification circuit, C9ii adopts a fully symmetrical circuit architecture.

Fully-symmetric Headphone Amplifier

In HiFi audio amplifier design, fully symmetrical circuit architecture contributes to higher audio fidelity and better performance by reducing distortion, enhancing dynamic range, increasing channel separation, and providing a stable and consistent performance.  Fully symmetrical circuits are expensive to implement, especially in compact portable applications, since their advantages align with the expectations of audiophiles, this provides a lot of incentive for Cayin to fit this technology into the C9ii portable headphone amplifier.

 

Cayin has designed a three-stage amplification circuit to embrace the fully symmetric design into C9ii: a differential input stage, a voltage amplification stage, and a power driver stage.

       Stage One:  Symmetric JFET Common-source Differential input amplifier.

The differential input stage has been fully upgraded to use TWO ultra-low noises, audio grade dual JFETs per amplification channel: Linear Systems’ LSK489 and LSJ689.  Be reminded that there are 4 channels in a fully-differential circuit.  While these JFETs arrived as factory-matched pairs, Cayin uses a transistor characteristic curve tracer to measure the matched JFET and selects the best pairs to ensure the performance consistency of the C9ii audio circuit

     Stage Two: Symmetric BJT Common-emitter Push-Pull Differential Voltage Amplifier.

We employ Toshiba audio-grade BJT (PNP) transistors in this voltage amplification stage.  We ensure stable operation and high-quality output signals by finely adjusting peripheral circuit parameters. This will optimize the harmonic distribution and result in improved sound quality.

       Stage Three: Symmetric BJT Common-collector Push-Pull Parallel Current Amplifier (emitter follower).

The power driver output stage uses Nexperia’s paired high-power bipolar transistors in parallel output, known for their excellent thermal stability and superior audio performance. This dramatically enhances the load-driving capacity and compatibility of the amplifier.

From a user perspective, the overall improvement of the C9ii headphone amplification circuit over the first-generation C9 is comprehensive:

• Stronger load-driving capability, making the C9ii more adept at driving larger headphones or multi-driver in-ear monitors. It provides better bass control, treble extension, and imaging.
• A more transparent sound presentation with balanced musicality, and a more complete and clear soundstage outline.

When we initialize the C9ii development project, we expect our R&D Department to push the envelope on the original C9, After all, pure analog signal amplification technology hasn’t seen significant advancements in many years and the C9 is a very well-received product with impressive track record in past 4.5 years. When we finally finalized the circuit and industrial design of the C9ii, it’s hard to believe the C9ii has outperformed our expectations and completely outshines its predecessor in sound quality.   We are excited and we are looking forward to hearing the impressions from our users, reviews, and fellow hobbyists.

Negative FeedBack (NFB)

The C9ii has an integrated precision control circuit that allows the headphone amplifier to operate in various modes such as NFB/LFB, Class A/Class AB, Hyper, single-ended, and balanced input/output modes.

NFB (Negative Feedback) is a function that is being introduced on the C9ii. This feature applies to the three-stage headphone amplification circuit mentioned above.

When NFB is set to the OFF position, the headphone amplification circuit will be regulated by large-loop cross-stage negative feedback. The feedback signal is sent from the power output stage to the negative signal terminal of the differential input stage, controlling the overall gain of the power circuit.

When NFB is set to the ON position, the headphone amplification circuit will be regulated by local negative feedback.  The feedback signal is sent from the voltage output stage to the negative signal terminal of the differential input stage, mainly controlling the voltage stage gain and altering the sound characteristics.

In analog signal amplification circuit design, feedback circuit design is an indispensable tool as it directly impacts overall gain, performance indicators, and output impedance. At the same time, the placement and structure of the feedback circuit within the overall design serve can only be determined through length tuning and optimization according to the experience and know-how of the Engineers.

On the C9ii, when NFB is on (indicator light on), the audio output presents more spatiality with stellar dynamic range, excellent transient response, and relatively fewer harmonics, delivering a more neutral sound with impressive detail and speed,  audiophiles frequently associate these changes as “higher resolution”.  On the other hand, when NFB is off (indicator light off), the audio output displays richer harmonics.  You can hear a more cohesive and full-bodied presentation, with stronger imaging and more textured and elastic mid-bass frequencies.

There is no right or wrong in NFB adjustment, it’s a matter of personal preference, and adjustable NFB will allow user to pick their preference on the go.

We have gone through all the features and circuit designs that offer certain levels of sound-shaping capability to our users.

•       Timbre: 3 options (Classic Tube, Modern Tube, Solid-state)
•       Amplification Mode: 3 options (Class A, Class AB, Hyper)
•       Anode: 2 options (High, Low)
•      NFB: 2 options (On/Off)
•      Input mode: 2 (Line, Pre)

Can you calculate how many sound variations are available with C9ii?

Power Supply Design

The power supply design of the C9 series is pivotal to its performance. The discrete headphone power amplification circuit of both C9 and C9ii are powered directly by batteries. The advantage of such design is that it avoids any DC/DC voltage boosting conversion, which is considered the most ideal power environment in analog circuit.

The power system is carefully filtered and decoupled using four WIMA metalized film capacitors, 35 Panasonic POSCAP polymer tantalum capacitors, four Panasonic OS-CON polymer aluminum capacitors, and numerous COG-type MLCC capacitors.

We know you guys are not particularly interested in the ‘component arms race’, especially when it comes to cramming dozens or even hundreds of capacitors onto a single circuit board in what might seem like a component arms race. So, we’ll keep this part brief: we used plenty WIMA metalized film capacitors, plenty Panasonic POSCAP polymer tantalum capacitors, plenty Panasonic OS-CON polymer aluminum capacitors and plenty COG-type MLCC capacitors.

Actually there are particular reasons why we used these capacitors. We’re not trying to break a Guinness World Record for how many capacitors we can fit on a single board, nor did we include this many different types just for fun. Each capacitor serves a specific purpose and brings certain benefits to the sound, unfortunately due to space constraints. We’ll highlight just two of them for you.

1. Panasonic POSCAP Polymer Tantalum Capacitors:

POSCAPs offer high capacitance in a compact form factor, which is precisely why we managed to fit 35 of them in there. Yup, you heard that right—35. They are perfectly suitable for densely packed circuits where space is at a premium.

2. Panasonic OS-CON Polymer Aluminum Capacitors:

These capacitors offer long-term reliability, maintaining their performance over extended periods, which is crucial for ensuring the longevity of a portable, sometimes palm-heating headphone amplifier like the C9ii. Imagine this: In 2077, your grandson asks about the mysterious black box in your display case. You proudly pull out your C9ii, play him a song, and he’s moved to tears—’buy better, last longer,’ perhaps lasting as long as your favorite pair of Levis.

The C9ii utilizes four Samsung INR18650-35E battery cells, forming a series connection that directly powers the headphone amplifier circuit. The C9 used four Sony US18650VTC6 cells. With nearly identical internal resistance, the Samsung cells onC9ii offer a nominal capacity of 3400mAh, compared to the 3120mAh of the C9. Depending on the operating mode, the C9ii offers a maximum battery life of 17 hours (in solid-state timbre, single-ended input/output, AB mode) and a minimum of 8.5 hours (in classic timbre, balanced input/output, Hyper mode). This marks a significant improvement over the first generation, driven largely by the upgraded battery cells.

It’s worth noting that different brands and models of 18650 battery cells can have a direct impact on sound quality, because there are variations in internal resistance, discharge capacity, and other performance parameters. You are welcome to try out different brands of batteries and see what are the differences.

Both Samsung and Sony batteries have a cutoff voltage of 4.2V. This means that unless voltage boosting is applied, the power output remains unchanged between the two generations of the C9, as both rely on direct battery power for the circuit. In other words, the output power of both C9 generations is almost the same and will decrease over time as the battery voltage drops.

However, thanks to architectural changes, the C9ii significantly improves load-driving capability compared to its predecessor. The dynamic output current increases significantly in response to changes in music signals and backend load, resulting in stronger driving capability, fuller sound presentation, a more complete soundstage, and a more natural and fluid musicality. Audiophiles can easily discern these differences by comparing the sound of the first and second generations using the same source, track, load, and operating mode.

It’s important to emphasize that rated power output figure, the number alone do not equate to actual driving capability. Many products on the market boast high output power achieved through voltage boosting, yet they may still produce a terrifying sound that’s dry, lacks transient response, or sounds harsh or hollow highs when driving high loads. The root cause is often limited dynamic output current and load-driving capability.

This brings us to why the C9ii does not include a P+ mode, which would require voltage boost conversion (DC conversion). Implementing DC conversion would undermine the benefits of direct battery-powered amplification, and the direct battery-powered amplification is a fundamental design philosophy of the C9ii that sets it apart from other products. The C9ii is designed for purer performance, it retains the Hyper mode introduced in the N30LE. This mode further adjusts the static operating current under Class A mode, pushing the power circuit to its limits in various working modes.

This concludes the core design changes of the C9ii. For those interested in further technical details, please refer to the design block diagram below.

Complete Structure, Heat Dissipation, and Standard Recording Cable

 Next, let’s explore additional changes and adjustments. The C9ii continues to build on the strengths of the first-generation C9, such as the Class A/AB operation modes and the pure power amplifier mode (ideal for use with a DAP that supports Pre out). Volume control remains precise, utilizing the MUSES72320V as you can find on many of our products.

In terms of appearance and structure, the C9ii features two small handles that are not merely decorative. Due to differences in function switches and components between the C9 and C9ii (the C9 required two-position switches, while the C9ii uses three-position switches), the toggle switches are now raised above the panel. The handles on the sides of the C9ii provide protection, ensuring that any accidental bumps only affect the exterior without compromising the switches or impairing performance. The side panels have been redesigned with a curved structure, thus improving grip and increasing surface area for heat dissipation.

Heat dissipation in the C9ii has been significantly optimized. The primary heat source is the transistors in the four-channel discrete high-power headphone amplification circuit. To efficiently transfer this heat to the metal casing with minimal thermal resistance, the internal structure incorporates graphene thermal conductive sheets with high thermal conductivity. Additionally, aluminum alloy heat sinks are used to enhance heat absorption and ensure uniform temperature distribution. This heat is ultimately transferred through the graphene thermal conductive sheets to the aluminum alloy casing for radiation via a low thermal resistance path.

Another major structural change from the first-generation C9 is in battery replacement. While the original C9 required external tools (screwdriver, actually) for this task, the C9ii introduces a self-locking push-button switch and a precision-molded battery base. The battery module features a tool-free self-locking design with a self-locking push-button switch for easy assembly and disassembly. It uses a SATA slot to connect the battery module and C9ii, ensuring a three-point secure fixation that is both sturdy and convenient for replacement.

 

Given that portable headphone amplifiers need to be paired with a source device, the quality of the source’s line out (LO) or preamp signal has a significant impact on overall sound quality. The interconnect cable included with the C9ii has also been upgraded following extensive subjective listening tests. The new cable is made of eight-core OCC (Oxygen-free copper) (the original used a mix of four-core OCC and OFC). It features a Litz structure with mixed twisting, and each core is individually shielded with OFC (oxygen-free copper). The 4.4mm-to-4.4mm interconnect cable utilizes four-core balanced transmission, with the shielding layer independently grounded to greatly reduce interference.

 

Cayin, Never Be the Same Again!