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Cayin C9ii - Reaching New Heights
The fundamental purpose of a portable headphone amplifier lies in its core function: improving sound quality and expanding its ability to drive a wide range of loads. Today, when many DAPs feature significant improvements in circuit performance, can the C9ii still achieve the same level of popularity as the C9 did three and a half years ago?
This is a question that concerns many audiophiles. Some even ask us which amplifier offers better sound quality, the N30LE or the C9ii? The C9 portable amplifier became a phenomenon three and a half years ago, mainly because of its innovative design and unique and powerful features. Today we take a closer look at the new design, features and updates of the newly announced C9ii.
1. Gen5Nutube 6P1 Vacuum Tube Audio Circuit
Cayin has used this vacuum tube in several products, starting with 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 voltage (plate voltage), in addition to the highly acclaimed Classic and Modern two-tube timbres. This adjustable anode voltage offers two levels: high and low.
1.1 Dual Tube Timbre circuit
The Nutube Timbre circuit in the C9ii uses a three-stage amplification design, the core components of which are a matched pair of Nutube 6P1 Dual Direct-Heated Triode (DHT) vacuum tubes and sixteen audio-grade Toshiba 2SK209 JFETs. The first stage is a discrete buffer amplifier consisting of four pairs of selected Toshiba 2SK209 low-noise JFETs, one pair per differential amplification channel.
The second stage is the tube voltage gain stage, which provides the primary voltage gain and determines the overall sonic character of the timbre circuit. It is the heart of the C9ii's 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 have used a source follower circuit with a pair of Toshiba 2SK209 JFET transistors. Instead of conventional power resistors, 4 pairs of selected 2SK209 are used in the source load circuit to create a powerful constant current source. This greatly increases the power of the source follower and improves the current driving capability of the tube timbre circuit.
In addition, Cayin engineers have carefully designed two feedback and fine-tuning circuits to process the output signal of the tube sound circuit, resulting in two different sound characteristics leads:
ModernSound: Introducing a large loop negative feedback system, with the overall gain determined by the large loop negative feedback circuit. By incorporating the negative feedback across multiple gain stages, the total harmonic distortion (THD) is significantly reduced. The harmonic distortion characteristics and sound signature tend towards more of a modern style, with greatly reduced thermal noise and a slightly cleaner tube sound. The original C9 was designed using a similar NFB circuit.
Classic Sound : No large 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 amplifier circuit dominate the sound signature, resulting in a higher THD value. The harmonic distortion distribution and sound signature show a classic tube amplifier circuit with relatively high thermal noise and a richer tube representation. The N8 DAP was designed with a similar NFB circuit.
1.2 Anode (High/Low)
Cayin introduces a new "Anode" function to the C9ii. The adjustable anode (plate) voltage (high/low) expands and refines the sound palette of the vacuum tube. The principle of operation is that applying different operating voltages to the plate of the vacuum tube changes its operating state, which ultimately affects the sound signature of the timbre circuit.
This manifests itself primarily in changes in audio linearity, and results in an adjustment of the superposition, spectrum, amplitude and ratio of second-order harmonics, resulting in perceptible differences. Here's a practical scenario: For example, if you use the classic tube timbre to hear extremely detailed vocals, some audiophiles might find the sound too dense, with too much focus on vocal imaging.
Switching the board voltage to the L (Low) setting can make the sound characteristics relatively relaxed and more in line with the listener's preferences. The C9ii is not only a high-quality portable amplifier, but also offers numerous functions, gain status and tone adjustments to satisfy our users' subjective preferences, such as exploring their music library at different times/moods, with different IEM/headphones and different music genres.
The C9ii also features a solid-state (SS) tone. We designed a discrete tone circuit using four 2SK209 transistors in a Class A single-ended source follower circuit. A powerful constant current source is implemented using four discrete double triode transistors, finely tuned to deliver clear, smooth, and fatigue-free audio output without harshness. It is important to note that adjusting the plate voltage has no effect on the solid-state tone. Thus, the C9ii offers four tube tone colors and one solid-state tone, for a total of five variations, while the original C9 only offers two tone colors.
To put the tone options of our Gen5 Nutube 6P1 Vacuum Tube Audio Circuit into context, let's summarize these options:
- Classic + Anode(H): The richest tube sound, with intense emotional depth, suitable for vocal music with lots of detail.
- Classic + Anode(L): Slightly lighter tube sound, suitable for vocals with thicker tones.
- Modern+Anode(H) : Lesstube character, with moderate smoothing in the midrange and a slight sweetness.
- Modern+Anode(L): Minimal tube coloration without affecting resolution and definition.
- Solid-state sound: Balanced across the entire frequency range, musical without tube coloration.
2. Fully discrete, fully symmetrical differential headphone amplifier
The headphone amplifier circuit of the C9ii is significantly different from the first generation C9.While both amplifiers use a fully discrete, fully differential four-channel headphone amplifier circuit, the C9ii uses a fully symmetrical circuit architecture.
2.1 Fully balanced headphone amplifier
When designing high-fidelity audio amplifiers, a fully balanced circuit architecture contributes to higher fidelity and better performance by reducing distortion, improving dynamic range, increasing channel separation, and providing stable and smooth performance. Fully balanced circuits are expensive to implement, especially in compact, portable applications.
Since its advantages are in line with audiophiles' expectations, this is a great incentive for Cayin to incorporate this technology into the C9ii portable headphone amplifier. Cayin has developed a three-stage amplifier circuit to incorporate the fully symmetrical design into the C9ii: a differential input stage, a voltage gain stage, and a power driver stage.
Level one: Symmetrical JFET common source differential input amplifier. The differential input stage has been completely upgraded to use TWO ultra low noise audio grade dual JFETs per amplification channel: Linear Systems' LSK489 and LSJ689.
It is worth remembering that there are 4 channels in a fully differential circuit. While these JFETs are supplied as factory matched pairs, Cayin uses a transistor characteristic tracer to measure the matched JFETs and select the best pairs to ensure the performance consistency of the C9ii audio circuit
Second stage: Symmetrical BJT common emitter push-pull differential voltage amplifier. In this voltage amplification stage, we use Toshiba's audio-grade BJT (PNP) transistors. We ensure stable operation and high-quality output signals by fine-tuning the parameters of the peripheral circuits. This optimizes the harmonic distribution and leads to improved sound quality.
Third stage: Symmetrical BJT common collector push-pull parallel current amplifier (emitter follower). The power driver output stage uses Nexperia's high-performance paired bipolar transistors in parallel, known for their excellent thermal stability and superior audio performance.
This greatly improves the power handling and compatibility of the amplifier. From a user perspective, the overall improvement of the C9ii headphone amplifier over the first generation C9 is comprehensive: The increased power handling makes the C9ii better suited to drive larger headphones or in-ear monitors with multiple drivers. It offers better bass control, treble extension and imaging.
More transparent sound presentation with balanced musicality and a more complete and clear soundstage. When we began development of the C9ii, we expected our R&D department to push the boundaries of the original C9. After all, pure analog signal amplification technology has not made significant advances for many years, and the C9 is a very well-received product with an impressive track record over the past 4.5 years.
When we finally completed the circuitry and industrial design of the C9ii, it is hard to believe that the C9ii has exceeded our expectations and completely outshines its predecessor in terms of sound quality.We are excited and look forward to hearing the impressions of our users, reviews and other hobbyists.
2.2 Negative Feedback (NFB)
The C9ii has a built-in 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 feature 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 amplifier circuit is controlled by a wide-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 and controls the overall gain of the power circuit.
When NFB is set to the ON position, the headphone amplification circuit is controlled by local negative feedback. The feedback signal is sent from the voltage output stage to the negative signal terminal of the differential input stage, which mainly controls the gain of the voltage stage and changes the sound characteristics.
When designing analog signal amplification circuits, feedback circuit design is an indispensable tool because it directly affects the overall gain, power indicators and output impedance. At the same time, the placement and structure of the feedback circuit within the overall design can only be determined by length tuning and optimization according to the engineer's experience and know-how.
With the NFB on (indicator light on), the C9ii offers a more spacious sound with superior dynamic range, excellent transient response and relatively fewer harmonics, resulting in a more neutral sound with impressive detail and speed. Conversely, with the NFB off (indicator light off), the audio output has richer harmonics.
You will hear a more coherent and full-bodied reproduction with stronger imaging and more textured and elastic mid-bass frequencies. There is no right or wrong with the NFB setting, it is a matter of personal preference and the adjustable NFB allows the user to choose their preferences on the go.
We have thought through all the features and circuit designs that offer our users specific sound shaping options.
Timbre: 3 options (Classic Tube, Modern Tube, Solid-State) Gain 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 possible with the C9ii?
3. Power supply design
The power supply design of the C9 series is critical to its performance. The discrete headphone amplifier circuits of the C9 and C9ii are powered directly from batteries. The advantage of such a design is that it avoids DC/DC voltage boost, which is considered the most ideal power supply environment in analog circuits.
The power supply system is carefully filtered and decoupled, using four WIMA metal film capacitors, 35 Panasonic POSCAP polymer tantalum capacitors, four Panasonic OS-CON polymer aluminum capacitors, and numerous COG type MLCC capacitors.We know you're not particularly interested in the "component arms race," especially when it comes to putting dozens or even hundreds of capacitors on a single circuit board, which might look like a component arms race.
So let's keep it short here: we used a lot of WIMA metal film capacitors, a lot of Panasonic POSCAP polymer tantalum capacitors, a lot of Panasonic OS-CON polymer aluminum capacitors, and a lot of COG MLCC capacitors. There are actually special reasons why we used these capacitors.
We're not trying to break a Guinness World Record for the number of capacitors we can fit on a single board, nor did we use so many different types for fun. Each capacitor serves a specific purpose and brings specific benefits to the sound, unfortunately due to space constraints. We'll just highlight two of them for you.
1. Panasonic POSCAP polymer tantalum capacitors
POSCAPs offer high capacity in a compact form factor, and that's exactly why we managed to fit 35 of them in. Yes, you heard right: 35. They're perfect for densely packed circuits where space is at a premium.
2. Panasonic OS-CON polymer aluminum capacitors
These capacitors provide long-term reliability and maintain their performance over longer periods of time, which is critical to the longevity of a portable, sometimes palm-sized headphone amplifier like the C9ii. Picture this: In the year 2077, your grandson asks about the mysterious black box in your display cabinet. You proudly pull out your C9ii, play him a song, and he is moved to tears - "buy better, last longer," perhaps as long as your favorite pair of Levis.
The C9ii uses four Samsung INR18650-35E battery cells that form a series connection that directly powers the headphone amplifier. The C9 uses four Sony US18650VTC6 cells. With almost identical internal resistance, the Samsung cells in the C9ii offer a nominal capacity of 3400 mAh, compared to the C9's 3120 mA. 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 runtime of 8.5 hours (in classic timbre, balanced input/output, Hyper mode).
This is a significant improvement over the first generation, mainly due to the improved battery cells. It is worth noting that different brands and models of 18650 battery cells can have a direct impact on sound quality, as there are differences in internal resistance, discharge capacity, and other performance parameters.
Feel free to try different brands of batteries to see the differences. Both Samsung and Sony batteries have a cut-off voltage of 4.2V. This means that the output of the two C9 generations will remain unchanged unless a voltage increase is applied, as both rely on direct battery power for the circuitry.
In other words, the output power of both C9 generations is almost identical and decreases 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 the music signals and backend load, resulting in stronger drive capability, fuller sound representation, a more complete sound image, and more natural and fluid musicality.
Audiophiles can easily detect these differences when comparing the sound of the first and second generations with the same source, track, load and operating mode. It is important to stress that the rated output power and number alone do not equate to actual performance capabilities. Many products on the market boast of high output power achieved by voltage boosting, yet produce a terrifying sound that is dry, lacks transient response, or sounds harsh or hollow in the highs at high loads.
The cause of this is often limited dynamic output current and power handling. This brings us to the reason why the C9ii does not include a P+ mode, which would require voltage boosting (DC conversion). Implementing DC conversion would undermine the benefits of direct battery-powered amplification, and 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 and retains the Hyper Mode introduced in the N30LE. This mode further adjusts the static operating current in Class A mode and pushes the circuit to its limits in various operating modes. This concludes the main changes to the C9ii's design. For those interested in more technical details, we recommend checking out the block diagram below.
4. Complete construction, heat dissipation and standard recording cable
Next, let's look at other changes and adjustments. The C9ii continues to build on the strengths of the first generation C9, such as the Class A/AB operating modes and the pure power amplifier mode (ideal for use with a DAP that supports pre out). The volume control is still precise and uses the MUSES72320V as you find on many of our products. As for the look and structure, the C9ii features two small handles that are not just decorative.
Due to the 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 located above the control panel. The handles on the sides of the C9ii provide protection and ensure that accidental impacts only affect the exterior without affecting the switches or reducing performance.
The side panels have been redesigned with a curved structure, improving grip and increasing the surface area for heat dissipation. The C9ii's heat dissipation has been significantly optimized. The primary source of heat is the transistors in the discrete four-channel high-performance headphone amplifier circuit.
To transfer this heat to the metal casing efficiently and with minimal thermal resistance, the internal structure contains heat-conducting graphene sheets with high thermal conductivity. Additionally, aluminum alloy heat sinks are used to improve heat absorption and ensure even temperature distribution.
This heat is eventually transferred through the thermally conductive graphene sheets to the aluminum alloy chassis for dissipation via a low thermal resistance path. Another major structural change from the first generation C9 is the battery replacement. While the original C9 required external tools (a screwdriver, actually) for this task, the C9ii features a self-locking push-button switch and a precision-molded battery socket.
The battery module features a tool-free self-locking design with a self-locking push-button switch for easy assembly and disassembly. The connection between the battery module and the C9ii is via a SATA slot, ensuring a secure three-point attachment that is both robust and conveniently replaceable. Since portable headphone amplifiers must be paired with a source device, the quality of the source's line-out (LO) or preamp signal has a significant impact on the overall sound quality.
The interconnect cable included with the C9ii has also been improved after 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 mixed strand structure, and each core is individually shielded with OFC (oxygen-free copper).
The 4.4mm to 4.4mm interconnect cable uses four-wire balanced transmission, with the shielding layer independently grounded to greatly reduce interference.
We really hope that we can offer you a new and enjoyable experience with our new C9ii!
