When Cayin launched the original, flagship C9 portable amplifier in 2021, it quickly garnered attention among audiophiles for its innovative blend of tube and solid-state timbres, direct-drive battery-powered circuit, discrete Class A/AB DAO (Dual Amp. Operation), and pure power amplifier Pre-Amp input mode, among other unique design and amplification features.

The C9 was a highly specialized product within a niche market, but its unexpected success helped cement Cayin’s strong reputation, proving that there was a demand for audiophile-grade products aimed at perfecting sound quality.

At the heart of any portable headphone amplifier is its core function: improving sound quality and expanding its capability to drive various headphone impedances. With many DAPs now offering significant advancements in circuit performance, can the new C9ii live up to the market recognition the original C9 achieved three and a half years ago? This is a question many audiophiles are asking, with some even wondering which offers better sound quality: the N30LE or the C9ii?

The original C9 amplifier became a standout in the audiophile world due to its groundbreaking design and powerful features (insert link to HeadFi forum or official thread about the first generation). Now, let’s take a closer look at the updated design, features, and improvements of the newly launched C9ii.

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 numerous Toshiba 2SK209 audio grade JFETs as buffer amplifier and buffer driver stage before/after the Nutubes.

The Timbre circuit offers three timber options:
Modern Sound: 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: 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.
Solid State: A discrete timbre circuit with four Toshiba 2SK209 JFET transistors in a single-ended Class A source follower circuit. A clear, smooth, and fatigue-free audio output with no harshness.

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 vacuum tube's plate alters its operating state, ultimately affecting the sound signature of the timbre circuit.

The “Anode” 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. In contrast, the original C9 only offers two timbre options.

1. Classic + Anode (H)
2. Classic + Anode (L)
3. Modern + Anode (H)
4. Modern + Anode (L)
5. Solid State sound

Fully Discrete, Fully Symmetric Differential Headphone Amplifier

Cayin has designed a three-stage amplification circuit to embrace the fully symmetric design into C9ii:

• Stage One: Symmetric JFET Common-source Differential input amplifier.
• Stage Two: Symmetric BJT Common-emitter Push-Pull Differential Voltage Amplifier.
• Stage Three: Symmetric BJT Common-collector Push-Pull Parallel Current

From a user perspective, the C9ii offers a stronger load-driving capability and a more transparent sound presentation when compared to the first-generation C9

To further explore the discrete headphone amplification circuit, we have introduced the Hyper Mode on top of the regular Class A and Class AB options of the renowned DAO (Dual Amplifier Operation. The Hyper mode delivers the ultimate performance of the discrete amplification circuit in the C9ii at the expense of a shorter battery duration and and operates at a higher temperature.

A new NFB (Negative Feedback) feature is added to C9ii. This feature applies to the three-stage headphone amplification circuit mentioned above. When NFB is set to OFF, the headphone amplification circuit will be regulated by large-loop cross-stage negative feedback. When NFB is set to the ON position, the headphone amplification circuit will be regulated by local negative feedback

The effect of NFB is fairly noticeable. In our opinion, there is no right or wrong in NFB adjustment, it's a matter of personal preference, and adjustable NFB will allow the user to pick their preference on the go.

Inputs:

• 3.5mm TRS
• 4.4mm TRRRS

Timbre Circuit (Tube):

• KORG Nutube 6P1 x2

Outputs:

• 3.5mm TRS
• 4.4mm TRRRS

Timbre Circuit (Solid State):

• Toshiba 2SK209 JFET x4

Charging:

• USB Type-C (QC3.0/PD2.0 9V-2A)

Input Differential Amplifier:

• LSK489 x4 + LSJ689 JFET x4

Dimensions:

• 165 x 88.5 x 30 mm

Electronic Volume Control:

• MUSES72320V x2

Net Weight:

• 650g (±5g)

Battery:

• Samsung INR18650-35E (3400mAh) x4

Battery Duration (C9ii Battery Mode and Duration)

Charging Condition

• Standard Charger (5V-2A)
• Quick Charge QC3.0/PD2.0 (9V2A)

Charging Time:

• Standard Charger: - 6.5 Hours
• Quick Charge: - 3.8 Hours

Note:

1. Test conditions: Line in, volume at 10 o'clock position, low gain, high anode (plate) setting, connected to 320 ohm headphones.
2. Charging time may be extended if the four lithium batteries have different voltages (capacities).

C9ii AIL Headphone Output Parameters

Note:

1. Unless otherwise stated, all parameters are measured at 320Ω loading, under high anode (plate) voltage with NFB (Negative Feedback) turned off.
2. The tube specifications are based on measurements of the Modern Tube Timbre.
3. All parameters are measured when the batteries are fully charged.
4. The parameters are based on measurements from the Cayin laboratory and are for reference only.

A fully balanced and fully discrete 4-channel high-fidelity headphone amplifier, packed with features, delivers up to 4,100 mW per channel at 16Ω and 2,600 mW per channel at 32Ω.

The Gen5 Nutube 6P1 Vacuum Tube Audio Circuit offers dual tube timbres—Classic Tube and Modern Tube—along with a Solid State option for both balanced and single-ended inputs. The vacuum tube timbre circuit is built around a matched pair of KORG Nutube 6P1 miniature, low-power, directly heated triode (DHT) vacuum tubes.

The ANODE (plate) voltage of the Nutube can be adjusted (High/Low) to further refine and enhance the vacuum tube timbre range in the C9ii.

The three-stage, fully symmetric differential headphone amplifier improves audio fidelity by reducing distortion, boosting dynamic range, enhancing channel separation, and providing stable, consistent performance.

Dual Amplification Operation (DAO) allows users to toggle between Pure Class A and Class AB amplification modes. Hyper Amplification mode unlocks the ultimate performance from the discrete amplification circuit within the C9ii.

The NFB (Negative Feedback) control gives users the option to switch between large-loop cross-stage feedback or local feedback in the three-stage headphone amplification circuit.

The amplifier supports both 3.5mm single-ended and 4.4mm balanced inputs and outputs. It converts single-ended input into a balanced audio signal before processing it through the differential amplification circuit.

In addition to the regular LINE input mode, the C9ii features a PRE-amp input mode (pure power amplifier mode), making it compatible with source devices that include volume control.

The highly linear volume control system incorporates a 4-channel ALPS potentiometer, paired with a MUSES72320 low-noise, low-distortion resistance ladder-based stereo electronic volume.

A removable battery module holds 4x Samsung 35E 3400mAh rechargeable lithium batteries, allowing users to easily replace the batteries and extend the battery life when needed.