Cayin C9ii Flagship Portable Headphone Amplifier

The Cayin C9ii is Cayin’s flagship portable headphone amplifier: fully discrete, fully balanced and fully symmetrical, with a pair of fifth-generation KORG Nutube 6P1 tubes or a discrete JFET solid-state stage. Adjustable tube anode voltage, three amplifier modes, switchable feedback and two input modes let you shape the sound in many ways, and it delivers up to 2,600mW into 32Ω from its 4.4mm output, running straight from four 18650 batteries with no voltage boost.

Gen5 Nutube circuit with adjustable anode voltage

After years of experiment and refinement, Cayin gave the C9ii an adjustable anode (plate) voltage on top of its Classic and Modern tube timbres. The Nutube 6P1, developed by KORG with Noritake Itron in 2015, is a flattened double directly heated triode built like a vacuum fluorescent display: compact, energy-efficient and long-lived, but very prone to microphonics. Cayin brought it to portable audio with the N8 player in 2018, used a balanced pair in the original C9 in 2021, and added Classic and Modern timbres in the N30LE in 2023.

To tame microphonics, each Nutube sits in Cayin’s suspended installation: a custom silicone housing, a spring-loaded or vertically suspended shock-absorbing mount and a flexible circuit board. That gives a darker background and more detail, and lets you hold the C9ii and walk around while you listen.

A “heavy-duty” discrete timbre circuit

The tube circuit is a fully balanced, three-stage low-gain buffering design, available to both the 3.5mm and 4.4mm outputs, using 16 selected Toshiba 2SK209 audio JFETs:

• Stage 1, buffer amplifier: four pairs of 2SK209 JFETs, one pair per differential channel.
• Stage 2, voltage amplifier: the matched pair of Nutube 6P1s as a low-gain voltage amplifier.
• Stage 3, buffer driver: instead of ordinary power resistors, four more pairs of 2SK209s form a high-performance constant-current source load, greatly increasing drive.

The solid-state timbre circuit is just as serious: four selected 2SK209 JFETs, one per differential channel, in a discrete single-ended Class A source-follower with a constant-current source, tuned to match the tube circuit’s level and quality for a clear, smooth sound with no harshness.

Five sound signatures

Classic tube mode has no large-loop feedback: each stage works with local feedback, for the higher, low-order distortion of a classic tube amplifier, a little more thermal noise and a richer presentation. Modern tube mode adds a large-loop feedback circuit for much lower distortion and noise and a slightly cleaner tube sound. Changing the plate voltage reshapes the even-order harmonics, so with solid state there are five timbres:

• Classic, high anode: the richest tube sound, with emotional depth for vocals and plenty of detail.
• Classic, low anode: slightly lighter, still with thicker tones, suits vocals.
• Modern, high anode: a lighter tube sound with moderate smoothing in the midrange and a touch of sweetness.
• Modern, low anode: minimal tube colour, without affecting resolution and definition.
• Solid state: balanced across the range, musical without tube colour.

A fully symmetric headphone amplifier

The headphone amplifier has four discrete differential channels in a fully symmetrical multi-stage circuit, for better handling of peaks, accurate timing, low noise and a strong stereo image. Its input stage uses eight ultra-low-noise Linear Systems LSK489 and LSJ689 JFETs, two pairs per channel, each measured by Cayin on a curve tracer for consistency. Toshiba audio-grade PNP transistors form the push-pull voltage amplifier, and the output stage uses high-power Nexperia bipolar pairs in parallel for stability and current, so it drives a wide range of headphones and IEMs. The output is direct-coupled.

Class A, Class AB or Hyper

A three-way switch selects Class A (low distortion and silky, warm, lush sound with lots of detail, for sentimental music), Class AB (higher efficiency, clear, tight, extended and authoritative, for music with a wide dynamic range) or Hyper, which puts performance first: the amplifier runs in Class A with the static current set to its maximum and other parameters tweaked, for better dynamics and a grander soundstage than Class A. There is no clear winner, only synergy and preference.

Feedback and input modes

Two independent feedback circuits shape the amplifier’s dynamics, distortion, transients and output impedance. With the NFB indicator on, it uses local feedback, for richer harmonics, a fuller, warmer sound and more textured bass; with it off, large-loop feedback gives more space, better dynamics and transients, and a cleaner sound.

In Line mode, the C9ii takes a fixed-level signal and controls the volume itself, and slightly shapes the overall sound. Press and hold the PRE button for 3 seconds for Pre mode, where the C9ii’s volume is disabled and your player or pre-amp controls it, preserving the source’s character. If your player has a clean headphone output but no line out, use it as a pre-amp output; lower-powered players often suit Pre mode.

Cayin suggests three starting points: warm and tubey (Classic tube, Class A, high anode, NFB off); technical (Modern tube, Class AB, low anode, NFB on); and neutral and dynamic (solid state, Hyper, NFB off).

High-precision volume

A pair of JRC MUSES72320V two-channel resistor-ladder volume chips keeps the circuit fully differential all the way, with excellent channel matching, low noise and minimal distortion, in 131 steps. The ALPS potentiometer only senses the knob position, so there is no channel imbalance even at low volume. High and low gain suit a wide range of IEMs and headphones.

Driven directly by its batteries

Four Samsung 35E 18650 cells, chosen for their high energy density, high discharge rating and low internal resistance, form a ±8.4V supply for the amplifier with no DC/DC conversion, key to the C9ii’s strong transient current. WIMA film capacitors, C0G MLCCs, Panasonic POSCAP and Panasonic OS-CON capacitors filter the supply for low noise and low impedance.

The upgraded battery tray releases without tools: press the self-locking latches and slide it out. A SATA connector on the battery board and the two latches give three-point fixing. Spare trays can be bought separately and also work as a standalone 18650 charger, and different brands of 18650 cell can noticeably change the sound, so you can experiment.

Cool, solid and carefully finished

A high-conductivity graphite pad and CNC-machined heat sinks spread the heat. The chassis is 6063-T6 aluminium alloy, about 200W/m·K, heat-treated for strength and conductivity, and Cayin enlarged the body from 160mm to 165mm to add CNC-machined heat sinks to both side panels. Two windows show the Nutube glow, the volume knob has the C9 series’ mirror-polished, PVD gold-plated ring, the curved sides improve grip and cooling, four LEDs show the battery level (and flash while charging), and small guards on each side protect the toggle switches from knocks.

Inputs, outputs and cables

4.4mm balanced and 3.5mm single-ended inputs and outputs cover every portable set-up, and a single-ended input is converted to balanced inside to make full use of the balanced amplifier. The supplied CS-44C44II balanced interconnect has eight Litz conductors, each of eight strands of enamelled OCC copper in a braided jacket and shielded by enamelled OFC strands tied to ground, for a darker background and clearer imaging; a CS-35C35II single-ended interconnect is included too.

Good to know

• The C9ii is an analogue amplifier with no DAC. Use it with a player or DAC that has a line or headphone output.
• In the box: C9ii with battery module fitted, 4 × Samsung INR18650-35E batteries, CS-35C35II 3.5mm and CS-44C44II 4.4mm interconnects, USB-C cable, Hi-Res stickers and user manual.
• See also the original Cayin C9.