FIIO K15 Complete User Manual

1. Product Introduction

(1) Overview: The FiiO K15 is a decoding headphone amplifier integrated machine featuring a retro structural appearance, designed to complement the product line and offer users a high cost-performance product. It boasts rich interfaces, including dual front and rear USB inputs, coaxial/optical inputs, Bluetooth input, streaming, local USB input, balanced input, RCA input, and eight other input methods. The device utilizes a dual power supply design with a built-in high-performance switching power supply module and an additional DC port for an external power supply. The DAC employs two newly reproduced AK4497S chips, and the headphone amplifier circuit retains a discrete design with JE243G-JE253G transistor pairs. High-end components include ACCUSILICON low-phase-noise femtosecond crystal oscillators, Nichicon audio-specific capacitors, ELNA brown coupling capacitors, Rubycon film capacitors, wafer resistors, and Emerald solid-state capacitors.

(2) Supported Sampling Rates:

• USB DAC Mode: Supports PCM 768K 32Bit, DSD512, and MQA Full Decode.
• Coaxial Decoding Mode: Supports PCM 192K 24Bit and DOP64.
• Optical Decoding Mode: Supports PCM 96K 24Bit.
• Bluetooth Reception: Supports SBC/AAC/aptX/aptX LL/aptX HD/aptX Adaptive/LDAC.
• Streaming Decoding: Supports up to 384kHz/32bit and DSD256 (DOP).
• Local Playback: Supports up to 384kHz/32bit and DSD256.

Note 1: The sampling rate for USB decoding depends on driver settings and computer configurations.

Note 2: Mac computers do not require driver installation. However, specific software like Audirvana is needed to support DSD playback, with a maximum support of DSD256.

(3) Basic Appearance Information:

• Dimensions: Approximately 244.6 x 213 x 66.8 mm.
• Colors: Black, Silver.
• Weight: Approximately 2100g.
• Material: Front panel CNC-machined aluminum alloy, middle frame metal stamping.

Download Links:

• Quick Start Guide: Click here
• USB DAC Driver Download: Click Here
• K15 PEQ Adjustment Webpage: https://fiiocontrol.fiio.com/ (Note: Apple's default Safari browser cannot open the PEQ webpage. Please use a Google-based browser like EDGE or Chrome.)
• FiiO Control App: Click Here
• K15 Firmware: To be added.

2. Operation Guide

(1) Power On, Power Off, and Standby:

For AC power, ensure the POWER switch on the rear panel is set to AC. Then, toggle the rocker switch to the "-" position. The power indicator on the front panel will light up red, indicating power is connected but the device is off. In this state, the infrared remote cannot turn on the K15. For DC power, ensure the POWER switch on the rear panel is set to DC.

• Standby, remote control offers instant power-on (Indicator: Green light)
• Power off, power supply connected (Indicator: Red light)
• Power on (Indicator: White light)

Power On: After power is connected, turn the front panel power switch to the ON position. The indicator will turn white, the display will light up, and the K15 will power on.

Standby: In the powered-on state, pressing the standby button on the remote will put the K15 into standby mode, where the main control and microcontroller units operate in low-power mode.

Long-term Non-use: Turn off the rear rocker switch or disconnect the external DC power.

(2) Touchscreen Operation: The K15 features a 3.93-inch touchscreen display.

Interface Navigation:

• Slide left and right: Switch between working modes (USB, Optical, Bluetooth, Streaming, Coaxial, Line Input, Balanced Input, Local Playback).
• Slide up and down: Access settings for playback, equalizer, channel balance, themes, backlight, etc.

Main Interface Example (USB DAC working mode): The display shows USB, DSD512, EQ:R&B, PO+PRE, G:L, Volume 75, DSD, DSD512.

Mode Selection Interface: Icons for USB, OPTICAL, COAXIAL, LINE IN, BALANCED IN, BLUETOOTH, STREAMING, LOCAL PLAY.

Menu Settings Interface: Icons for Play settings, Equalizer, Filters, Max volume, Balance, Network, Brightness, Screen time, RGB Lights, Theme, Idle time, U-AUDIO, Language, Reset option, Others, About K15.

VU Meter Interface: Available in streaming or local playback modes. DSD is also unsupported.

3. Detailed Description of Buttons and Interfaces

(1) Analog Output Interfaces:

a. Headphone Output ports:

• 6.35mm single-ended output: Outputs left and right positive signals (R+, L+), directly parallel to the balanced output. Simultaneous connection of single-ended and balanced headphones may increase load; avoid excessive volume + gain to prevent abnormal operation.
• 4.4mm balanced output: Outputs left/right positive and negative signals (R+, R-, L+, L-).
• XLR4 balanced output: Outputs left/right positive and negative signals (R+, R-, L+, L-), internally parallel to the 4.4mm balanced output.

Warning: Incorrect adapters (e.g., balanced-to-single-ended cables that short L- and R- to ground) may trigger overload protection.

b. Line Output ports:

• RCA single-ended output: Stereo line output (R+, L+ and ground). Maximum output is 2.5Vrms; reduce volume in PRE mode if distortion occurs.
• XLR3 balanced output: Balanced line output (R+, R-, L+, L-), suitable for active speakers or amplifiers.

(2) Knob Functions:

• a. Power on/off Selector Switch: Controls power on/off. Remote wake-up is disabled when the knob is in the off position.
• b. Analog Output Selector Switch: Switches output between PO+PRE, PO, PRE, and LO.
  • PO+PRE: Simultaneous headphone and line output, with volume and gain adjustments affecting both.
  • PO: Only headphone outputs are active (6.35mm, 4.4mm, XLR4).
  • PRE: Only line outputs are active (RCA, XLR3), with adjustable volume (unaffected by gain).
  • LO: Fixed maximum volume line output (volume and gain controls disabled). Ensure connected devices are set to appropriate levels to avoid loud noises.
• c. Gain Switch: Five levels—L (Low), M (Medium), H (High), SH (Super High), UH (Ultra High). Gain can be achieved through software gain (controls internal volume output level via software) or hardware gain (changes amplification factor of the hardware circuit).

The Ultra high and Super High gain modes are hardware high gain modes. The L/M/H modes are hardware low gain modes.

• d. INPUT/MENU Knob: Rotating displays an input selection menu. If no action is taken for 3 seconds, the device enters the selected operating mode. Pressing the button immediately enters the selected mode.
• e. VOLUME Knob: Rotate to adjust volume. A short press turns the screen off or on. (Note: In Settings > Other Settings, the "Short press volume knob" setting can modify its function to "Turn off/wake up" or "Mute/unmute".)

Current Volume Display: A visual representation of the current volume level from 0 to 100.

(3) Digital Input Interfaces:

• a. USB IN (Front/Rear): For connecting computers, phones, or players. The front USB takes priority if both are connected.
  • Note: Windows 10+ requires driver installation (Win7 or below is not supported); macOS does not.
  • Note: PS4/PS5 requires UAC1.0 mode (set in the K15 menu).
• b. RS232: For PEQ adjustment or firmware upgrades via PC.
• c. COAXIAL IN: Coaxial input.
• d. OPTICAL IN: Optical input.
• e. USB HOST: Connect a USB flash drive for local playback. Current limit is 500mA. For external hard drives, use an external power supply. Tested USB flash drive capacity up to 256GB (HDD up to 2TB). Supported formats: FAT32, NTFS, and exFat.
• f. LAN: Ethernet port (10/100/1000Mbps).

(4) Other Interfaces:

• RF Antennas: Right (near power input) for Bluetooth; left (near coaxial port) for Wi-Fi (2.4G/5G dual-band). Avoid blocking antennas.

4. Main Interface Display

(1) Status Bar (USB Mode as Example): The status bar displays current mode (e.g., USB DAC mode), sample rate (e.g., DSD512), current EQ type, network status (WiFi connected/disconnected, Ethernet connected/disconnected), output mode (PO+PRE/PO/PRE/LO), gain level (L/M/H/SH/UH), and volume level (0-120).

(2) USB DAC mode interface: Displays current working mode (USB DAC decoding), current playback track format, current EQ type, and current playback track sampling rate.

(3) Streaming Mode Interface: Displays streaming connection mode (Airplay or Roon Ready), song title and artist information, playback order options, shuffle play status, song duration and progress bar, previous track, and play/pause controls.

(4) Local Playback Interface: Displays song title, song duration and progress bar, playback mode options (sequential, loop, shuffle, single-track), add/remove from favorites, jump to current playlist, previous track, play/pause, next track, and cover display information (JPG format covers up to 1.2MB supported).

5. Working Modes

• (1) USB DAC Mode: PCM: 44.1kHz–768kHz, 16–32-bit. DSD: DSD64–DSD512. MQA: 8x full decode. Multichannel WAV or other formats: Unsupported. Mono tracks: Only ≥44.1kHz supported; use software SRC for tracks below 44.1kHz.
• (2) Optical Input Mode: PCM: 44.1kHz–96kHz; DSD not supported.
• (3) Coaxial Input Mode: PCM: 44.1kHz– 192kHz; DSD: DSD64.
• (4) Single-Ended Line Input Mode: Supports single-ended headphone output and balanced headphone output (front output switch set to PO+PRE or PO), as well as rear panel line output (front output switch must be set to PRE or LO).
• (5) Balanced Line Input Mode: Supports single-ended headphone output and balanced headphone output (front output switch set to PO+PRE or PO), as well as rear panel line output (front output switch must be set to PRE or LO).
• (6) Bluetooth Receiver Mode: Uses QCC5125 Bluetooth receiver chip, version 5.1, supporting AAC/SBC/aptX/aptX-LL/aptX-HD/aptX-Adaptive/LDAC encoding formats.
• (7) Streaming Mode: Supports Airplay and Roon Ready. Requires K15 and streaming device on the same local network.
  • Airplay: Stream via Airplay in Apple Music or QQ Music.
  • Roon Ready: Connect K15 and device to the same network, enable FIIO K15 in ROON CORE Settings > Audio. Supports 44.1kHz–384kHz; 16bit-32bit, DSD64-DSD256 (DOP).
  Note: K15 does not support DLNA (UPnP).

  How to confirm if the device is on the same local network? Check IP addresses in K15 Settings menu > Device Info and compare with phone's IP address. If the first three segments match, they are on the same local network.

• (8) Local Playback Mode: Active only in local playback mode. Scans and plays songs from a USB drive. Supports AAC/AIF/AIFF/APE/FLAC/MP3/WAV/DSD formats, with maximum sampling rates of PCM384K and DSD256. Does not support OGG format, CUE track splitting. Cover support: JPG format embedded covers under 1.2MB.

6. Menu Introduction

Playback Settings:

• 1. Playback Mode: Sequential Playback, Loop Playback, Shuffle Playback, Single-Track Playback, Single-Track Loop.
• 2. Resume Playback: Off (manual playback), Position (resumes from last position in USB mode), Song (restarts from beginning of last track in USB mode).

Equalizer: Includes 10 preset EQ curves (unmodifiable) and 10 customizable EQ slots.

Filter: 6 digital filters.

Max Volume: Limits maximum output volume.

Balance: Adjusts left/right channel volume compensation.

Network: Switch between wired and wireless connections.

Brightness: Adjusts screen brightness.

Screen Time: Sets screen auto-off timer.

RGB light: When set to "Color Follows Audio," the sampling rate follows the table below.

Theme: K15 offers two themes. Selecting a theme triggers an automatic restart.

Idle time: If no data is input within the set time, K15 enters power-saving mode. Playback resumes automatically when data is detected. Note: Smart idle is unavailable in RCA and balanced line input modes.

U Audio: Supports UAC1.0 and UAC2.0.

• UAC1.0: No driver required. Compatible with SWITCH, PS5, PS4, etc. Supports 44k–96k.
• UAC2.0: Typically used for players, phones, and PCs. On Windows, download the FiiO driver from: https://forum.fiio.com/note/showNoteContent.do?id=202105191527366657910&tid=17. Supports PCM44K–768K, DSD64–DSD512, MQA. macOS does not require drivers.

Language: Supports Chinese, English, and Japanese.

Reset Options: Allows partial resets for network settings, EQ, Bluetooth, song library, or a full "Restore Factory Settings".

Others: Includes Press the volume knob, FW update, reset options, and factory debugging.

• 1. Press the volume knob: Set to toggle screen on/off or mute/unmute.
• 2. FW Update: Includes SOC online update, SOC local update, and MCU update. The K15 employs a dual-master architecture: SOC handles playback, streaming, VU meter calculations, UI display, and operations. MCU manages power control, mute functions, DAC control, PEQ adjustments, etc. Separate firmware upgrades are required for SOC and MCU.
  • (1) SOC Online Upgrade: Allows online updates when connected to the internet.
  • (2) SOC Local Upgrade: Insert a FAT32 formatted USB drive with the unzipped SOC firmware (.zip file) into the rear USB-A port. Navigate to Settings > Firmware Upgrade > SOC Upgrade. Key Notes: Do not rename or extract the .zip file. Place the file directly in the USB root directory. macOS users must disable Safari's auto-unzip feature.
  • (3) MCU Upgrade: Download the MCU firmware (.bin file) to a computer. Connect K15's RS232 port to the computer via USB. Navigate to Settings > Other Settings > Firmware Upgrade > MCU Upgrade. A virtual USB drive will appear. Copy the .bin file into this drive, then eject via "My Computer". The K15 will auto-reboot.

7. Network Connection

The K15 supports both WiFi (2.4G/5G dual-band) and wired network connections. In the "Network Settings" menu, select the desired network.

Wired Network: Plug the network cable into the Ethernet port. Select "Wired Network" in the menu. The K15 will automatically connect. Supports 10M/100M/1000Mbps speeds. The network icon in the status bar will display as connected. Network IP can be viewed in "Device Information".

Wireless Network: In "Network Settings", click "Wireless Network". The K15 searches for available WiFi networks. Click the desired network to enter the password. After entering the correct password, confirm with the checkmark icon (✓). The K15 will connect to the WiFi hotspot. Once connected, the WiFi icon will show "connected", and the network IP can be viewed in "Device Information".

Notes:

• The K15's character keyboard is simplified and may not support certain special characters.
• The WiFi password field displays a maximum of 17 characters.
• The K15 determines network connection, not internet access.
• For router WiFi encryption, select WPA2-PSK. WPA3-PSK will cause connection failure.

8. Mobile Remote Control App: FiiO Control Installation and Usage

After installing FiiO Control on your smartphone, you can remotely control the K15's local playback, PEQ adjustments, and other functions.

1. Search for the "FiiO Control" app in your app store, download, and install it.
2. Ensure both the K15 and your smartphone are connected to the same network. (Confirm by checking IP addresses in K15's "Device Information" menu and comparing with your phone's IP address. If the first three segments match, they are on the same local network.)
3. Open the FiiO Control app. Grant necessary permissions. Follow these steps to connect to the K15:
   • After opening the app, click "Add New Device" in the center.
   • Click the "Player" icon at the bottom right.
   • Click the refresh icon at the top right. Once the K15 is detected, click to connect.
   • Connection successful.

Notes:

• If there are multiple K15 devices, check the IP address in "Device Information" and select the correct one.
• If no devices appear, click the refresh icon.
• If the device still does not appear, manually enter the K15's IP address by clicking "Manually Enter Linker Control IP".
• To control USB playback, ensure the K15 is switched to "Local Playback" mode first.

After connecting, you can adjust local playback, EQ settings, volume, and more.

9. PEQ Function

FiiO engineers have developed a PEQ adjustment feature based on the XMOS XU316 platform, supporting 10 customizable frequency bands. Users can tune EQ frequency points, gain, and bandwidth via FiiO's apps or a web interface.

(1) Adjust EQ in K15: K15 includes 10 preset curves (Jazz, Rock, R&B, Hip-Hop, Pop, Dance, Classical, Retro, Sibilance Reduction 1, and Sibilance Reduction 2). Access these via the menu. Custom curves start flat and can be saved and selected directly on the K15 after adjustment via FiiO Control App or the web interface.

(2) App Adjustment (FiiO Control): Connect to the app, navigate to Equalizer > PEQ Settings. Adjust, save, or rename curves. Note: Curve names support letters and numbers only. Preset curves cannot be modified or renamed.

(3) Web Control Adjustment:

1. Use a USB-A-to-C cable to connect K15's RS232 port to a computer.
2. Open the PEQ page: https://fiiocontrol.fiio.com/ > Click "Connect" > Select "Serial Port". Note: Requires Chromium-based browsers (e.g., Edge, Chrome). Safari is unsupported.

After connection, the interface displays "FiiO Audio DSP".

6. Key Notes for PEQ Parameters: Gain Adjustment: Raising custom curves above 0dB may cause digital clipping (distortion). Reduce "Global Gain" to compensate.

(4) Decoder Compatibility (with PEQ Enabled):

• 1. USB Decoding: PCM: 44.1–192 kHz (direct processing); 352–768 kHz: Unsupported. DSD: Unsupported. MQA: Unsupported.
• 2. Coaxial Input: PCM: 44.1–192 kHz (direct processing).
• 3. Optical Input: PCM: 44.1–96 kHz (direct processing).
• 4. Line-In (RCA/4.4mm): PEQ adjustment not supported.
• 5. ROON Streaming: PCM: 44.1–192 kHz (direct); 352–384 kHz: Downsampled by ROON. DSD: Supported via sample rate conversion by ROON.
• 6. Local Playback: PCM: 44.1–192 kHz (direct); 352–384 kHz: Supported via sample rate conversion. DSD: DSD64–DSD128 (converted). DSD256: Unsupported.

(5) Other Notes for PEQ Usage:

• 1. Browser Compatibility: The PEQ web interface requires Chromium-based browsers (e.g., Edge, Chrome). Safari is not supported.
• 2. Format Limitations: Enabling PEQ disables MQA decoding entirely.
• 3. Playback Interruptions: Switching PEQ ON/OFF causes:
  • USB Decoding: Reconnection (playback stops; manual resume required).
  • Streaming Modes:
    • ROON Ready: Auto-reconnects.
    • AirPlay: Manual reconnection needed.
    • Local Playback: Playback pauses (resume manually; DSD256 files show "Unsupported" error).
    • Coaxial/Optical Input: Brief audio interruption (resumes automatically).
• 4. Bypass Mode: Engages a flat EQ curve (no DSP processing). Avoids playback interruptions when switching between PEQ presets. Note: MQA remains unsupported in this mode.

10. Streaming Features Introduction

Preface: The K15 supports two streaming methods, AirPlay and Roon Ready. Both require the K15 and the casting device (e.g., a phone or computer) to be connected to the same network for casting.

(1) AirPlay: The K15 supports AirPlay 1st generation. After connecting to an Apple device, you can remotely cast music from Apple Music or other APPs via AirPlay.

Note: The K15 runs on the AirPlay 1 protocol. After updating to iOS 18.4, the album cover may freeze on the first song's artwork and not refresh when switching tracks. This is an iOS issue, and it is hoped it will be resolved in future updates.

(2) Roon Ready: Install the Roon music software on your phone or computer. In the settings under "Audio," select the Roon Ready device as "FiiO K15." Once connected, you can cast music to the K15 for playback.

(3) NAS Playback via Roon:

1. Ensure the NAS, phone (or computer), and K15 are on the same local network.
2. Install Roon Server on the NAS and the Roon music player on your phone (or computer). In Roon's settings, select the NAS server.
3. Add songs from the NAS to Roon.
4. Set the K15 to streaming mode and connect to your phone via Roon Ready to access songs on the NAS.

11. Protection Mechanisms

(1) Overload Protection: When the headphone output interface exceeds a certain power level, overload protection is triggered. A warning message "Overload anomaly, please power off and restart" will appear. Check headphone wiring and insertion. If the issue persists, power off and restart the K15. Short circuits between channels can also trigger overload protection.

(2) DC Protection: If the internal circuit is damaged, direct current (DC) may be generated, potentially damaging headphones. If a DC protection pop-up appears, turn off the AC power supply and then turn it back on to restart the K15. If the issue persists, contact customer service.

(3) Temperature Protection: The K15 has a built-in temperature detection circuit. When the device overheats, an over-temperature protection alert will appear. Power off the K15 and let it rest. It can be used again once the temperature has slightly decreased.

(4) AC Fuse: The K15 has a built-in fuse above the AC power socket. If the device shows no response when plugged in and powered on, check the fuse. The fuse specification is 250V 0.5A.

12. Firmware Upgrades Introduction

K15 supports firmware upgrades for SOC, MCU, Bluetooth chip, and XMOS316.

FW update options: SOC online upgrade, SOC local upgrade, MCU upgrade.

SOC Upgrade:

• 1. OTA Upgrade: After connecting K15 to the internet, a notification for new firmware will appear. You can choose to cancel, remind later, or proceed. Alternatively, navigate to Menu > Other Settings > Firmware Upgrade > Main Controller Online Upgrade. If the OTA upgrade is interrupted, reconnect to the same network to resume. After completion, K15 will reboot. For persistent "Updating" issues, use USB drive recovery: Download the latest firmware, insert a USB drive into the rear USB-A port, copy the compressed file "K15_update_20250317_Vxxx" to the USB drive, manually extract it, and rename the extracted folder to "K15_update_images".
• 2. USB Drive Upgrade: Tutorial video available. Download the latest firmware and guide from FiiO's website, then follow instructions to copy files to the USB drive. Navigate to Menu > Other Settings > Firmware Upgrade > Main Controller Local Upgrade. Note: Some USB drives may have compatibility issues.

MCU Upgrade:

• Tutorial video available.
• Notes (Windows): After copying firmware, right-click the USB drive in "This PC" and select "Eject". Using the system tray's "Safely Remove" option will cause MCU upgrade failure.