KGUSS Topping A90 Discrete Headphone Amplifier Preamplifier User Manual

1. Introduction

The KGUSS Topping A90 Discrete is a high-performance headphone amplifier and preamplifier designed for audio enthusiasts. It features fully discrete NFCA (Nested Feedback Composite Amplifier) modules, precise R2R relay volume control, and versatile output options including 6.35mm, 4.4mm balanced, and 4-Pin XLR headphone outputs. This manual provides detailed instructions for setting up, operating, and maintaining your A90 Discrete unit.

KGUSS Topping A90 Discrete Headphone Amplifier Preamplifier in silver finish

Figure 1.1: KGUSS Topping A90 Discrete Headphone Amplifier Preamplifier.

2. What's in the Box

Carefully unpack your A90 Discrete and ensure all components are present:

A90 Discrete Unit
Remote control
AC cable
6.35mm to 3.5mm adaptor
User manual

Contents of the A90 Discrete box including the amplifier, remote, cables, and manual

Figure 2.1: Package Contents of the A90 Discrete.

Video 2.1: Unboxing the TOPPING A90 Discrete headphone amplifier, showing all included accessories.

3. Product Overview

3.1 Front Panel

Front panel diagram of the A90 Discrete showing output channel indicator, remote control receiver, power indicator, input channel indicator, volume, 4-Pin-XLR headphone output jack, 4.4mm balanced headphone output jack, 6.35mm headphone output jack, and volume knob/multi-function button.

Figure 3.1: Front Panel Layout and Controls.

Output Channel Indicator: Displays the active output channel.
Remote Control Receiver: For receiving signals from the remote.
Power Indicator: Shows the power status.
Input Channel Indicator: Displays the active input channel.
Volume: Digital volume display.
4-Pin-XLR Headphone Output Jack: Balanced headphone output.
4.4mm Balanced Headphone Output Jack: Balanced headphone output.
6.35mm Headphone Output Jack: Single-ended headphone output.
Volume Knob/Multi-function Button: Controls volume and navigates menus.

3.2 Rear Panel

Rear panel diagram of the A90 Discrete showing firmware upgrade interface, extender input interface, balanced XLR input, single-ended RCA input, single-ended RCA output, balanced XLR output, trigger interface, ground lift switch, power input, and power switch.

Figure 3.2: Rear Panel Layout and Connections.

Firmware Upgrade Interface: For updating device firmware.
Extender Input Interface: For connecting external modules like Ext90.
Balanced XLR Input: For balanced audio input.
Single-ended RCA Input: For single-ended audio input.
Single-ended RCA Output: For single-ended audio output.
Balanced XLR Output: For balanced audio output.
Trigger Interface: 12V trigger for synchronized power with other devices.
Ground Lift Switch: To eliminate ground loop noise.
Power Input (AC 100-240V 50/60Hz): Power connection.
Power Switch: Turns the unit on/off.

3.3 Remote Control

Remote control layout with buttons for standby, volume up/down, input switching, output switching, user settings, headphone amp gain, mute, OK, and brightness settings.

Figure 3.3: Remote Control Functions.

Standby: Power on/off.
Volume Up/Down: Adjusts volume.
Switch to Previous/Next Input: Cycles through input channels.
Output Channel Switching: Toggles between headphone amp and preamp outputs.
User Setting 1/2: Customizable settings.
Headphone Amp Gain: Adjusts gain level.
Mute: Mutes audio output.
OK Button: Confirms selections.
Brightness Settings: Adjusts display brightness.

4. Setup

4.1 Power Connection

Ensure the power switch on the rear panel is in the OFF position.
Connect the supplied AC cable to the power input on the rear panel of the A90 Discrete and then to a suitable power outlet.

4.2 Audio Connections

The A90 Discrete supports various input and output configurations:

Inputs: Balanced XLR, Single-ended RCA, Extender Input (for optional modules like Ext90).
Outputs: Balanced XLR, Single-ended RCA, 4-Pin-XLR Headphone, 4.4mm Balanced Headphone, 6.35mm Single-ended Headphone.

Connect your audio source (e.g., DAC) to the appropriate input on the A90 Discrete using high-quality cables. Connect your headphones to one of the front panel headphone output jacks or connect to a power amplifier via the rear panel RCA or XLR outputs.

Diagram showing connection of A90 Discrete with a D90SE/D90LE DAC and an optional Ext90 for additional inputs.

Figure 4.1: Example connection with a D90SE/D90LE DAC and Ext90.

Video 4.1: Overview of the TOPPING A90 Discrete Headphone Amplifier, demonstrating various connections and features.

5. Operating Instructions

5.1 Power On/Off

Flip the power switch on the rear panel to the ON position. The unit will power on. To power off, flip the switch to the OFF position.

5.2 Input/Output Switching

Input Switching: Press the volume knob on the front panel to cycle through available input channels (XLR, RCA, EXT).
Output Switching: Double-click the volume knob on the front panel to switch between headphone amplifier output (HPA), preamplifier output (PRE), or both (PRE+HPA).

5.3 Volume Control

Rotate the volume knob on the front panel to adjust the volume. The A90 Discrete uses an R2R volume control composed of relays and resistor networks for precise and matched channel balance.

Internal view of the A90 Discrete showing the R2R relay volume control components.

Figure 5.1: R2R Relay Volume Control.

A safe volume can be set to avoid sudden loud sounds after power on or input switching.

Front panel display showing volume level and input/output indicators.

Figure 5.2: Digital Volume Display.

5.4 Gain Settings

The A90 Discrete offers two gain options to accommodate various headphones:

Low Gain: Suitable for high-sensitivity IEMs (In-Ear Monitors).
High Gain: For driving full-sized over-ear headphones.

Adjust the gain using the remote control or the multi-function button on the front panel. The ultra-low noise of 0.2uVrms ensures sensitive IEMs do not suffer from noise.

Diagram showing balanced and single-ended gain settings for low and high gain modes.

Figure 5.3: Gain Settings for Balanced and Single-Ended Outputs.

5.5 Preamp Functionality

The A90 Discrete can function as a dedicated preamplifier. In preamp mode, it delivers the same high performance as the headphone amplifier mode. The 20ohm SE and 40ohm balanced output impedances are suitable for various power amplifiers. You can switch between headphone amp only, preamp only, or simultaneous output from both.

A90 Discrete unit placed between two speakers and a headphone stand, illustrating its use as a preamplifier.

Figure 5.4: A90 Discrete in Preamp Mode.

6. Key Features

6.1 Fully Discrete NFCA Modules

The A90 Discrete utilizes fully discrete components to redesign the NFCA (Nested Feedback Composite Amplifier) module. It incorporates four built-in NFCA-Discrete modules, each using 39 transistors to form a fully balanced structure. This optimized Voltage Current hybrid feedback architecture, combined with UHGF (Ultra High Gain Feedback) technology, ensures excellent DC and AC performance, resulting in unparalleled distortion and noise performance.

Internal circuit board of the A90 Discrete highlighting the NFCA-Discrete modules.

Figure 6.1: Internal View of NFCA-Discrete Modules.

6.2 High Output Capability

The A90 Discrete delivers high output voltage, high current, and low output impedance, allowing it to drive a wide range of headphones effectively. High output voltage easily drives high impedance headphones, while high output current provides sufficient power for low-sensitivity headphones. The low output impedance prevents unwanted frequency response effects when driving low-impedance headphones.

A90 Discrete with headphones, illustrating its high output voltage, current, and low impedance capabilities.

Figure 6.2: Driving Various Headphones with A90 Discrete.

6.3 Ground Loop Noise Terminator

To address ground loop noise that may occur when multiple devices are connected, the A90 Discrete includes a GND/LIFT switch. Set it to GND for normal use. If ground loop noise is present, switch it to LIFT to eliminate the noise.

Close-up of the rear panel showing the GND/LIFT switch for ground loop noise termination.

Figure 6.3: Ground Lift Switch.

6.4 12V Trigger Interface

The A90 Discrete is equipped with a 12V Trigger interface, which can be set to active control (Trigger out) or passive control (Trigger in). This allows for synchronized power on/off with other devices equipped with a 12V Trigger interface, simplifying system operation.

Diagram showing the 12V Trigger interface on the rear panel and its connection to other devices.

Figure 6.4: 12V Trigger Interface.

7. Troubleshooting

This section addresses common issues you might encounter with your A90 Discrete. For more detailed troubleshooting, refer to the video below.

Video 7.1: TOPPING A90 Discrete Troubleshooting Guide.

Phenomenon	Solution
Can't be turned on	Check the power connection. Check if the rear panel power switch is in the ON position. Try with another power cable. Try with another power outlet.
The volume too low	Press the B button on the remote to switch to volume curve 1. Turn up the volume of the source. Set the headphone gain to high gain by the GAIN button on remote. Turn up the volume of the downstream devices if you are using the Line out output of the unit.
No sound	The device did not power on. Check the power supply. Check that the proper input source is selected. Press the knob to switch input channel. Check the output setting. Double-press the knob to switch output channel. Check the connections for all devices. Insert connection cables all the way in. Check the volume of all devices and whether they are muted.
Noisy playback	Try with other songs. Try to switch to "LIFT" on rear panel. Check to see if the problem is with the source, downstream equipment and cables.
One channel with noise/no output	Check to see if the problem is with the source, downstream equipment and cables. You can try swapping the left and right channel connections.
Channel imbalance	Check to see if the problem is with the source, downstream equipment and cables.
Remote control not responding	Enable remote control function in the setup menu first. Try to insert two fresh AAA batteries. Aim at the remote control receiver. Bring the remote control close to the device. Use your Android phone's camera to look at the transmitter at the top of the remote; if it doesn't blink when you press the remote button, the remote is defective. If you have more than one Topping device, check that you are using the correct remote control.
Negative and positive voltage abnormal (-2/-3/-4)	Please update the firmware first. May be caused by high temperature. Turn the power off, wait for the unit to cool down sufficiently, and then turn the power on again. Please place the unit in a place having good ventilation. Probably overloaded. Try turning down the volume and then reboot the unit. Too low load impedance or short-circuit load. Check the connections with the speaker. Disconnect other devices and reboot. If the problem still persists, your unit may be defective. Please contact the store where you purchased it. Amplifier circuit outputs abnormal DC voltage.
Overload (-5)	Turn down the volume of this unit, turn it off and let it cool down for 1 minute before restarting it. Disconnect other devices, turn this unit off and cool it down for 1 minute and then restart it. If the problem still persists, your unit may be defective. Please contact the store where you purchased it.
Firmware related issues (-7, LARGE.TXT, ERRFW.TXT, UNKOWN.TXT, FAILED.TXT)	Please re-flash the correct firmware. Reboot the computer or try with another computer for firmware updates.

8. Specifications

Specification	Value
Voltage	240 Volts
Maximum Supply Voltage	240 Volts (AC)
Minimum Supply Voltage	240 Volts
Mounting Type	Surface Mount
Number of Channels	4
Output Power	19600 Watts
Material Type	Silver, Metal
Specification Met	CE, Energy Star, FCC, RoHS
Model	A90 Discrete
Brand Name	KGUSS
Unit Count	1.0 Count

Detailed specifications table for A90 Discrete Headphone Amplifier and Pre-AMP, including THD+N, output level, input sensitivity, output power, and load impedance.

Figure 8.1: A90 Discrete Detailed Specifications.

9. Warranty and Support

The A90 Discrete is sold by HIFI College. HIFI College offers a 30-day money-back guarantee for any reason and more than a 12-month warranty on all quality defects. They also support 30-day price matching.

For any questions or support, please contact HIFI College directly.

Contact Email: HIFICOLLEGE@gmail.com

KGUSS A90 Discrete