Independent channel volume knob tube amp

The xDuoo TA-68 is a fully balanced desktop tube headphone amplifier with independent volume controls for the left and right channels. Its signal path combines 5U4G tube rectification, 12AU7 voltage amplification, a four-tube 6C19 current buffer, and capacitor-coupled headphone outputs. Separate channel circuits and isolated transformer supplies maintain electrical separation from input to output, while balanced and single-ended connections support source equipment and headphones using XLR, 4.4 mm, RCA, and 6.35 mm interfaces. With a recommended load range of 60 to 600 ohms, the TA-68 is intended primarily for full-size, medium- and high-impedance headphones.

Separate balanced circuits for each channel

The left and right channels use independent tube circuits throughout the amplifier. Balanced sources can be connected through stereo XLR or 4.4 mm inputs, with the signal carried through the corresponding channel circuitry to the 4-pin XLR or 4.4 mm headphone outputs. A stereo RCA input and 6.35 mm headphone output provide single-ended operation.

Channel separation reaches 80 dB in balanced mode and 71 dB through the single-ended output. The separate channel layout is supported by two low-noise EI transformers, which provide isolated power feeds and reduce coupling between the left- and right-channel supplies.

Tube rectification and voltage amplification

Two 5U4G tubes rectify the high-voltage supply, with independent anode voltage delivered to the left- and right-channel circuits. Tube rectification is part of the amplifier’s power architecture rather than an auxiliary display stage, placing the rectifier tubes directly within the operating supply for the audio circuit.

Voltage amplification is handled by two matched and screened 12AU7 dual-triode tubes. These feed the 6C19 output buffers while providing 24 dB of gain in balanced operation and 18 dB through the single-ended path.

Four-tube 6C19 current buffer

Each channel uses two matched 6C19 power tubes for current amplification. The four-tube buffer stage supplies up to 330 mW per channel into 300 ohms from the balanced outputs, with rated balanced power of 120 mW at 75 ohms, 160 mW at 150 ohms, 200 mW at 250 ohms, and 240 mW at 600 ohms.

The single-ended output delivers 60 mW at 75 ohms, 100 mW at 150 ohms, 130 mW at 250 ohms, 210 mW at 300 ohms, and 160 mW at 600 ohms. This output profile is aligned with the TA-68’s recommended headphone impedance range of 60 to 600 ohms rather than low-impedance portable headphones that require higher current at lower voltages.

Capacitor-coupled headphone outputs

The output stage uses a capacitor-coupled topology, which blocks direct current from reaching connected headphone drivers while retaining the tube-based amplification circuit. Vishay capacitors are used for signal coupling, and Nichicon audio-series capacitors provide power-supply filtering.

Independent left and right volume controls

Separate volume knobs adjust the two channels independently. This allows the user to correct for differences in hearing sensitivity, compensate for channel imbalance in a headphone or recording, or shift image centering without changing the level of the opposite channel.

The control arrangement differs from a conventional stereo potentiometer, where both channels are tied to a single shaft. Independent attenuation provides finer manual control over channel balance, though matching overall listening level requires adjusting both controls.

Input switching and headphone connections

The rear panel includes stereo XLR and 4.4 mm balanced inputs alongside a stereo RCA input. A front-panel selector switches between the balanced and line-level input paths without requiring cables to be disconnected.

Headphones can be connected through a 4-pin XLR balanced output, a 4.4 mm balanced output, or a 6.35 mm single-ended output. The three connections cover common full-size headphone terminations and allow both balanced and conventional cables to be used directly.

Startup protection and replaceable tubes

A relay-controlled protection circuit delays output engagement for 45 seconds after power-on. This gives the rectifier, preamplifier, and buffer tubes time to reach stable operating conditions before the headphones are connected to the active circuit. The relay also reduces switching transients during startup and shutdown.

All eight tubes are socket-mounted. The supplied complement consists of two 5U4G rectifiers, two 12AU7 preamplifier tubes, and four 6C19 buffer tubes. Electrically compatible replacements can be installed without modifying the amplifier, allowing worn tubes to be serviced and compatible alternatives to be evaluated within the requirements of the original circuit.

Chassis and power configuration

The TA-68 is housed in an aluminum alloy chassis with a sandblasted front panel and a ventilated protective cage over the tube assembly. The enclosure measures 13.0 × 11.8 × 7.3 inches and weighs 24.3 pounds, reflecting the use of two EI power transformers and a fully tube-based amplification circuit.

Power versions are available for AC 100–120 V or AC 220–240 V operation. The supplied package includes the amplifier, two 5U4G tubes, two 12AU7 tubes, four 6C19 tubes, and the protective tube cage.

Availability

The xDuoo TA-68 is now available on Apos.