Cayin HA-300MK2

Single-ended 300B Class-A headphone & speaker amplifier with external power supply

Like its predecessor, the HA-300MK2 is a transformer-coupled headphone and speaker amplifier built around a single-ended triode Class-A topology. A selected pair of 300B direct-heated triodes (output tubes) and a selected pair of 6SN7 driver tubes amplify the signal, which is then delivered through output transformers to single-ended headphone outputs, balanced XLR-4 and 4.4 mm headphone outputs, and a pair of speaker terminals.

Many tube enthusiasts consider the Direct Heated Triode (DHT) the holy grail for audio. DHT refers to triode tubes that use the cathode as the filament and heat it directly during operation. These tubes can be seen as the most fundamental form of amplification and have a devoted following thanks to their exceptional sonic qualities.

They are known for a rich, full midrange with harmonically dense texture and a holographic presence that renders instruments and performers with lifelike dimensionality. Technically, the 300B offers very high linearity in amplification, which is a key reason for its natural, realistic musical reproduction.

Balanced headphones are very popular among audio enthusiasts. Because Cayin amplifiers are transformer-coupled, both single-ended and balanced operation can be realized within the same circuit. In balanced headphone amplification, signals of opposite polarity are delivered to the left and right earcups. Thanks to the transformer winding arrangement, positive (+) and negative (-) polarity are routed in opposite directions, creating a fully balanced output from a single-ended tube amplifier circuit. Both the single-ended and balanced headphone outputs are “native” transformer outputs, not balanced after the fact.

The HA-300MK2 offers two separately optimized balanced headphone outputs: 4.4 mm and XLR4. The XLR4 output is taken directly from the output transformer, while the 4.4 mm winding is level-matched, lower-noise, and lower-power while maintaining the same sonic character—making it better suited to portable, high-sensitivity headphones.

A particularly useful feature is selectable impedance matching for the headphone outputs. The HA-300MK2 provides three impedance settings to optimize output performance for your headphone’s impedance. Each matched output is fed by an independent secondary winding of the output transformers:

• Low (L): recommended for 8 Ω ~ 64 Ω — target impedance (XLR4 & 4.4 mm): 32 Ω
• Medium (M): 65 Ω ~ 250 Ω — target impedance (XLR4 & 4.4 mm): 64 Ω
• High (H): 251 Ω ~ 600 Ω — target impedance (XLR4 & 4.4 mm): 300 Ω

Correct impedance matching contributes to a natural sound with well-defined bass. The recommendations are a guideline—depending on your headphones and preferences, experimenting with other settings can be worthwhile.

The HA-300MK2 output transformers are extremely complex multi-winding designs. They feed three impedance-matched headphone outputs (6.35 mm, 4.4 mm, XLR4) and also provide power to the speaker outputs.

The separate power supply uses a custom toroidal transformer and includes multiple circuits to feed different gain stages. Four NOS RCA 22DE4 rectifier tubes convert incoming AC to DC and ensure the amplifier stage receives exceptionally clean power with minimal interference. The 22DE4 tubes can typically serve reliably for 5 to 10 years in the HA-300MK2.

Design & Features

• Single-ended direct-heated triode design that highlights tube charm with natural, realistic musical reproduction.
• Driver tubes: selected pair of Tung-Sol 6SN7GTB
• Output tubes: selected pair of Genalex PX300B
• Rectifier tubes: 4× NOS RCA 22DE4
• Supports balanced (XLR) and single-ended (RCA) inputs; balanced input is converted to single-ended via a pair of input transformers.
• The 300B output feeds Cayin’s in-house, high-grade EI output transformers for speakers, single-ended (6.35 mm) and balanced (4-pin XLR and 4.4 mm) headphone outputs.
• Three impedance-matched headphone outputs (L, M, H) ensure satisfying compatibility with a wide range of headphones.
• High-power headphone output with ample headroom, even for demanding headphones.
• Custom silicone-gel damping as an anti-shock buffer between critical tube parts, chassis frame, and tube sockets—effectively reducing 300B microphonics for quiet, realistic playback even with sensitive headphones.
• Enhanced transformer shielding to minimize interference.
• 60-second start-up delay circuit protects tubes and circuitry and reduces turn-on pops.
• One set of speaker terminals providing 8 watts per channel in Class-A for speaker connection.
• External power supply delivers clean, stable power with minimal interference.
• Device-specific, in-house produced premium toroidal transformers and EI output transformers.
• High-voltage rectification using four NOS RCA 22DE4 tubes.
• Two-chassis design with multi-stage, tube-regulated power supply and independently regulated stages reduces interference and improves audio performance.
• Point-to-point wiring ensures the shortest signal path for increased transparency and resolution.
• High-precision electronic volume controller JRC MUSES72320V with a 41-step ALPS potentiometer for low-noise, precise volume control.
• Premium MultiCap coupling capacitors for optimal resolution.
• High-quality, shielded, silver-plated internal wiring minimizes interference in signal routing.
• Beautifully finished, high-grade VU meter.
• Aerospace-grade copper lock-on quick connectors with silver-plated Teflon cable for the external power supply.
• Removable tube cage—easy to remove and install.

Technical Specifications