Allnic Audio HPA-15000 OTL/OCL

Reference-Class Tube Amplifier with True OTL/OCL Design, Class A Operation and up to 10 Watts of Output Power

The Allnic Audio HPA-15000 OTL/OCL takes a radically different approach from most high-end tube amplifiers. Instead of routing the output signal through an output transformer or a large coupling capacitor, Allnic eliminates both.

OTL stands for Output Transformer-Less, while OCL means Output Capacitor-Less. The output tubes are therefore connected to the headphone output without these conventional coupling elements in the signal path. The goal is an exceptionally direct connection, with high transparency, fine dynamic resolution and as little sonic coloration as possible.

In the HPA-15000, Allnic combines this unusual circuit concept with substantial power reserves. With up to 10 watts available, the amplifier is not limited to traditional high-impedance dynamic headphones, but can also drive demanding planar magnetic designs.

OTL and OCL – As Direct as Possible

In a conventional tube amplifier, the output transformer provides the interface between the high-impedance tube circuit and the much lower impedance of a pair of headphones. Other designs employ large output capacitors to block DC from reaching the load.

Allnic follows a more elaborate route with the HPA-15000, eliminating both output transformers and output coupling capacitors. This means fewer frequency-dependent components are placed between the output stage and the headphones.

That is precisely the attraction of the OTL/OCL concept: fast transients, exceptional microdynamic resolution and an open, immediate presentation, while retaining the dimensionality and harmonic character of a sophisticated tube circuit.

Class A and 10 Watts of Tube Power

The HPA-15000 operates in pure Class A and delivers a maximum output power of up to 10 watts.

That is exceptional for an OTL/OCL tube amplifier and considerably broadens its range of suitable headphones. Allnic specifies an optimal headphone impedance range of 15 to 600 ohms – from classic dynamic designs to modern planar magnetics.

The output stage uses four powerful 6C41C triodes. Together with the preceding tube stages, they form a fully analogue amplifier architecture whose priorities are a short, controlled signal path and musical transparency rather than excessive feedback or headline-grabbing specifications.

Harmonic Balance Mode for Planar and Ribbon Headphones

One of the HPA-15000's distinctive features is Allnic's Harmonic Balance Mode. It was developed particularly with ribbon and planar magnetic headphones in mind and can be activated directly from the amplifier.

The function is designed to complement the tonal balance of these exceptionally fast driver technologies without sacrificing their characteristic advantages in resolution, speed and transient response.

The HPA-15000 is therefore not simply a larger HPA-10000, but a design specifically conceived to accommodate a wider range of modern high-end headphones.

61-Step Constant Impedance Attenuator

For volume control, Allnic does not rely on a conventional potentiometer. Instead, the HPA-15000 uses the company's proprietary 61-step Constant Impedance Attenuator – CIA, a technology also found in Allnic's high-end preamplifiers.

The circuit maintains a defined impedance throughout the volume range, helping preserve channel balance, dynamics and spatial definition even at lower listening levels.

This is especially beneficial with more sensitive headphones, allowing precise volume adjustment without being confined to a very small usable portion of a conventional potentiometer.

Four Headphone Outputs and Five Inputs

The HPA-15000 is also unusually generous in terms of connectivity. On the input side, it can accommodate a total of five analogue sources:

• 3 × stereo RCA
• 2 × stereo XLR

For headphones, the amplifier provides no fewer than four connections:

• 2 × 6.35 mm headphone outputs
• 2 × 4-pin XLR headphone outputs

This makes the HPA-15000 particularly convenient not only for frequent headphone changes, but also for direct comparisons between multiple models within a high-end listening system.

Optional RAAL Direct Drive

On request, one of the balanced headphone outputs can be configured as a RAAL Direct Drive connection, allowing compatible RAAL ribbon headphones to be driven directly by the HPA-15000.

For this dedicated output, Allnic uses a 0.2-ohm transformer with silver windings and a Permalloy core. This optional RAAL output is therefore the one exception to the otherwise transformerless OTL/OCL architecture; the standard headphone outputs remain entirely free of output transformers and coupling capacitors.

Also a High-End Tube Preamplifier

The HPA-15000 can also be used as a line-stage preamplifier. Its rear RCA pre-out allows the signal to be routed directly to a power amplifier or active loudspeakers.

This means the Allnic can serve as the centre of a complete analogue system: DACs, phono stages and other sources can be connected centrally, while both headphones and a separate loudspeaker power amplifier benefit from the same high-quality tube line stage.

A Tube Amplifier Without the Traditional “Tube Filter”

The HPA-15000 is not primarily aimed at listeners who expect a tube amplifier to add maximum warmth or deliberately soften the upper frequencies.

Its OTL/OCL architecture instead pursues the opposite objective: combining the naturalness and spatial presentation of tubes with as much transparency, speed and control as possible.

With highly resolving headphones, the result can be a fascinating combination: physical and dimensional, yet open, fast and exceptionally immediate.

Allnic Audio HPA-15000 OTL/OCL – Highlights

• Allnic Audio flagship tube headphone amplifier
• True OTL/OCL circuit architecture
• No output transformers or output coupling capacitors in the standard signal path
• Pure Class A operation
• Up to 10 watts of output power
• Designed for headphones from 15 to 600 Ω
• 4 × 6C41C output tubes
• Harmonic Balance Mode for planar magnetic and ribbon headphones
• 61-step Constant Impedance Attenuator
• 2 × balanced 4-pin XLR headphone outputs
• 2 × 6.35 mm headphone outputs
• 3 × RCA and 2 × XLR analogue inputs
• RCA pre-out for use as a line-stage preamplifier
• Optional RAAL Direct Drive output
• Remote-controlled volume and input selection

Technical Specifications