Audio-GD HE-27 - All-In-One Headphone Amplifier/Pre-Amplifier

Delivery time approx. 2 weeks

The HE-27 is a truly symmetrical, fully discrete all-in-one headphone amplifier and preamplifier with integrated R-2R DAC.

Highlights

• Regenerative power supply
• Uses 4 pieces of the most realistic R-2R DA chip PCM1704UK
• Truly symmetrical, fully discrete headphone amplifier/preamplifier, ultimate all-in-one device
• 32 Bit / PCM384K asynchronous transmission Amanero 384 uses the FPGA sync clock
• All digital inputs and digital parts are fully isolated from the analog parts.
• Supports external 10 MHz clock input.

Please note: The PCM1704UK does not support DSD playback. The user must convert DSD to PCM in the source device.

Design overview

• A truly symmetrical current transfer design uses fully discrete components.
• Four pieces of the R-2R DA chip PCM1704UK are used, delivering the most realistic sound.
• Two premium Accusilicon TCXOs with frequencies of 90 and 98 MHz synchronize the entire device and are used for playback of all data rates without PLL up-conversion.
• All digital inputs and digital components are fully isolated from the analog components.
• A firmware update can convert the coaxial IN2 input into an external 10 MHz clock input (75 Ohms).
• All settings for the digital mode can be changed live via the control panel on the front.
• At the back of the device, there is a port for firmware upgrades (the cover does not need to be opened to update the firmware).
• The unique, genuine, feedback-free ACSS headphone amplifier features four channels with feedback-free output stages, each consisting of 8 transistors, each with a power of 15 A/150 W.
• The total device capacitance reaches up to 50,000 uF.
• Four R-core transformers with a total power of 160 W are used.
• High output power of 10 W at 25 ohms and 1000 mW at 300 ohms, which can easily drive most headphones.
• Unique, fully analog regenerative power supply technology, similar to a built-in private power generator, providing an extremely clean power supply and making the sound quality more transparent, but analog, much better than the external digital power supply processor, let alone the passive LC power filter output.
  　

Unique technology in the HE-27

1, Unique jitter elimination technology:
Jitter has always been considered the main reason why the sound quality of digital systems is inferior to that of high-quality analog systems.
In some traditional DAC designs, the clock processing method consists of following and restoring the clock of the signal source. Some designs also have the ability to reduce jitter, but with limitations when the jitter level is high.

This common solution allows good measurement results in laboratory tests. However, in practice, most signal sources have a high jitter level, so the resulting sound quality is not satisfactory. To completely eliminate the influence of jitter in the input signal, the HE-27 uses an asynchronous clocking process.

First, it reads a considerable amount of signal source data and stores it in RAM, discards the clock of the signal source, and directly uses the integrated powerful Accusilicons to clock the output from the RAM. Since both data and clock are processed by the FPGA, the influence of transmission on jitter is negligible. The jitter level essentially corresponds to that specified for the TCXO clocks used by the HE-27.
               
The key to this processing method lies in ensuring the integrity of data transmission, which is our technical secret. One must realize that even the loss of a single data sample out of a million during transmission results in a sharp and clear crackling sound during playback, similar to interference noise or a scratch on a vinyl record.

Mr. He is a passionate music lover, and for him, listening sessions are a source of inspiration for new design ideas. Our company not only has the advantage of having full control over the software and hardware technology, but we also have a correct understanding of music playback. Since we do not rely on outsourcing for research and development, we can listen to the devices at any time during development and thus aim for and realize the most realistic sound reproduction.

Since the local clock and the signal source clock are completely isolated from each other and run independently, traditional jitter measurement methods cannot be applied. Instead, a special data acquisition circuit is required to evaluate the level. For humans, the best test method is always the listening test. A jitter-free sound is very realistic and natural, clear like pure water, and very engaging.

2, Unique true ACSS headphone amplifier/pre-amplifier technology without feedback:

The HE-27 features a true symmetrical design using the latest ACSS design. It may seem complex, but it is actually the simplest design ever, with many components for power supply processing.

The amplifier built into the HE-27 has only a single "voltage gain stage" for signal processing but operates in ACSS mode, so it is actually a current gain stage instead of a voltage gain stage, whereas normal OPA chips already contain two or three voltage gain stages. The simplest and shortest signal path ensures the most neutral and least colored sound.

The HE-27 uses a single-stage ACSS amplifier with extremely low distortion, directly soldered onto the mainboard. The gain stages and the Diamond-Cross output stage operate in pure Class-A mode.

ACSS is a feedback-free technology realized with fully discrete amplifiers. Most people know that global feedback designs can offer better specifications in test measurements, while feedback-free designs do not perform as well in tests but provide better sound for the human ear. Here lies a conflict between classic circuits. But ACSS opens a new field; it can offer a sound with minimal coloration that is more neutral, with very low distortion and high linearity. Thus, it can maintain dynamics, detail, and neutral sound without sounding bright or harsh.

The entire signal amplification is done in the current domain and is achieved through the unique, excellent open multi-output current mirror. The result is an incomparably precise and extremely low-distortion amplification of the signal. The signal path is fully symmetrical and all transistors operate in Class-A mode.

The HE-27 features four channels with feedback-free output stages, consisting of 8 transistors, each with a power of 15 A/150 W. The total device capacity reaches up to 50,000 uF. Four R-core transformers with a total power of 160 W are used.

3, Unique, fully analog regenerative power supply technology:

A regenerative power supply is like a private generator integrated into the device. It can largely filter out interference on the power line, supply the audio components with extremely pure power, and return a harmonious and powerful bass as well as a highly analog signal to the audio system and end users, which is far better than an external digital power supply processor, let alone a passive LC power filter output.

In the implementation of the regenerative power supply, the 120 V/220 V AC is converted into DC via the regenerative mains input transformer, which is then supplied to the regenerative waveform generator and the driver through a parallel stabilized Class A power supply.

One set of symmetrical regenerative waveform generators produces an extremely low-distortion 50 Hz sine wave. After a symmetrical amplifier stage and a powerful driver stage, three regenerative power output transformers output a pure voltage that is stable in both amplitude and frequency.

Two sets of parallel regulated Class A power supplies independently power the analog output stages of the left and right channels as well as the digital audio components with their different processing functions. Both the regenerative waveform generator and the driver are powered by a stabilized Class A power supply, so their operation is not affected by fluctuations in the power grid and interference.

The Class A shunt regulator has a very high input impedance to block interference and prevent disturbances from the power grid from entering the audio circuit and affecting sound quality. Additionally, crosstalk between the left and right signals is avoided through the power supply, improving separation.

The low output impedance exhibits excellent linearity and speed, which also contributes to an outstanding design of the regulated power supply. Arguments from Audio-gd on the design of the regenerative power supply:

1. Through internal design analysis and actual test comparison of the AP SYS-2722, the distortion of the purely analog regenerative sine wave generator is much lower than that of digital synthesis generators.

2. The digital driver stage can have a higher energy conversion efficiency, but the fully analog driver stage has a more analog character.

3. The regenerated waveform frequency of 400 Hz has a higher efficiency in energy conversion. However, the regenerated waveform frequency of 50 Hz is further from the sensitive range of human hearing than 400 Hz, so the sound quality at 50 Hz is softer and more natural.

4, Unique I/V conversion volume control technology:

Volume control also differs from conventional technologies. In conventional volume control technologies, volume is controlled in the analog amplifier input stage via a volume control to reduce the signal. The quality of the volume control directly affects sound quality.

Low-quality volume controls lead to loss of detail and channel imbalance, causing distortion in the sound image. Even with a high-quality volume control, detail loss and degradation of the signal-to-noise ratio occur in the attenuation and amplification modes of the signal.

In the HE-27, we use an I/V conversion for volume control. Volume control is done via a variable passive I/V conversion at the output of the ACSS amplifier, where the output signal is a current signal, not a voltage signal. Volume control is achieved through I/V conversion, and the volume depends on converting current (I) into voltage (V). (Similar to R-2R D/A chips that output a passive I/V conversion.) This keeps the signal frequency band flat and no details are lost. Sound quality is not affected at any volume.

The quality of volume control is very important in a true balanced device. It must ensure exact signal control of the four channels to achieve the performance of a true balanced device. If the cold and hot signals of the balanced signal cannot be precisely guaranteed, strong distortion occurs at the balanced output, resulting in sound quality and performance even worse than a single-ended device, wasting the balanced device design and cost.

The HE-27 uses a four-channel digitally controlled relay volume control, where volume is adjusted by swapping resistors with a tolerance of 0.05% to avoid channel imbalance and achieve the best performance and sound quality of the device.

The HE-27 features a 100-step volume control with a super-exponential volume curve that can be easily adjusted to the volume of different power amplifiers and speakers. When changing the volume, various relays are opened or closed, which may cause a slight switching noise at the output, but this disappears once the volume change is complete.

Technical data