About Current Mode Technology
This is a story about the creation of a Hi-End audio amplifier, and about a quest, a vision and perseverance as well.
It has been nearly a century since the audio amplifier came into being, where the first application of a tube audio amplifier was involved. From then until now, audio amplifiers have undergone updating, development and have progressed, for our sense of hearing is one of the most important and complex of our senses, and also the most basic one. In a desire to accurately play back music via a high fidelity audio amplifier, thousands of electronics engineers and audiophiles are making efforts to improve on audio amplifiers.
Wang Fengshuo (Jason Wang), founder and CEO of Questyle Audio, was deeply attracted to Hi-Fi audio from the time in which he listened to it, for the first time, when he was a seven-year old boy. Days later, like many other electronics enthusiasts, he bought all kinds of electronic components, picked up his soldering iron and tried every kind of circuit and device, immersing himself in his own world. He was also a guitar lover. Imagine what it was like, he was embraced by music on one side, and electronics engineering on the other, it’s a similar experience to the story of Steven Jobs or Bill Gates, who had crushes on computer programming, when they were little. He kept on this way for 15 years. When he went to university, he chose to major in Electronic Engineering without hesitation. He was excited to have an advanced lab in which to experiment, a wonderful library to study in, and kind teachers to consult, while at university. After having thrown himself in for some time, he found that there were many defects in the traditional audio amplifier circuit, which blocked his quest for true Hi-Fi sound.
Wang Fengshuo (Jason Wang) at Senior
High School and his first self-made amplifier
Let’s begin with transient intermodulation distortion (TIMD). Some amplifiers have excellent test specs, but practically, when they process high speed audio signals, their negative feedback amplifier, while working in an open loop, will lose control, which is called transient intermodulation distortion. It lowers the sound quality, making the sound harsh and unpleasant. Some top manufacturers, like Gryphon, PASS and MOON, have been aware of this problem and have adopted the approach of little negative feedback or non-negative feedback to avoid TIMD. However, that either doesn’t solve the problem or is extremely costly. Successfully producing a non-negative feedback amplifier requires a very strict selection of components, with only a 1% or even 0.1% chance of successful matching. Meanwhile, there is the stability issue, which comes along, when amplifiers operate in a non-negative feedback state.
Next, the bandwidth of many traditional amplifiers is seemingly wide enough, while practically speaking, when dealing with audio signals of high frequency and large amplitude, it is insufficient. For a general audio amplifier, its slew rate (SR), power frequency response (fp(max)) and output voltage amplitude(Vo(max)) follow the law below:
Therefore, if the SR is not high enough, the bandwidth value of a high amplitude signal is far lower than that of a low amplitude signal. Take the μA741, a general operational amplifier, as an example, the gain bandwidth product(GBWP) of a μA741 is 1MHz, the SR is 0.5V/μs, and at an output voltage of 10V, we get:
Thus it can be seen, the bandwidth of this μA741, when used for audio amplification of a high amplitude signal, is no more than 8 kHz. Well within the human hearing range of 20k，two thirds of the sound cannot be heard, let alone that of DSD, with a frequency range of 100kHz! Those same top manufacturers also realized this problem and took measures to increase the SR of their amplifiers (such as NHB108 of darTZeel, FBI series of Krell, and flagship amplifiers of Gryphon), widening their frequency response, in order to ensure their amplifiers can track transient signals without delay and guarantee the real speed and transparency of the amplifier, otherwise, Hi-Fi is impossible.
Total harmonic distortion (THD+N) is another issue. The 24Bit DAC, mainly used as a digital signal source, can achieve THD+N below 0.001%. Working under a real driving load, traditional amplifiers at full band, especially at high frequency, can rarely make a THD+N below 0.001%, which then becomes the choke point, of the electronics part, of an audio system.
Sometimes chance comes along quietly, without being noticed. In the spring of 2004, while performing an experiment, on a current mode communication circuit, in the university lab, Wang Fengshuo (Jason Wang) accidentally discovered a circuit, which failed to process communication signals, because it was mistakenly assembled, had very low TIMD and could be used for high speed amplification. This inspired him to begin research on current mode circuits, looking through all the books that could be found on the subject, to learn whether current mode technology could be used for audio amplification. He turned to one of his teachers, Yan LiZhong, a very strict but respected teacher in the Electronic Engineering Department. Mr. Yan was moved by the dedication and persistence of this young man, which led him to decide to support and participate in the project.
During the summer holiday of 2004, Wang Fengshuo (Jason Wang) stayed at the University to work on his quest, rather than going home. Sweating and full of ambition, he led three of his classmates in an effort to develop an audio amplifier, of unparalleled performance, in the lab. One of the teachers reminded them kindly, saying, “You guys should be at home during holidays, you know that people all over the world have spent so many years studying audio amplifiers, but have only made small changes, nothing big and no essential breakthroughs.” Perhaps, because they were young, adventurous and fearless, they just laughed and continued fighting. During the holidays, they found that some companies had already applied current mode technology to the audio field. For example, Krell was using CAST（Current Audio Signal Transmission）technology for audio transmission, Japanese amplifiers were adopting the SATRI technology BPM7110 module, both of which were non-negative feedback solutions, but, still pioneers of current mode audio circuits. The focus of these technologies was on transmission rather than amplification. The circuit was non-feedback and testing specs were not good. Furthermore, it was not compatible with other equipment, because the interface was pure current mode. So, on their own, these young men decided on a design to solve the problem: voltage input and voltage output; while having all signal amplification occur in current mode, becoming high-performance and compatible at the same time.
At the end of summer and the autumn which followed, in the lab, was born the first current mode audio amplifier, designed with new ideas and topological structure. To their surprise, this amplifier had a bandwidth of 1MHz working at a 2*100W output power, a frequency response far beyond other amplifiers, while the TIMD was too low to be tested. How amazing! Their Dean was so shocked at the news that he came and checked it out in person. Wang Fengshuo (Jason Wang) and his small team were excited and published their research results in Audio Technology, the national core periodical, releasing their Current Mode Audio Amplification Technology to the world, then applied for, and received a patent.
After they enjoyed some exciting times, there came a problem. Their amplifier was just an experiment, not a product. The Transliner-TL loop inside the Current Mode Amplifier had strict requirements for the matching of transistors. According to the special requirements, dozens of transistors used inside were carefully selected from thousands of transistors. They knew they couldn’t make a product this way. Microsystems International Limited, a Canadian semiconductor manufacturer, hoped to produce current mode amplification integrated circuits, but failed in the end because of the precise nature of the technology. A Transliner-TL loop with six transistors would cause about 0.01% of distortion for 2 times as long as there was a mismatch of 5uV Vbe. This remained a problem unsolved until they graduated from university.
After graduation, Wang Fengshuo (Jason Wang) worked for a US IC design company where the most sophisticated of testing equipment was employed and talented analog amplifier engineers were gathered. He continued his study while working. At that time he became fascinated with Hi-Fi headphones and bought a pair of Sennheiser HD800s. However, he was very disappointed with the officially recommended amplifier for matching with the HD800, for the sound was dry, empty and emotionless. So he decided to design an amplifier for himself to match the HD800 on his own.
He went to his boss Eddie, a most senior engineer of audio IC design, who had once worked for American National Semiconductor (NSC). Eddie had helped design the LM3886 and LM1875, the classical monolithic audio power AMP ICs, which were released ten years ago and are still sold in large quantities to this day. Also, Dr. Charles, from the University of California Los Angeles, assisted him, sharing his wonderful ideas about complementary semiconductor technology. The use of a current mode design was well supported by everyone involved. To meet the demanding requirements for semiconductor technology and circuit, as well as solve the problem of matching components, was a challenge they set for themselves, with the goal of designing a top headphone amplifier, in their spare time.
After many long hours of debugging, the CMA800, the world’s first Current Mode headphone amplifier, was born in 2007. CMA is short for Current Mode Amplification, whose THD+N is 0.0004%, and whose TIMD is so ultra low that it can hardly be tested. Technically, the CMA800 is a perfect amplifier, because the SR grows linearly with input signals, without any limit, and the bandwidth is up to 700kHz driving a real load. What is most important of all is that the CMA800 has solved the technology issue and can be mass produced with excellent and stable performance.
Well, what is Current Mode Amplification?
Current Mode Amplification is a shortened form, referring to a Current Mode Audio Amplifier, which features, the use of transistors to amplify, with a disaggregate structure designed in, to wit: voltage input and output, while the core amplification takes place in the current domain operating in a pure class A state, and allowing the output stage (OPT) to be changed to either class A or AB. It is completely different from the traditional voltage mode amplifier, in the structure of the amplification circuit. Capacitance between the transistors, which affect the speed and bandwidth, operate at a very low impedance, and the full-power bandwidth of the entire amplifier loop is very close to the bandwidth of a closed loop while, the SR grows linearly with input signals, together with a strong and speedy negative feedback loop (the speed of the circuit is hundreds of times faster than that of conventional voltage amplifiers). All these attributes are quite different from that of a traditional voltage amplifier. It not only eliminates the TIMD, it also easily achieves amplification with a wide bandwidth and ultra low distortion.
Then, how does this Current Mode Amplifier work?
The diagram above shows a current mode amplification circuit. It is basically made up of voltage-controlled current source (VCCS), current amplification unit(A(i)), I/V converter, output stage（OPT）and negative feedback. In the science of electronics, voltage and current exist side by side, power (Watts) is the product of the two, so you can amplify either current or voltage. In the current amplification stage, signals are amplified in current to achieve higher performance, while the input and output stages remain voltage. This kind of design is easily compatible with other advanced sources and loads. It is an open system for the demanding user who likes Hi-End products.
As shown by the diagram, the input signal goes into VCCS first and the VCCS converts input voltage to current which goes into Ai for current amplification, then it is converted to voltage by I/V converter to drive load through OPT.
Negative feedback is applied to guarantee stability. Because of the low impedance of the negative feedback and the high-speed performance of the amplifier, the processing speed of the feedback loop is hundreds of times faster than that of voltage mode amplifiers. So the issue of TIMD as produced in conventional amplifiers doesn't exist here.
High performance doesn’t necessarily equal high quality sound, it is only the foundation upon which good sound is built. Wang Fengshuo (Jason Wang) has developed an extremely sensitive and picky ear for sound. During the 4 years from 2008 to 2012, in addition to improving the circuit continuously, he adjusted and improved the sonic characteristics of the CMA800 22 times and requiring 8 different versions of the PCB. He was not satisfied with it until he tried every component, revised every routing and even evaluated the influence coming from the casing structure including mechanical vibration. These improvements tested out the same via AP, but the sound was greatly improved, almost perfect.
In 2012, audiophiles around him noticed this unusual headphone amplifier, offering him very positive feedback after listening tests. Then orders for the unit started coming in, increasing dramatically as time went on. Obviously, the manual welding of products at home was no longer going to suffice. So, at the end of 2012, Wang Fengshuo (Jason Wang) left his comfortable working environment, and started up Questyle Audio in the CBD of Shenzhen.
In 2013, when Wang Fengshuo (Jason Wang) was researching top speaker systems, an interesting idea came into his mind: top power amplifiers were often designed as mono blocks to improve performance, why not do the same for a headphone amplifier? With separate chassis and power supply systems, along with a full balanced amplification configuration, would it not provide excellent performance and sound? Therefore, based on the design of the Questyle Audio CMA800, he began work on the Questyle Audio CMA800R, improved the circuit and added mono full balanced XLR input and output, thus the Questyle Audio CMA800R was born, taking the place of the Questyle Audio CMA800. Operating in dual mono mode, the THD+N of the Questyle Audio CMA800R was 0.00026%, which challenged the limits of physical testing. The resulting sound has captured the praise of some of the most demanding headphone lovers and sound mixers in the world. The Questyle Audio CMA800R has gained a very good reputation. At the same time, Wang Fengshuo (Jason Wang) successfully applied for a PCT patent for Current Mode Technology.
Following the introduction of the Questyle Audio CMA800R, the Questyle Audio CMA800i, an all-in-one DAC and Current Mode headphone amplifier and pre-amp, as well as the Questyle Audio Q192, a low priced DAC/amplifier/pre-amp were launched, both of which are highly praised and rewarded by the global media and audiophiles alike.
Current Mode Amplification Technology will go on to be used in other Questyle Audio products. The 150i, an integrated amplifier shown at CES in January 2015, driving a 4Ω load at 100 Watts of output power, has the ultra-low THD+N of 0.0003%, and less than 0.0005% within 20Hz-20kHz, with a bandwidth of 1MHz . There is no rival among integrated amplifiers with equal performance and price point to compete with the 150i, globally.
What are more attractive still, are the QP1/QP1R portable music players released in May of 2015, whose headphone amplifier sections, are pure class A, discrete component, current mode amplifiers. It is rare feat to incorporate a discrete component design, usually reserved for large hi-end audio amplifiers, into the tiny chassis of a portable device. In this era of competition between portable music players and smart phones, a design such as this, focused on sound quality, will become the real stand out offering, far ahead of its opponents.
As more people understand the true meaning of high fidelity, it is the revolutionary breakthrough of Current Mode Amplification Technology and the uncompromising concept in sound tuning of Wang Fengshuo (Jason Wang), founder and CEO of Questyle Audio, that ensure Questyle products can provide really accurate and reliable Hi-Fi sound and a unique brand value.
The story of Current Mode and Questyle Audio is on going, let's keep focusing and following!