Audiophile amplification gains from GaN

Exceptional switching characteristics of GaN FETs underpin a new era in high-fidelity amplification

BY LEO AYZENSHTAT FROM ORCHARD AUDIO

FOR MORE THAN half a century lovers of hi-fidelity have argued over the best technology for making audio amplifiers. Sitting on one side of the divide are the valve aficionados, who claim that tubes are the key to providing an engaging, unfatiguing and rewarding listening experience. In the other camp are the fans of the transistor, who view this as by far the better option – one that delivers a realistic, powerful and faithful delivery of the recorded medium.

For those that prefer solid-state technology to valves, a decision now awaits. Do they hold on to their cherished amplifier that sports silicon transistors, or do they trade it in for a new breed, built around wide bandgap devices? It is not a difficult decision, because if they do invest in the future, they will reap many rewards.

The benefits are not limited to a better sound for the outlay, but extend to practical gains, such as a far smaller footprint and a higher efficiency that trims household bills.

GaN devices from GaN Systems lie at the heart of the Orchard Audio Starkrimson Ultra product range.

Over the last decade or so, there has been an increase in sales of all forms of Class-D audio amplifier, which operate at high switching frequencies. With this mode of operation it is challenging to realize an acceptable level of total harmonic distortion + noise (THD+N), due to the need for faster, cleaner switching transitions. When class D amplifiers are based on silicon MOSFETs, they incorporate substantial feedback circuitry to compensate for the poor open-loop performance and subsequent noise. While it is possible to reduce this with larger devices, this comes at the expense of higher switching losses, diminished efficiency, an increase system size and higher material costs.

All these issues are not just of concern to high-end audiophiles, who will pay thousands and thousands of dollars for an amplifier. Over the last few years home audio has changed, partly due to the pandemic-driven shut-down of traditional entertainment sources, such as movie theatres and live music venues. Many of us are now spending more time with our audio systems – whether we are streaming movies, playing games, or listening to music – and this has heighten our awareness of how high-quality home audio systems can enhance our listening experience.

Market analysis supports this view. Those in the know are pointing out that demand for high-quality audio is fuelling the growth of the Class-D audio amplifier market, which is tipped to reach $4.92 billion by 2026. This class of amplifier is being deployed in ever more audio applications, including home theatres, high-power smart speakers, pro-touring amplifiers, portable speakers, automotive, marine, and power sports.

At Orchard Audio of Succasunna, NJ, we are playing our part in the audio revolution by launching a portfolio of products that feature amplification with GaN transistors. We have adopted these devices because they have exceptional switching speeds. Several benefits result from this attribute: very fast slew rates, which are much valued in a class D amplifier, because this narrows the gap to the ideal square wave (see Figure 1); incredibly precise timing, critical to realizing high-quality audio; and improved efficiency, with amplifier topologies being able to operate with a much shorter dead time, an approach that leads to a much lower cross-over distortion (see Figure 2).

Figure 1. The Starkrimson Streamer Ultra operates with a very fast slew rate of around 10,000V/ΩS. This narrows the gap to the ideal square wave.

Figure 2. GaN devices result in a far lower crossover distortion than silicon incumbents.

Another benefit of using GaN transistors is that it makes it much easier to design a circuit with no or very limited ringing. Minimising ringing is highly valued, because it eradicates EMI issues and prevents noise. By turning to GaN, our amplifiers combine decreased noise with less distortion, better transient response and a higher bandwidth.

But what does all this mean when it comes to sound quality, the most important metric of all? Well, quite a lot – our amps are renowned for their reduced harshness, cleaner highs, better transparency, and greater audio detail.

Like other electrical units, such as power supplies, using GaN rather than silicon also delivers benefits at the system level. There is a trimming of the cost of other system components, including capacitors, heat sinks, and inductors.

As well as a reduction in the total bill for these components, they are smaller and lighter. Thanks to this, amplifiers built with GaN can be around one-quarter the size of silicon equivalents. That’s a big selling point for potential customers with small homes, and for those that don’t want a stack of large audiophile units in their living room.

Figure 3. The Starkrimson Streamer Ultra combines a digital-to-analogue converter, an amplifier and a streamer inside an easy-to-use enclosure.

Why GaN trumps SiC
You may be wondering why we are making our amplifiers with GaN rather than SiC, the other commercialised wide bandgap semiconductor. Well, there are several reasons – some are related to audio, and other due to cost and practicality.

One of the downsides of SiC is that it is not that good at switching at high frequencies. What’s more, it’s hard to drive this class of device at a high frequency. In comparison, that’s not an issue with GaN, thanks to its much simpler, lower-voltage gate drive. Additional attributes of GaN are its low gate charge, zero reverse recovery and flat output capacitance; all of which yield a high-quality switching performance.

Where SiC has enjoyed most of its success is at high voltages, typically 1200 V. Audio amplifiers do not require such high voltages – and for the mid- and low-range voltages where they do operate, GaN has far lower switching losses. For example, at 650 V, switching losses for GaN are at least three times lower than those for SiC.

Even if SiC devices were as good on paper as those made from GaN, there are plenty of reasons to shy away from them. SiC devices are more pricey, and compared with those made from GaN there are limitations associated with both their supply and the supply chain. Amplifiers built with GaN can also enjoy a greater power density than those made from SiC, delivering savings in size and weight.

An expanding portfolio
With a goal of delivering the ultimate sonic listening experience, we are continuing to expand our product portfolio. Our range currently includes expertly designed high-performance digital-to-analogue converters (DACs), streamers, and amplifiers. These products are helping consumers to elevate the sound in their home theatres, listening rooms, and recording studios.

Our belief is that every aspect of sound can be measured, a philosophy that underpins our research and development efforts. We are focussed on achieving the best possible objective measurements – and delivering the ultimate, subjective results.

Many of our products incorporate devices made by GaN Systems. This chipmaker produces very fast, true enhancement-mode (E-mode) GaN devices with a simple unipolar gate drive, a feature that makes them close to ideal for audio applications. In comparison, devices made by many other GaN manufacturers require a more complex or slower gate drive, or are cascode, which makes it much more difficult to control the timing of the switching. Further strengths of the products by GaN Systems are a low on-resistance, and a form of packaging that makes it easy to incorporate these devices in circuit boards for audio amplifiers.
Figure 4. To offer the same level of facilities as the Starkrimson Streamer Ultra requires an amplifier, preamplifier, DAC, and a streamer.

One of our most recent products is the Starkrimson Streamer Ultra. It represents a new kind of high-end audio system. Miniaturization enabled by GaN allows a digital-to-analogue converter to be united with amplifiers and a streamer, inside an easy-to-use enclosure (see Figure 3). This level of functionality is typically realised with a rack of components (see Figure 4). A comparable system, including an amplifier, preamplifier, DAC, and a streamer, would typically command a price tag two-to-three times higher than that of the Starkrimson Streamer Ultra. Measurements of our Streamer Ultra confirm the pedigree of this fully balanced unit, which can drive speakers with an impedance as low as 2 Ω. Testing confirms extremely low noise and distortion (Figure 5), ultra-low jitter (Figure 6), and native playback up to 24Bit/192k.


Figure 5. Using a measurement bandwidth of 22 kHz, the total harmonic distortion of the Starkrimson Ultra Amp as a function of power at 1 kHz.

Like the Starkrimson Streamer Ultra, its cousin, the Starkrimson Stereo Ultra amplifier, delivers less harshness, cleaner highs, and better overall transparency and detail, alongside vanishingly low noise levels. This amplifier delivers a power of up to 500 WRMS (1,000 WPEAK) and up to 20 A of current, while maintaining extremely low noise and distortion. What’s more, this unit has enormous reserves of energy for extended transition. Expanding linearly with load, it is capable of delivering 125 watts into 16 Ω, 250 watts into 8 Ω, and 500 watts into 4 Ω. This culminates in powerful, unrestrained music.

Figure 6: Starkrimson Streamer Ultra Jitter Spectrum and Noise, 256kFFT 16 Averages

A significant part of every Class-D amplifiers is its filter. Thanks to the high-speed switching of GaN Systems’ transistors – they ensure two-to-three times faster switching than traditional Class D amplifiers using silicon transistors – our design employs a simple LC inductor and capacitor filter. Equipped with these components, our Starkrimson Stereo Ultra amplifier produces practically no phase shift from DC (0 Hz) to 20 kHz.

In our view, high-quality audio is now a ‘must-have’ across all segments, from pro-audio to home-audio and portable audio. The best approach to this is a Class-D audio systems with GaN devices. Armed with this technology, audio delivers a superior sound quality from smaller, lighter units; and there is no need for active cooling – there can be either no or minimal heart sinking.

How does it sound?
Orchard Audio is winning fans within the audiophile and music lover communities. Those that have heard these audio products are saying: My speakers are very efficient and tend to make a feature of any noise in the electronics. The Starkrimsons are super-quiet and my music now plays against a silent background. Not only that, but the sound is wonderful. Separation and clarity are improved and the bass is noticeably tighter and more forceful. They are a clear step up from the power amp..

Rich and detailed without being overly analytical, drive and grip, brilliant low end, gorgeous midrange and extra levels of transparency that lift a veil from your music.
His amp is a realization of the benefits of GaN. The Ultra has improved harmonic integrity, dynamics, and resolution over traditional silicon.

With the Starkrimson driving them, the sound became more lively and dynamic but without a trace of harshness. The bass was on a par with the $6k amp, as was the treble smoothness, but the Starkrimson was more open. Playing reference tracks that I’ve heard dozens of times on the .7s, I was frequently startled — literally — by the realism of instruments and vocals. Micro-detail, textures and subtle dynamics that I hadn’t noticed before became clear.