Amplifier FAQ
What is an Analog Switching Amplifier? What's the difference between NuForce's amplifier and other digital amplifiers?

NuForce's switching amplifier is a drastic departure from conventional approaches to switching amplifier design. Most class-D amplifiers use a fixed sawtooth waveform to modulate an audio signal, and suffer from the 180-degree phase shift of the LC reconstruction filter which would normally cause a feedback from the load to the error amplifier to oscillate unless phase compensation is used. That compensation network drastically reduces the amplifier bandwidth to below the corner frequency of the LC reconstruction filter. Thus most class-D amplifiers have low bandwidth and high distortion due to limited gain of the phase-corrected error amplifier at audio frequencies.

NuForce's amplifier technology is based upon the principle that a power oscillator can be modulated by an audio signal so that it produces an amplified audio signal obtained with a reconstruction filter, without the bandwidth limitation of a fixed frequency carrier-based conventional PWM control. It uses analog modulation technique and close-loop control systems. Therefore NuForce refer to its audio amplifier as Analog Switching Amplifier.

What are the problems with traditional Class-A and A/B amplifiers?

Traditional linear amplifiers such as Class-A and Class-A/B amplifiers are bulky and inefficient. The inefficiency compromises the reproduction of music signal's full dynamic range. Its resulting higher operating temperature also shortens the useful life of the electrolytic capacitors used in abundance in these amplifiers. To get around that problem, today's better amplifiers employ bulky heatsinks and costly linear power supplies to provide enough headroom to handle the full dynamics. These huge power supplies are unregulated and could add noise and ripples at low volume. Besides being inefficient, linear amplifier depends on transistors or MOSFET devices to generate power. Big (high-power) bipolar transistors or MOSFETs have inherently low bandwidth and do not provide adequate audio performance. Therefore, smaller (up to 20+) MOSFETs with decent audio bandwidth performance are paralleled to provide sufficient power. Each MOSFET has an inherent junction noise - actually worse audio low frequency noise than bipolar transistors - and the aggregated noise corrupts music reproduction. What you hear is haziness and a lack of clarity in music reproduction. MOSFETs are used in parallel because technically, they are easier to drive although they have inherently higher distortion than bipolar transistors, which are much harder to drive when they are paralleled. Class-AB amplifiers - the most popular amplifier circuit - have to overcome the inherent crossover distortion that occurs when the audio signal goes from negative to positive and vice-versa, crossing the zero region where gains of transistors are much reduced. They are actually down to zero when the transistors stop conducting current. Close-loop system designers know that lower gain means higher inaccuracy of the amplification loop.

What are the problems with Class-D digital switching amplifiers?

Digital Switching Amplifiers (commonly known as Class-D) have been around for years. Nevertheless, it is nearly impossible to engineer a conventional Class-D amplifier that handles the full requirement, 20-20,000Hz, for full-bandwidth music reproduction. A Class-D amplifier works by utilizing a high-frequency sawtooth waveform to modulate the music signal (to learn more about how Class-D amplifier works, click here). The constant presence of the sawtooth waveform, which is very high in frequency spectrum and its inevitable frequency jittering, can mask or corrupt low-level music signal. The output filter designed to filter out noise and overtones caused by the sawtooth waveform adds a 180 degree phase shift to Class-D output stage, causing possible instability and adding distortion due to its own inherent non-linearities. Additionally, the output filter presents frequency-variant output impedance that can interact with a speaker's complex impedance. Variants of Class-D amplifie rs with the addition of Digital Signal Processor claim to improve music reproductions. However, because of their lack of close-loop design, especially from the speaker's terminals, spurious interaction between the speaker's complex impedance and back-EMF with the amplifier's resonant output filter can result in harsh sound reproduction. The fundamental flaws of conventional Class-D amplifiers remain unresolved.

Does NuForce's amplifier experience crowding-out phenomenon as in conventional audio amplifiers?

Many audiophiles have observed that the main voices in a recording when produced by amplifiers that have impressive specification numbers are crowded out or submerged in the presence of strong basses. Yet these amplifiers boast about 100kHz bandwidth. Is this a human hearing characteristic or the amplifiers have something strange going on?

While it is true that human ears are highly non-linear vs. amplitude and frequencies, it is also true that no datasheet of transistors contain straight lines on any parameter such as gain (or rise time) vs. any other parameter such as drain/collector current (or the voltage across the transistor terminals). In particular, the current gain of a bipolar transistor or the transconductance of a MOSFET varies not only with the collector current or drain current but also the frequency of the input signal. For example it is well known that the transconductance of a bipolar transistor is at first approximation proportional to the collector current but on the other hand its current gain decreases with collector current. Therefore when a bass note appears during a continuous main voice, the gains of the transistors become actually lower than during the absence of the bass note, especially a strong one, because the transistor currents are higher during the vibration of the bass note, the refore its F.sub.t also go lower, causing the higher frequencies of the main voice to be actually less amplified.

Thus in conventional amplifiers, especially class-AB amplifiers using bipolar transistors, the crowding-out or submersion of high-frequency voices is real.

NuForce's amplifiers on the other hand do not suffer from this crowding-out phenomenon. This is because the output MOSFETs only switch between ON and OFF regardless of the amplitude of the audio input signal, the gain in such amplifiers is independent of the transistor current level. NuForce's amplifiers have exceptionally huge bandwidth exceeding 100kHz and very low phase shift in the same frequency range. Therefore all the audio frequencies and their pitch-defining harmonics are faithfully reproduced. Additionally its extremely high damping factor (exceeding 4000) guarantees very tight bass reproduction.

Why is switching amplifier better than linear amplifier in reproducing music? Music is a blend of sine waves, isn't it?

While any waveform theoretically consists of sine waves, that mathematical decomposition assumes that the waveform is periodic, in other words, a repeating waveform. Musical instruments, on the other hand, produce waveforms with full sharp edges that even trained eyes have a difficult time figuring out what the fundamental frequency is, because even a single note produced by any instrument is full of attacks and decays (except for instruments that are based on natural resonance such as the pipe organ or the flute). A violin produces very complex waveforms full of high frequencies called harmonics. Harmonics are so named because their frequencies are multiples of the fundamental frequency of the note being played. Faithful reproduction of such complex waveforms requires an amplifier capable of very high bandwidth, and more importantly, with no crossover distortion. The most popular amplifiers use a class of amplifying circuit called class-AB amplifier. It is actually a compromise between the huge inefficiency of class-A amplifiers - the simplest circuit universally used in low power amplification - and the high distortion but higher efficiency of class-B amplifier. Class-A amplifiers theoretically have no crossover distortion. This is the main reason why audiophiles are willing to pay sky high prices for some class-A amplifiers. Likewise, NuForce's analog switching amplifier circuit has no crossover distortion. And while typical linear amplifiers have a bandwidth that is barely over 20 kHz - lower still in boomboxes and in conventional switching amplifiers -NuForce's analog switching amplifier uses proprietary technologies to achieve bandwidth up to ten times higher than typical linear amplifiers.Its huge bandwidth allows it to amplify complex music faithfully, much beyond the concept of high-fidelity. A good side effect of zero crossover distortion and huge bandwidth is a huge sound stage most music lovers never experienced be fore, because now spatial information contained in a stereo program is completely reproduced.

NuForce amp uses a lot of Surface-mount components (SMD). Are SMD worse than traditional through-hole discrete components?

SMD components are rated and performed as well and sometimes better than through-hole components in several ways: (1) The proximity of lead-less components reduces parasitic inductances, capacitances, and spurious induced noise, so the circuit performs closer to theoretical ideal even when operating at high speed/frequencies; (2) SMD manufacturing technology, driven by PC and telecommunication industries, are more reliable and consistent to produce quality circuit boards. Additionally, our rigorous component selection and scientific circuit design methodologies coupled with extensive computerized simulations and analyses, made possible by our deep insight of high-performance high-precision analog design, make very high fidelity a scientific reality and not marketing hypes of voodoo electronics. Using SMD with the resulting minimum parasitic inductances, capacitances, and minimum antenna effects, NuForce's analog switching amplifiers achieve the highest bandwidth at lowest distor tion of a magnitude higher than that of linear and conventional switching amplifiers.