Class D amplifiers are fundamentally different from analog amplifiers, not only in their circuitry, but more importantly in the way they operate. Yet, engineers continue to test Class D amplifiers using the same tests and test procedures that were developed generations ago specifically for analog amplifiers. Because these tests were designed for analog amplifiers and are focused on analog particulars and capabilities, it is no wonder that we have Class D amplifiers that test well in the lab, yet reproduce inferior sound under real world operation. Clearly, new tests that are specifically designed with Class D operation and behavior in mind are needed if the test results are ever going to be relevant.
Reconsidering THD in a Class D Environment
Analog amplifiers modulate power continuously in accordance with an analog input signal, and generate continuous wave (analog) outputs. Class D amplifiers generally rely on the principle of Pulse Width Modulation, and, by virtue of their use of switching power devices, generate outputs comprised of discrete quantitized samples. So, while both amplifiers perform the same general task, the ways they accomplish this are as different as the way LPs and CDs play back sound. As we know, LPs drag a stylus over plastic, and CDs read a digital signal with a light source.
Some of the traditional tests used to validate the performance of the venerable but ancient Class A and Class A/B amplifier architectures such as Total Harmonic Distortion plus Noise Floor (THD+N) will reveal some useful performance or sonic characteristics in a Class D amplifier. However, these tests were originally conceived to show known common limitations of the analog amplifiers of that era. At that time, audio distortion was primarily harmonic, and more importantly, dynamic range was limited (and defined) by the noise floor.
Digital switching amplifiers, by their very nature, provide a completely new set of strengths and weaknesses. For example, distortion may be harmonic or enharmonic in nature, due to interactions with the sampling rate. Amplifier jitter rejection can also be critical. On the other hand, dynamic range is rarely, if ever, limited by the virtually non-existent noise floor of a digital system, especially when the incoming audio signal is digital and therefore also virtually noiseless. To compound the issue, most audio test equipment was designed to test within the confines of analog amplification and an analog sound source, therefore, this equipment - by design and definition - must give inaccurate results when confronted with inaudible switching frequencies.
The most significant analog amplifier measurement has now become the least reliable and least significant measurement for a digital amplifier. Since the test equipment was designed before switching amplifier systems were conceived, THD+N readings may easily be unreliable or misleading. In other words, in the world of switching amplifiers, THD is at least not as relevant as other tests and at worst is totally irrelevant as a standard by which to evaluate the performance of a digital amplifier. In fact, most THD measurements of digital amplifiers are no more representative of sonic performance than wow and flutter measurements are of a CD player. A single arbitrary ‘harmonic’ distortion category by itself is meaningless in the context of digital amplification without additional and more appropriate tests, such as intermodulation distortion and spectral display. Designing a Class D amplifier solely for good THD test results does not mean it sounds good, let alone qualifies it as “audiophile”.
Dynamic Range: The Acid Test for Class D Amplifiers
Dynamic range, or the ratio of the largest accurately amplified signal to the smallest, has never been more relevant than in today’s world of 16 to 24-bit digital audio sources. A wide dynamic range is what makes music sound live, and 3-dimensional. With analog amplifiers, the popular shorthand for this measurement has become the ratio of the onset of clipping to the noise floor. This is practical for Class A and Class A/B amplifiers because any signal below the noise floor is largely masked. This does not hold true for Class D amplifiers. Here, the noise floor is generally much lower than Class A/B amplifiers, as a result of the noise immunity of digital circuitry. This does not mean that the amplifier can actually reproduce signals near that noise floor because most digital amplifiers, in fact, cannot. It is a limitation inherent in the architecture itself.
The critical fact to bear in mind is that all Class D amplifiers have outputs that are comprised of discrete power increments, a notion that can more easily be thought of as resolution steps. When the definition of dynamic range is applied in the context of quantitized increments, the result is that dynamic range of a Class D amplifier is defined as the ratio of the smallest discrete power level the amplifier can accurately output to the largest.
Given this clarity of understanding, the obvious question becomes: How do we determine the smallest output power level in a Class D amplifier? The answer can be seen in its linearity.
The Key Is Testing for Linearity
The blue trace of Figure 1 shows Power Out (right Y axis) versus Input (X axis) of an E-Bridge', True Fidelity' amplifier. The red trace shows deviation (left Y axis) from linear operation over the same input range (or error from linearity). Although the expected -30 dB output is seen with a –30 dB input, the difficult part for most digital amplifiers lies at the bottom end of the amplitude range. The further down the amplifier maintains linear operation, the greater the dynamic range.
Note that this test shows actual operation and doesn’t rely upon what we will call, ‘magic numbers’ (how other companies measure dynamic range often has nothing to do with the actual definition). The noise floor of this amplifier is around –120dB, so the distinction between the two is obvious. The sonic implications of a broad dynamic range become more striking with the observation that while the upper amplitude regions impart what a musical instrument is, the lower regions impart where it is. Spatial location of sound is the driving force behind the entire home theatre movement, which makes these low-level signals even more important. Here, the dynamic range of this True Fidelity amplifier extends to about 102dB.
Fig. 2: Conventional Class D Amplifier Linearity
By contrast, conventional Class D amplifiers show their limitations when the same linearity test is applied. Figure 2 shows the dynamic range of a conventional Class D amplifier. Curiously, the company whose amplifier was used in this test boasts a dynamic range in excess of 100 dB, where the graph clearly shows that that the amplifier ceases to operate linearly at levels below –80 dB. This is traceable directly to inadequate resolution of the amplifier’s output stage. While the processing portion of the amplifier is capable of handling 16 or more bits of digitized data, it is the output stage that actually determines resolution.
Because Class D amplifiers are fundamentally different than Class A and Class A/B amplifiers, different tests need to be conducted to show performance. For more meaningful predictors of a switching amplifier, dynamic range is the critical test in an arsenal of new tests aimed specifically at digital amplification, since it shows the amplifier’s ability to reproduce the full range of signals it receives. A simple and effective way to measure dynamic range in a Class D amplifier is by its linearity in actual operation. With this test, you can be assured that great performance in the lab will equate to great performance under real world operation.
About the Author
Jim Shanahan is a co-founder of JAM Technologies, Inc. JAM Technologies is a fables semiconductor company that architects and designs digital audio amplifiers for a variety of consumer electronics products. The company is headquartered in Boston, MA. Jim can be contacted at email@example.com