EFFECTIVE RESOLUTION
The digital version of Dynamic Range (sometimes referred to as DNR) essentially indicates the effective “resolution” of a digital audio device. When used for amplifier measurement, it provides a number related to SNR in that the numbers should be pretty close (for example, 102 dB SNR and 100 dB DNR).
DNR is mostly used to measure digital devices, such as A/D (analog to digital) and D/A converters, and has recently become prevalent in the measurement of digital input amplifiers. It can be used to measure analog amps as well.
The typical method for measuring DNR is to input a small signal (-60 dB, referring to 0 dB for maximum output) and measure THD+N in dB, and then add 60 dB to the measurement. The -60 dB input is used to prevent auto turn-off of any signal stages that might contribute to noise.
If you notice SNR and DNR measurements that aren’t close, a trick is probably being used. The trick in this case is usually some form of gating, as mentioned previously. Gating results in an erroneous SNR figure, but doesn’t effect DNR, so you might notice a much higher SNR than DNR in this case. Exaggerated maximum power ratings also artificially inflate DNR and SNR figures due to increased headroom that’s not really there.
Another trick used to get higher power ratings is to drive very low impedance loads. This has been used on conventional pro audio amps for many years, but for all the right reasons, because “paralleling” several loudspeakers for more output is common.
However, some Class-D manufacturers rate power output at 4 ohms or even 2 ohms, then rate distortion at 8 ohms. Read the fine print.
In car audio, using low impedance has gone to the extreme, mostly with Class-D, but sometimes with conventional amps as well. For example, some car amps, especially subwoofers, are made to drive loads of less than 1 ohm.
Of course, this is impractical with any appreciable wire distance due to simple resistance. As long as the other specifications are rated with the same load as the power output, this is not a problem. Most power ratings are based on continuous output of a sine wave, typically 1 kHz.
Yet another ratings trick is used to indicate outrageously exaggerated power output. “Peak power” is sometimes specified, but rarely for pro audio amps, though this is a common consumer market trick. Peak power is measured several different ways.
One way to measure it is to use a burst tone at the input and measure momentary power output. This method is not dishonest, even though the result can be deceptive. If the conditions of the test are stated to adequate detail, the measurement is on the up-and-up.
Another way to measure peak power is to double the continuous power measurement. Theoretically, the “instantaneous peak” power is the power to the load at the top of the sine wave. Continuous output is most common, and usually most honest, but music is not continuous.
IN RELATION TO ANALOG
Switching power supplies have come a long way, but linear supplies are still the cheapest option for amplifiers. One trick employed throughout history in the power amp business is the use of underrated transformers.
For example, you might buy a 500- watt amp that has a 300-watt transformer in it. How can you get 500 watts from a 300-watt transformer, even at 100 percent efficiency? Simple – consider the phrase “not for long.” A 300-watt linear transformer can deliver 500 watts for short bursts, but it begins to heat up (and eventually blow out) if asked to put out that kind of power continually. If we take advantage of the fact that audio is transient in nature, we can downsize the transformer.
Class-D benefits more from this than conventional amps due to increased efficiency. In most cases, a well-designed Class-D amp could use a transformer half the size of one required for a conventional amp with equal power output.
The caveat is that some Class-D designs have poor power supply rejection ratio (PSRR), a spec that the chip guys often neglect to provide. Linear supplies suffer from ripple, and the frequency of the ripple is double the line frequency, typically 100 Hz or 120 Hz (very audible spots in the spectrum). Some designs require regulated supplies due to this problem.
Although Class-D amps provide definite advantages over conventional units, the methods used to measure these new products should be carefully considered. Fortunately, measurement tricks are decreasing in popularity for professionals, and increasing in popularity for consumers. But – always be sure to read the fine print.
Tommy O’Brien is a power amplifier designer.
