"MEASURING ULTRA LOW-LEVEL MUSICAL AMPLIFIER SIGNALS
This may be the most important letter I have ever received from a reader (which is why it is located here and not below). The letter speaks for itself, and is unedited, with my bold. (My "personal comments", plus a sceptical and cautionary perspective from another source, will follow below).
"My name is Steve Keiser, the 'K' of B&K components, and presently design engineer with Luminance Audio. I have developed a measurement system which is able to quantify distortion levels at micropower quantities down to 1/1000th of a watt. These measurement techniques are unprecedented, and have revealed a number of revelations of amplifier distortion characteristics, at micropower levels, which are in direct opposition with traditional and scientific assumptions up unto this point.
The emphasis of my work is to definitively quantify low level signal linearity measurements of power amplifiers, and attempt to correlate these measurements with subjective listening results, as well as establishing the significance of low level distortion. Conventional test equipment generally does not resolve meaningful distortion measurements below 100mw, since the measurements become predominated by noise.
I have modified a spectrum analysis software program, which uses time-averaging to effectively cancel out noise products, leaving an identifiable signal and its related harmonics. This time-averaging approach is to identify extremely weak signals from spacecraft, amid a very high noise ambient background. Using this method, I am able to resolve a standardized total harmonic distortion measurement down to 1/1000 watts, and an approximate measurement down to 1/500,000th of a watt.
My measurement results oppose common engineering supposition, in that it is commonly believed that very low signal linearity is essentially 'virtually perfect', and that only high level signal linearity is a relevant parameter. My measurements indicate exactly the opposite is true of this common held assumption, particularly for amplifiers employing solid state devices.
To give you an example: the Halcro DM58 amplifier measures .007%THD at 2 watts, whereas at 1/1000th watts, THD measures 8.9%! By contrast a Wavac SH833 measures .57%THD at 2 watts and 1.6%THD at 1/1000th watts. The tube Wavac exhibits significantly lower THD at low signal levels by orders of magnitude than the Halcro. I have measured numerous amplifiers, both solid state and tube, which I will provide to you as well as any other information you may want pertaining to this work.
Correlative Listening Tests
...I will now provide a supplementary addition regarding correlative listening tests with a panel of 5 evaluators. Some of the tests were conducted using a blind A/B comparison method, in order to satisfy militant objectivists. The two amplifiers compared were a Wavac SH833 and Halcro DM58. In 10 trials, with listeners blindfolded, every listener on the panel preferred the Wavac by several orders of magnitude, with commentary such as describing the Halcro as sounding: transistory, thin, harsh, dark, closed in spatially, as well as having poor sound floor resolution.
Every listener described the Halcro as being 'unlistenable', while the Wavac enjoyed universal positive accolades. These listening tests correlate exactly with the comparative measurements I outlined.
My research into this characteristic is currently ongoing, and I would enjoy sharing my results with audio enthusiasts, editors, and designers. If this correlation between measurement technique and listening impressions holds up consistently, it could mean a whole new approach to audio engineering could be opened up resulting in significant breakthroughs in design performance. The main idea is to let our ears continue to be the final arbitrator of component performance and allow objective science to enhance our subjective appreciation."
