Behind the Scenes at Secrets – Number 3 – December, 2008

In reading our reviews that contain bench tests, you may have noticed in the past that we usually had a large number of graphs. More graphs in fact, than most other publications. A few years ago, there were pages and pages of graphs in certain reviews.

Trying to examine lots of graphs and then remember what you saw in all those graphs is problematic. I kept adding more tests, and the number of graphs got even larger.

In order to maximize the information shown, but reduce the number of graphs, I have begun to consolidate. I recently completed some tests on affordable receivers and found that the frequency response did not really change much at all, regardless of the voltage output (5 volts vs. 20 volts), or whether it was into an 8 ohm or 4 ohm load. I had to show this information on two graphs. So, after those reviews are published, future receiver reviews will just have one graph that shows 20 volt output performance into an 8 ohm or 4 ohm load. The problem will be that the graph lines may very well completely overlap, but I will deal with that as it occurs.

I used to show THD+N at numerous individual frequencies in preamplifiers, power amplifiers, and receivers. When I purchased our Audio Precision SYS 2722 analytical instrument, I was able to consolidate this by plotting THD+N vs. Frequency across the entire audible spectrum, resulting in one graph that showed it all. Although this test had not previously been used for speakers, I thought that it should work there just as well, so I tried it, and it did work just fine. So far, Secrets is the only publication to plot THD+N vs. Frequency for speakers and subwoofers. This has eliminated the need to show distortion spectra for numerous individual frequencies in subwoofers (each in its own graph), although I now still show the 20 Hz spectrum just to be able to see the relative contributions of the even vs. odd ordered harmonics. For full range speakers, only individual spectra for 50 Hz (from the woofer), 1 kHz (from the midrange driver), and 10 kHz (from the tweeter) need to be shown (in the past I sometimes showed distortion at other frequencies as well). The THD+N vs. Frequency plot will indicate the distortion at any other frequency we might be interested in.

For example, here is the THD+N vs. Frequency for Velodyne’s new Optimum-10 compact subwoofer (review is underway). I placed the microphone 1 foot from the center of the driver cone, set the volume at 50 Hz to 100 dB, then ran the test. See how distortion stays at 10% or below from about 16 Hz up to 200 Hz. Many people consider 10% distortion in a subwoofer the high limit of acceptability, so this graph is extremely informative (not to mention very impressive for a compact subwoofer).

If you have not paid attention to the bench test graphs, you should go back and take a look, specifically at these THD+N vs. Frequency tests for subwoofers and speakers. They are very revealing, and to me, perhaps the most important graph in the tests. Even compared to frequency response, this test is important, because the emergence of new room correction technology such as Audyssey will allow you to smooth out the response. Velodyne’s SMS-1 will smooth the response in subwoofers. But, there is no circuit that can be built into a preamp, power amp, receiver, or speaker that will completely eliminate inherent distortion. Negative feedback can reduce it in amplifiers but not eliminate it. Servo-feedback in subwoofers also can reduce distortion, but not eliminate it. Our THD+N vs. Frequency plots will give readers the bottom line.

So, if you have been a little numb from the amount of graphs we have been publishing in the past, be at ease. We will be trimming things down to just show what is really important, and put it all in fewer graphs.