Electronic Design

Bob's Mailbox

Hi Bob: Just read Bob's Mailbox from the April 12 issue with great interest—in particular, the letter from Terry Hosking about audio capacitors and your reply. It seems to me that the audio field above all others is one beset by pseudoscience and pure wooly thinking. (I tend to agree. /rap) As someone who earns his living as an electronics engineer, but also was a classically trained musician in the dim and distant past, I am very much aware of just how subjective any judgement of audio "quality" is likely to be. In particular, I cringe inwardly when I read or hear audiophiles raving over this amplifier's total harmonic distortion (THD) figure being 0.001% lower than that amplifier's figure. It may well be, when measured under certain conditions. But—and this is a big but—in a completely blind test, would anyone be able to consistently judge one as better than the other? I somehow doubt it. (I have actually seen cases where distortion as small as that can be heard and appreciated. If you look at the LM4702 data sheet, there is a basic audio amplifier, where the negative input of the LM4702 is coupled to ground through 20 µF and 1.8k. If you put 200 µF across the 20 µF, it sounds a lot better because electrolytics can generate distortion. Now, I measured the distortion at 20 Hz caused by the electrolytic, and it's about 2 ppm. Yet I have heard people with good ears say that the music sounds (slightly) noticeably better if the 200 µF is bypassed with a wire shunt. (Also, I want to do some tests with 2000 µF, but everybody's too busy to listen.) I am sure the distortion they hear is not the 2 ppm of distortion I measured, so it must be something else. But still, it is probably very small. The LM4702 has a THD of about 3 ppm at its output. Some people say it sounds as good as any power amplifier they have ever heard. That may well be so. It sure is clean with low distortion, with a clean transient response. /rap ) What I would be interested to know, and maybe you or your readership may be able to help with it, is whether any such tests have ever been carried out in proper, controlled circumstances. And if so, what were the results?
Chris Hill
Pease: Ihave tried to set up some controlled A-B tests, but most people seem to be too busy to do such tests. Thanks for the comments.

Hi Bob: Can you suggest any reasonably comprehensive sources of information on the analog characteristics of "passive" components—for example, the dielectric absorption of surface-mount capacitors? I recently ran into a problem where a supplier of assembled printed-circuit boards (PCBs) substituted a polyester cap for an obsolete polycarbonate in an integrator circuit. Changing the PCB layout was not an option, so the cap was predefined as 0.1 µF in a 1218 package. The available parts were Mylar (polyester—cheap but bad), PEN (polyethylene napthalate—better), and PPS (polyphenylene sulfide—good enough). (My experience is consistent with yours. PPS is the least bad of all the ones you mentioned, as wave-solderable. The polypropylenes are much better, but not so small, and not wave-solderable. /rap) My setup was cheap and dirty: ±5-V power supply/single-pole double-throw (SPDT), center off switch/1k resistor/cap under test/LPC661 buffer/DMM. The procedure was to switch the device under test to +5 for 1 minute, to 0 for 1 second, then open, and watch the DMM. (That is basically the right test, but you might need to change the times if you were using the cap in a different timeframe. /rap) I also learned that all of the surface-mount film caps that I tested were useless after reflow soldering, unless they were immediately and properly cleaned with hot DI water.
Ben Barnett
Pease: Huh. I guess I didn't know that.

Bob: I assume from your comments in "\\[\\[What-s-All-This-Cold-Toes-Stuff-Anyhow-15245|What's All This Cold Toes Stuff, Anyhow?\\]\\]" (April 27, p. 20) that you have diabetes. (Correct. /rap) If so, you might want to think about changing doctors. Peripheral neuropathy is indeed a common complication of diabetes, common enough that all diabetics should be warned about it and what consequences it might have, even if they don't yet have it. (I guess I should ask for a good contact for a diabetes specialist. Since I was diagnosed with diabetes, I have hiked for more than 12 years, and in cold weather, and in insane conditions, for thousands of miles and never had any trouble until January. /rap) Furthermore, it's possible (even easy) to test for peripheral neuropathy during a regular checkup. This should be done, and any patient who develops signs of peripheral neuropathy should be warned again, more specifically. (Uh, yeah! I will check into this. Note: hiking hard and burning off the sugar (and taking in extra sugar) has given me some of the best health of my life. So I have still done a lot of things right. /rap) What happened to you shouldn't have happened, unless you were warned and ignored those warnings.
Fred Webb
Pease: I was not warned. But what happened to me was a gradual deal. How the hell do you give a frog a warning about warmer and warmer water? Just today, the pain went out of my left foot. Don't ask me to explain it. The right foot lost the pains a week ago. So while I am still angry at myself, and only myself, I am not as mad as I was. Thanks for the advice.

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