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?" (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.
Comments invited! rap@galaxy.nsc.com —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090