mzkmxcv351 posts01-14-2019 1:28pm@elizabeth
Hearing a difference is not the same as there being a difference.
Say what?!
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John Curl Interview Page 12/18
[Quote]
Q: Okay, what about capacitors?
JC: I learned how to measure a capacitor for the first time about 25 years ago working with Tektronix. They were making a piece of equipment which measured capacitors and they were really worried about the values. They were doing it with one of their pieces of test equipment -- It’s called a curve tracer. They modified it so it would measure the value of capacitors. They found that ceramic capacitors, for example, had another characteristic they had never been able to see on the screen before and it actually affected the measurement of the value of the cap. It showed a tremendous non-linearity. Interestingly enough, in this particular test and this method of measurement, only ceramic capacitors showed up to be really bad. We found that ceramic capacitors really were bad guys. Later we found that we could emulate the same problem indirectly using normal test equipment, but we had to operate the capacitor in some sort of real way. It couldn’t just be sitting there with zero volts across it; it had to be working with some kind of a signal like rolling it off high frequencies, low frequencies or something.
John Curl Interview Page 13/18
I published a paper in 1978 and Audio magazine article in 1979 that showed this problem with ceramics, and we also found that Tantalum capacitors did almost the same thing. With this particular test (with normal test equipment) you could see the non - linearity of the Tantalum capacitors as well as the ceramics. This still allowed us, in theory, to use aluminum electrolytics -- we couldn’t find any real problem with them as long as they were used properly -- or any kind of metal film capacitor. A third type of distortion, which has been known for many years, but had been forgotten about, is called dielectric absorption. This particular problem used to be very important back in the 50s when people used to solve many engineering problems with analog computers. These analog computers would emulate mathematical equations with capacitors, resistors, or amplifiers. Music will also evoke dielectric absorption. Music tends to not be completely symmetrical at all times, and even though it averages out in the long run it isn’t necessarily a test tone. If you put a symmetrical test tone through a mylar capacitor for example you won’t find any real problem. However if you use an asymmetrical signal you’ll find that it does have dielectric absorption. This is where the dielectric absorbs part of the signal and then spits it back later. Well this can’t be good. Invariably you never get the musical peak, it cannot be completely passed by the capacitor because the capacitor has to take some of the energy from the musical peak. It stores it like a battery. Fortunately, this material property isn’t shared by Polystyrene, Teflon, or Polypropylene, which is why we tend to use these caps instead of mylar. Tantalum, aluminum, and mylar are pretty bad in this area. As a result of all this, we have to exclude many types of capacitors
John Curl Interview Page 14/18
because they all have some problems to a greater or lesser degree. Ultimately we wind up with polystyrene, polypropylene, and Teflon. And that’s why we tend to prefer these capacitors when we can. Except for the use of aluminum electrolytic for power supplies , the more capacitors we can eliminate the better it its. Q: You’ve always been a proponent of trying to keep the signal path free of inductors and capacitors. Why is this so important?JC: It’s like this - it is easy enough today to design out capacitors between stages. It is rather redundant and wasteful to add capacitors between stages. First of all, they do not help the size of the unit. They’re not very reliable. If anything is going to go bad, the capacitors will probably go bad first ... unless you have catastrophic failure. In short, they don’t really do you any good so. The best capacitor is no capacitor...we don’t need them anymore. In the old days, when we didn’t have complementary circuits, we needed capacitors. When you look at vacuum tubes there is no such thing as a complementary vacuum tube device. So you almost invariably needed transformers and capacitors. Then again, one of the advantages of the vacuum tubes is that they are very high impedance devices, so the capacitors could be small in value even though they might have to be high voltage. Now, when you use capacitors in solid state transistor equipment, you generally need fairly large value capacitors, but their voltages don’t have to be so high. These situations would seem perfect for aluminum or Tantalum electrolytics. However, these are the ones that are not very reliable and they have all these secondary distortion characteristics ...
John Curl Interview Page 15/18
dielectric absorption, nonlinear distortion, and that sort of thing. So, if you can eliminate these capacitors, why put them in in the first place? Now, some people can say ’what about leakage or safety or something like that?’ Well, of course, you have to be careful, and that is what modern protective circuitry is good at. It shuts down the amplifiers if they are behaving abnormally, yet it doesn’t impact the signal when the amp is behaving normally. We also use servos, which are basically very precise well matched IC devices. In the factory, they laser trim them down to one or two millivolts and then we simply use these to compare the output to ground and then adjust very slowly to zero out any offset that might be inherent in the amplifier or preamplifier. It’s easy to do these things now. Thirty years ago it wasn’t easy because we didn’t have FET input ICs, much less very well matched FET input ICs. For example, the JC -2 didn’t have servos because they weren’t practical in 1973 when it was designed. Maybe the military could’ve done it, but the real world had to wait until about 1978 or so. Also, we couldn’t use mylar capacitors, which are fairly efficient coupling capacitors. While mylars are fairly efficient from a size and cost point of view, we realized they have problems with dielectric absorption. I didn’t believe it at first.
I was working with Noel Lee and a company called Symmetry. We designed this crossover and I specified these one microfarad Mylar caps. Noel kept saying he could ’hear the caps’ and I thought he was crazy. Its performance was better than aluminum or tantalum electrolytics, and I couldn’t measure anything wrong with my Sound Technology distortion analyzer. So what was I to complain about? Finally I stopped measuring and started listening, and I realized that the capacitor did have a fundamental flaw. This is were the ear has it all over test equipment. The test equipment is almost always brought on line
John Curl Interview Page 16/18
to actually measure problems the ear hears. So we’re always working in reverse. If we do hear something and we can’t measure it then we try to find ways to measure what we hear. In the end we invariably find a measurement that matches what the ear hears and it becomes very obvious to everybody. [End of Quoted material.]
http://www.parasound.com/pdfs/JCinterview.pdfSo first you have to admit you hear something.......
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