Electronic Design

Bob's Mailbox

Bob: Several years ago, an acquaintance bought a very expensive "high-end" audio system. The speakers were huge. The amplifiers were 350-W "mono blocks" using eight 6550s in a push-pull parallel configuration. The wires were about $100 per foot. Within a minute of first operating the system, the amplifiers were releasing smoke. He called the amplifier manufacturer. Their first question was "What cable are you using?" The strange impedance of the fancy cables interacted with the feedback system in the amplifier and caused an ultrasonic oscillation that destroyed them. So maybe the cable makes a difference, not because of its characteristics but its action on the amplifier and speakers. Not many amplifiers have a truly zero output impedance, especially vacuum-tube types.
Bob Bodmer
Pease: I have heard people say that low-impedance speaker cables have a lot of capacitance per unit length, if you just measure the cable open-circuit. "So when you run an amplifier that doesn't like capacitive loads, it can oscillate and cause great damage," these people say. First of all, vacuum tubes are supposed to be able to tolerate overloads in any good design. Second of all, a low-impedance speaker cable does not look like a capacitor unless you run it with no speaker. My favorite speaker cable is to parallel 20 strands of wire, going out to the speaker, and each wire serves as a twisted pair with one of the 20 (paralleled) return wires that come back to ground. If you run this with no 8-? speaker, it will indeed act capacitive, and the amplifier might be unhappy driving the many thousands of pf. It will look like an unterminated transmission line, and the reflections could be nasty. But when connected to an 8-? speaker, it looks like 7.5 ? because it has the characteristics of a 7-? transmission line. So every amplifier should drive this cheerfully. The cable and load will act like 7 or 8 ? at all frequencies, unlike lamp cord that acts like an 80-? transmission line at high frequencies. Audio amplifiers are not required to have a 0-? output impedance. Any low impedance that is consistent and predictable can work just fine. An audio amplifier that wouldn't drive an 8-? load sounds pretty flaky. And if a customer complains and the amplifier maker tells him "Oh, we forgot to tell you, our amplifiers don't like certain kinds of loads," but they didn't put it in their user's manual, that is a poor way to do business.

Bob: I saw your comments about Y5V dielectric in the May 10 issue. I have always avoided it, not only for its –80% voltage coefficient but also its poor temperature coefficient. For general-purpose use with 20% tolerance, I have always specified X7R dielectric or its more recent derivative X5R. However, recent cost-saving process technology changes mean you have to be wary. Time was when X7R capacitance was –20% at rated voltage. Now with modern thin-layer printing technology, the change of capacitance is –50%. So be careful. If your design needs 20% tolerance, you need to double the rated voltage of the X7R capacitors you use. For a 5-V supply, use a 10-V part, not 6.3 V. It is better to do this than to double the capacitance. The cost is about the same, and so is the capacitance at working voltage. But the reduced coefficient of the higher-voltage part makes for a more stable design.
Chris Green
Pease: Yeah, I believe it. Many "improved" processes are worse than ever. Your solution seems reasonable.

Dear Bob: I just wanted to write you a quick note commending you on your "What's All This Capacitor Leakage Stuff, Anyhow?" article (March 29, 2007). From the title I suspected that the devil of the measurement would be in the (attention to) details, and it was! When I read the sentence "I also keep my left hand grounded to +8.8 V," I was thoroughly pleased! You think like an electron, and that is a good thing.
Chuck Woringer
Pease: I tried grounding my left hand to +1 V. It made tiny errors, but when I did it nine times in a row, it made very small errors, and I didn't even want to make tiny errors. I thought about finding a teflon-insulated ball-hook or teflon-insulated alligator clip, and that ain't easy. By "grounding" my left hand to +8.8 V, the problem shrank to an acceptable error.

Dear Mr. Pease: One of your columns (Jan. 18, 2007) commented that it will be a long time before you will change to SI units. You are in good company. A few months ago, I was curious about the pace of conversion to SI. I noticed an important organization that tried metric units, and now it changed back to U.S. Customary units. The document is called "Caltrans Metric to U.S. Customary Units Transition Plan" (www.dot.ca.gov/hq/oppd/metric/TransitionPlan/Transition-Plan-021105.pdf).
Jim Marks
Pease: Yeah, I peeked at that 29-page document, and sent it to print out, and will read it later. The introduction says it will be less trouble to go back to "all U.S. units" and less likely to lead to the delays or extra expenses that happened in metric conversion—and safer, as of the era 2005-2007. Maybe we should share this philosophy with the Brits and Aussies.

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