Electronic Design

Bob's Mailbox

Hi Bob: Back in the early '70s, I was an EE major at Lowell Tech in Lowell, Mass. Within easy walking distance of the dorms, there was an electronics surplus store in one of the old textile mill buildings. One day I found some Philbrick P65AU op-amp modules there.

I bought several of them, for maybe $5.00 total. (A bargain. /rap) I decided to build a stereo hi-fi amp around them. It took the better part of the semester to build it, and I nearly flunked out as a result. But I ended up with a pretty good stereo amp at the end. I learned a lot by building the amp, a lot more than they were teaching me in class at the time!

Limited by the 30-V supply the op amp would run at, it didn't put out a lot of power. But with some efficient bass-reflex speakers, it was loud enough. (Check. The P66s were useful? If you had enough of them, you could parallel several of them or add other boost transistors. /rap)

A few years later I took the amp to a Tech HIFI audio clinic, where they got a good chuckle when they saw my homemade amp. But when they ran the frequency-response and distortion checks, they were quite impressed.

(The P65 wasn't very fast for slew rate. It had about 0.6 V/µs, so at 20 kHz, it could barely swing 8 V p-p. But most program material doesn't have a large amount of content at 20 kHz. The P65 also was a good low-noise preamp for phono cartridges, etc. /rap)

I used that stereo amp for many years and eventually gave it to a friend of mine, who might still be using it. I still have a "spare" P65AU module and a P66A booster follower, which I never did find a use for.

  • Steven Weber (via e-mail)
  • Pease: Take good care of them, Steven! We may need them someday! I've recently been running some P65s in a 1-ms delay line, using 10 H of inductance.

Hi Bob: My son is a chemist, so I sent him an e-mail asking him about the ozone reaction on rubber and rubber products ( ELECTRONIC DESIGN, Sept. 29, p. 18.). Here is his reply:

"Ozone is a highly reactive form of oxygen, which is why it can be dangerous to human health as well. Ozone is one of the primary causes of the degradation of rubber because it reacts with any unsaturated double bonds left in the rubber polymer. It can also react with regular covalent-type polymer bonds, but this is more of a minor reaction. Once the reaction has started, it basically goes down the polymer chain via free-radicals and 'unzips' the polymer molecule—effectively destroying it. Tire manufacturers minimize the effect of ozone degradation by adding antioxidants to their tire formulas and waxes that protect by blooming to the surface of the tire and forming a coating, effectively sealing the surface molecules of the rubber from the air and ozone. This is why the tire manufacturers discourage the use of products like Armor All on the sidewalls of the tires because the Armor All can solubilize the waxes and then remove them over time."

  • Dave Miller (via e-mail)
  • Pease: Hello, Dave. Well, that certainly seems to be the right story on ozone. If it beats up rubber, imagine what it does to our covalent bonds! That's what they were complaining about when Los Angeles was so smoggy! It's interesting how tire makers protect their tires. Too bad rubber belt makers can't do that. Or our lungs!

Dear RAP: Since you were quoting only the lyrics from "They All Laughed" (ELECTRONIC DESIGN, Sept. 15, p. 22.), and not the music, perhaps the credit should have gone to Ira Gershwin, instead of George. George just did the music...

  • Bruce Walker (via e-mail)
  • Pease: Hello, Bruce. Well, you are quite right. I must have goofed. My big reference book, American Popular Song by David Ewen, did say exactly that. I'm sorry!

Comments invited! [email protected] —or: Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

See the Figure

TAGS: Components
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