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What's Your EQ?
May 1967 Radio-Electronics

March 1967 Radio-Electronics

March 1967 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

The March 1967 issue of Radio-Electronics magazine presented these two circuit challenges in their "What's Your EQ?" feature. Arriving at the correct answer for the Crystal Mike Input problem requires an intimate knowledge of crystal microphones. I took a stab at a guess, and was kinda right, but for the wrong reason, technically. Failure admitted. Black Box type problems can be tricky because sometimes the requirements are not clearly defined (IMHO), and other times because more than one solution is possible. Other times, not getting it is due to being a dummkopf. I have been in that category on more than one occasion. Not so this time, though. My guess turned out correct. Pay attention to what components are excluded, and don't be afraid to think "outside the box" to figure out what is going on inside the box ;-)

What's Your EQ?

What's Your EQ?, May 1963 Radio-Electronics - RF CafeConducted by E. D. Clark

Two puzzlers for the student, theoretician and practical mono Simple? Double-check your answers before you say you've solved them. If you hove an interesting or unusual puzzle (with an answer) send it to us. We will pay $10 for each one accepted. We're especially interested in service stinkers or engineering stumpers on actual electronic equipment. We get so many letters we can't answer individual ones I but we'll print the more interesting solutions - ones the original authors never thought of.

Write EQ Editor, Radio-Electronics, 154 West 14th Street, New York, N. Y. 10011.

Crystal Mike Input - RF CafeCrystal Mike Input

A high-impedance crystal microphone is connected, as shown, to an amplifier with low input impedance. This looks plausible, as the transistor gets more current drive than if it had a higher input impedance (say with the emitter upbypassed). Therefore the hookup should produce more output voltage at the collector. Yet this method is undesirable. Why?

- C. S. S. Shenoi

 

Another Black Box - RF CafeAnother Black Box

Close the switch, and the lamp lights. Then open the switch, and the lamps remains lighted. The lamp can be turned off only by disconnecting the battery. No diodes or relays are inside the box. What is?

-Dale Terdan

 

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, Radio-Electronics, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

These are the answers.

Crystal Mike Input Solution - RF CafeCrystal Mike Input Solution

A crystal mike is a capacitive transducer at audio frequencies, and can be represented as shown below:

The voltage across the input resistance Rin, of the amplifier is almost equal to e0 at very high frequencies, when the reactance of the equivalent capacitor 1/2 2πfC is negligible in comparison to Rin. But at a low frequency when 1/2πfC = Rin, ein falls to e0/√2 (the 3-dB point). If Rin is high (say 100K) then the 3-dB point is a low frequency (say 30 Hz). But if Rin is low (say 1K) then the 3-dB point will be given by 1/2 2πfC = 1K at f = 3 kHz, and so the low-frequency response is badly affected. Thus to get a good low-frequency response Rin should be as high as possible.

 

Another Black Box Solution - RF CafeAnother Black Box Solution

The value of R equals the value of LDR when the latter is illuminated by the lamp. With the switch open, LDR has a high resistance which prevents the lamp from lighting. When the switch is closed, the lamp lights through R. The illumination decreases the resistance of LDR, locking it "on."

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About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2025
Webmaster: Kirt Blattenberger,
   BSEE - KB3UON - AMA92498

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a lady's voice announced "You've Got Mail" when a new message arrived...

Copyright  1996 - 2026

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