Audio Oscillators

Posted on Feb 6, 2014

It is a resistance tuned oscillator covering from 35 cps (frequency unit was the cycle at this time) to 35, 000 cps in 3 ranges. The major problem with such a resistor / capacitor tuned Bridge is to obtain a constant level output over a full decade frequency range. Bill Hewlett solved the problem by inserting a non-linear element in the feedback loop of the oscillator. This element controls the amount of feedback

Audio Oscillators
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in accordance with the amplitude of oscillation and consequently maintains the proper operating point in the system. And what makes the idea a genius one is that the non-linear element is nothing else than a simple 3 Watts light bulb. As shown in the wiring diagram below, two options with different frequency ranges were available: the 200A and 200B. A small modification of 6 resistors values resulted in different frequency ranges 35 to 35, 000 cps for the 200A and 20 to 20, 000 cps for the 200B. All other specifications are common for the two units. A two-stage power amplifier with transformer-coupled output delivers 1 Watt into a 500 ohm resistance load. The output voltage is controlled ahead of the output amplifier from a front panel level control. The total harmonic distortion is lower than 0. 5 %. The HP 200B with rounded corners shown in this photo was hand built, probably in the second garage ( 481 Page Mill Road ) in 1940, and may have been handled by Bill and Dave themselves. It is so old, they do not even have a HP company logo yet. Units with this particular case style are very rare today. Click on picture or link below for a PDF (2. 5 Mb) of the original instruction manual published in 1943. The manual include schematics, parts list, and chassis layout of the four different models produced by HP in 1943. The HP 200B with rounded corners shown in this photo comes from the Kenneth Kuhn collection. This unit was...

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