single pot Wien bridge audio oscillator
Hewlett-Packard started in the late 1930s with a Wien bridge audio oscillator product, the 200a.
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| the original HP product |
This was based on a clever design by Hewlett that used a light bulb's changing resistance to stabilize the gain of the oscillator. But the frequency was varied with a 4 gang variable capacitor that must have been fairly expensive even in 1939.
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| Wow, just like an SX-88 ! |
Later versions of the Wien bridge audio oscillator use a dual pot to vary the frequency. The dual pot is more expensive and harder to find than a single pot. Also with a dual pot, another dual pot would be used to provide fine tuning.
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| schematic of Wien bridge audio oscillator |
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| The dual pot Wien bridge audio oscillator, fine tune dual pot on right (actually there are two Wien oscillators, the one on the right has a fixed frequency) |
With a single pot Wien bridge audio oscillator either another single pot could be used for fine tuning or the expensive route: a 10 turn unit could be used to take the place of both the main and fine dual pots.
I had found a paper by Roelf Bakker PA0RDT of mini-whip-antenna fame: A PITCH SHIFTER FOR SELECTIVE LEVEL METERS: https://www.qsl.net/ko6bb/files/pitchshifter.pdf
What caught my eye was the Wien bridge audio oscillator with the frequency control by a single pot (the 2nd pot is to set amplitude).
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| the Wien bridge audio oscillator schematic from the above paper |
It looked interesting so I thought I'd give it a try, made a slight change with 0.047 uF caps in place of the 1000 pF ones in the original for lower audio frequency output.
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| a screen cap of the Wien single pot audio oscillator from the B&K 2190d scope |
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| the single pot Wien audio oscillator on the old breadboard |
It works pretty well, distortion goes up with frequency and amplitude, if the gain is turned down the distortion decreases, too. The LM324 could probably be replaced by another quad op-amp for better performance. I didn't do a distortion measurement but the wave looks pretty clean (symmetrical).
I also noticed this thread https://www.diystompboxes.com/smfforum/index.php?topic=83930.40 from a "fellow traveler" musician site which has several versions of the Wien audio bridge oscillator with the frequency controlled by a single pot. It's not clear exactly where the design originated but whoever the originator is, the design is a clever modification of a classic circuit.
This is definitely worth looking into further, wonder if I can use that extra op-amp section to key it somehow ? :)
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm