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Here is a two-transistor Wien bridge oscillator using an ordinary night-light bulb for stabilization. The output is about 6 volts p-p and can drive fixed loads as low as 2 or 3 thousand ohms without additional buffering. A 10 k amplitude potentiometer with the wiper going to a high input impedance output amplifier would make an excellent load.

Excellent distortion is achieved by adjusting the 1 k feedback potentiometer until the output amplitude is about a volt less than the maximum level (with the pot set to the highest resistance). Wait a few seconds between adjustments to give the bulb time to stabilize; the audio signal actually heats the bulb’s filament causing the resistance to go up which controls the loop gain. You will see the signal bounce a little as the bulb gains control. This simple version of the popular Wien bridge oscillator uses feedback to hold the junction of the two RC networks (base of first transistor) near zero volts (100 mV p-p) and the ends of the RC networks move in opposite directions like a see-saw.

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Here is a phase-shift audio oscillator with excellent distortion characteristics thanks to “softened” diode limiting provided by the 1N914 and resistor divider and degenerated gain provided by the 68 ohm emitter resistor. For minimum distortion, increase the 68 ohm resistor to a point just below where oscillation stops. A simple buffer may be added for driving lower impedance loads. The output amplitude will be about 5 volts p-p but one of the 1N914’s 10k divider resistors may be changed for a different output amplitude.

The circuit will work well with a power supply voltage other than 9 volts but the 68 ohm resistor may need adjustment

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior (with the exception of the amazing one-transistor reflex ). The TL431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor and the amplifier may be used for other projects, too. Higher impedance headphones and speakers may also be used.

An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms when using high impedance earphones to save battery power.

In the below circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present. If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply.

For example, an NPN common-emitter preamplifier may be added for very high gain and by connecting the differential inputs across the collector resistor instead of from collector to ground, destabilizing feedback via the power supply is greatly reduced.