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Two Transistor Wien Bridge Oscillator

admin — Sun, 27 Jan 2008 05:15:13 +0000

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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Audio Phase Shift Oscillators

admin — Fri, 25 Jan 2008 04:54:33 +0000

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

Crystal Radio (and other purpose) Audio Amplifier

admin — Wed, 23 Jan 2008 17:53:36 +0000

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.

Op-Amp Audio Amplifier

admin — Mon, 21 Jan 2008 17:33:10 +0000

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.

4-Transistor Amplifier for Small Speaker Applications

admin — Sun, 20 Jan 2008 17:21:00 +0000

The circuit below shows a 4-transistor utility amplifier suitable for a variety of projects including receivers, intercoms, microphones, telephone pick-up coils, and general audio monitoring. The amplifier has a power isolation circuit and bandwidth limiting to reduce oscillations and “motorboating”. The values are not particularly critical and modest deviations from the indicated values will not significantly degrade the performance.

Three cell battery packs giving about 4.5 volts are recommended for most transformerless audio amplifiers driving small 8 ohm speakers. The battery life will be considerably longer than a 9 volt rectangular battery and the cell resistance will remain lower over the life of the battery resulting in less distortion and stability problems.

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Computer Audio Booster

admin — Fri, 18 Jan 2008 16:54:52 +0000

Here is a simple amplifier for boosting the audio level from low-power sound cards or other audio sources driving small speakers like toys or small transistor radios. The circuit will deliver about 2 watts as shown. The parts are not critical and substitutions will usually work. The two 2.2 ohm resistors may be replaced with one 3.9 ohm resistor in either emitter.

Simple LM386 Audio Amplifier

admin — Wed, 16 Jan 2008 16:24:12 +0000

This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum.