AGC System For Audio Signals

This circuit is an AGC system for audio-frequency signals. AGC systems usually consist of three parts: an amplifier, rectifier, and controlled impedance. In this circuit the functions of an amplifier and a rectifier are performed by a single op amp. This makes the system simple and cheap. The rectifier is made with the output push-pull cascade of the op amp and Rs, RL, and CB. The transistor Q1 and D1 are used as a voltage-controlled resistance (Z). The input signal is (Z+R1)/Z times, diminished by the voltage divider and 1+R2/R1 times, amplified by the op amp. C2 eliminates influence of dc bias voltage. R3 protects Q1 and D1 from excessive current.

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Two Wire To Four Wire Audio Converter

This converter circuit maintains 40 dB of isolation between the input and output halves of a four-wire line while permitting a two-wire line to be connected. A balancing potentiometer, Rg, adjusts the gain of IC2 to null the feed-through from the input to the output. The adjustments is done on the workbench just after assembly by inserting a 1 kHz tone into the four-wire input and setting Rg for minimum output signal. An 82 ohm dummy-load resistor is placed across the two wire terminals.

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Stereo Amplifier With Gain Control

Excellent tracking of typical 0.3 dB is easy to achieve. With the potentiometer, Rp, the offset can be adjusted. For ac-coupled amplifiers, the potentiometer may be replaced with two 5.1 k ohm resistors.

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Speaker Amplifier For Hand Held Transceivers

The LM383 is an audio-power amplifier that is capable of producing up to 8 W of audio output. R1 is essentially a load resistor for the hand-held transceiver's audio output. R2 can be composed of two fixed resistors in a 10:1 divider arrangement, but using a potentiometer makes it easy to set the amplifier's maximum gain. When powered from a vehicle's electrical system, the amplifier's +12V power source requires filter L1 to eliminate alternator whine. The LM383 can be mounted directly on the heatsink because the mounting tab is at ground potential.

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Mini Stereo

This circuit is built around two chips: the MC1458 dual op amp, configured as a preamplifier, and the LM378 dual 4-watt amplifier. The gain of the preamp is given by R3/R1 for one side and R4/R2 for the other side, which is about 100. That gain can be varied by increasing the ratios. The left and right channel inputs are applied to pin 2 and 6. The left and right outputs of U1 at pins 7 and 2 are coupled through C5/R10 and C3/R6, respectively, to U2 to drive the two 8-W loudspeakers.

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Micro Sized Amplifier

Sound detected by electret microphone MIC1 is fed to IC1's input through resistor R2, and capacitors C1 and C2. Resistors R2 and R5 determine the overall stage gain, while C2 partially determines the amplifier's frequency response. To ensure proper operation, use a single-ended power supply. R3 and R4 simulate a null condition equal to half the power supply's voltage at IC1's noninverting input. The output of IC1 is transferred to emitter-follower amplifier Q1 via volume control R6. The high-Z-in/low-Z-out characteristic of the emitter-follower matches the moderately high-impedance output of IC1 to a low-impedance headphone load.

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Low Distortion Audio Limiter

The level at which the audio limiter comes into action can be set with the LIMIT LEVEL trimmer potentiometer. When that level is exceeded, the output from the LIMITER-DETECTOR half of the op-amp (used as a comparator) turns the LED which causes the resistance of the photoresistor to decrease rapidly.
That is turn causes the gain of the LIMITER half of the op-amp to decrease. When the signal drops below the desired limiting level, the LED turns off, the resistance of the photoresistor increases, and the gain of the LIMITER op-amp returns to its normal level-that set by the combination of resistors R1 and R2.
A dual-polarity power supply (+/- 12 volts is desirable) is needed for the op-amp.

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General Purpose Preamplifier

Suitable for general audio use, the preamp circuit uses a feedback pair. Current gain is set by the ratio of (R4+R6)/R4.

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Audio Distribution Amplifier 2

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20dB Audio Booster

The amplifier's gain is nominally 20 dB. Its frequency response is determined primarily by the value of just a few components-primarily C1 and R1. The values in the schematic diagram provide a response of 3.0 dB from about 120 to over 20,000 Hz. Actually, the frequency response is flat from about 170 to well over 20,000 Hz; it's the low end that deviates from a flat frequency response. The low end's rolloff is primarily a function of capacitor C1, since R1's resistive value is fixed. If C1's value is changed to 0.1 mF, the low end's corner frequency-the frequency at which the low end rolloff starts--is reduced to about 70 Hz.
If you need an even deeper low end rolloff, change C1 to a 1.0-mF capacitor. If it's an electrolytic type, make certain that it's installed into the circuit with the correct polarity--with the positive terminal connected to Q1's base terminal.

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Simple Electronic Metronome

Two complementary transistors form a simple oscillator whose frequency range is from about 0.5 to several Hz. This circuit is useful as a metronome, timer, or pacer for exercise equipment.

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Metronome 2

This simple circuit uses a multivibrator to generate the beats and a subsequent audio amplifier stage to increase the output level. Range of adjustment is approximately from 40 to 200 beats per minute set by the ganged potentiometer.

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Electronic Metronome

Ra sets the rate while Rb sets the volume of clocks in the speaker. The 555 is configured as a low frequency oscillator. The circuit is powered by a 6 V battery.

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Downbeat Emphasized Electronic Metronome

IC1a and IC1b form and astable multivibrator. The astable's signal is fed to IC1c, also to the clock input of IC2, a 4017B decade counter. That IC's Q0 through Q9 Outputs become high one at a time for each successive clock pulse received at pin 14. Switch S1 feeds one of those outputs to the 4017b's reset input.
Whenever the selected output becomes high, the 4017B restarts its counting cycle; that determines the number of beats per measure. The network composed of C2 and R6 sharpens the downbeat pulse, and the network composed of C3 and R7 sharpens the free-running pulses. By making C2 large than C3, the downbeat, receives greater emphasis.

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Variable Frequency Code Practice Oscillator

The variable frequency audio oscillator can be used as a low-level alarm sounder or a code-practice oscillator.

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QRP Sidetone Generator/Code Practice Oscillator

For use with low-power transmitters with a positive keying voltage. Q1/Q2/Q3 form a switching amplifier. When the key is pressed, the collector of Q3 goes to ground, turning on Q5 and activating IC1, an audio oscillator. Q4 drives the speaker. For use as a code practice oscillator, insert P1 and J1 and a key in J2.

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Electronic CW "Bug" Keyer

This keyer uses skin conductivity to simulate the old-fashioned mechanical CW bug keyer. When the "dit" paddle is touched the bias on the inverter, IC1-a is shunted to ground, and it produces a logic high, causing oscillator sections C&D to generate a low-frequency square wave keying Q1 for a series of "dits." When the "dah" paddle is touched, section b produces a logic high, driving keyer Q1 on.

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3 Channel Color Organ

The ac line power is brought back into the circuit through F1, a protective 5-A fuse. One side of the ac line is connected to one side of each ac outlet. The other side of the ac line is connected to each SCR or silicon-controlled rectifier. Each SCR is, in turn, connected to the other side of each ac outlet. An audio signal is brought into the circuit from a stereo speaker by transformer T1. This transformer has 500-ohm impedance on the primary and 8-ohm impedance on its secondary. Connect T1 so that the 8-ohm side is connected to the speaker and the 500-ohm side is connected to potentiometer P1. Potentiometer P1 is used as a level or sensitivity control. The signal from its wiper lead is applied to each RC filter stage. Because each SCR has a different RC (resistor/capacitor) filter on its gate lead, each will respond to different frequencies. The greater the capacitance in the filter, the lower the frequency that the SCR will respond
to.

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Two Door Annunciator

When the pushbuttons at either door are depressed, this circuit generates a different tone for each door. Tones are generated by phase-shift oscillator Q1/Q2. Q3 provides tone frequency change by changing the phase-shift network. U2 and U3 are timers for the tones and Q4/Q5 interface the timers with the pushbuttons.

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Sliding Tone Doorbell

When the doorbell is pushed, you'll hear a low tone that will "slide up" to a higher frequency. The frequency of the AF oscillator is determined by coupling capacitor, C1 and the value of the resistance connected between the base of Q1 and ground. That resistance, RBG is equal to (R1 + R2) R3. First, assume that S1 is closed and R2 has been adjusted to produce a pleasant, low-frequency tone. Capacitor C3 will charge through R6 until it reaches such a voltage that it will cause diode D1 to conduct. When that happens, the value of RBG is paralleled by R4. Thus, because the total resistance RBG decrease, the output tone slides up in frequency. Capacitor C3 will continue to charge until the voltage across D2 and D3 causes those diodes to conduct. Then RBG is paralleled also by R5, the total resistance again decreases, and the oscillator's frequency again increases.

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