Automatic Gain Control PreAmplifier Circuit Diagram

The preamp circuit uses an easily obtained 741 op amp set for an internal gain about 200.

A portion of the op amp’s output signal is rectified by the 1N4148 diodes, then filtered and fed to the gate of the FET input shunting circuit. As the output rises, more and more input shunting takes place. That is, more of the input signal is bypassed, effectively keeping the output level constant.

BF245

This preamp circuit offers a 100:1 limiting action. The input level can change over a 100:1 ratio with little or no effect on the output level.

741 PIN CONFIGURATION

The output output level itself can be set from less than unity all the way up to nearly the gain of the amplifier, making the circuit usable in other applications as well.

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FET Audio Mixing and Switching Circuit Diagram

Two or more signals can be switched and /or mixed without annoying clicks by using this fet audio mixer and switch circuit.

Junction-FETs such as the BF245 or 2N3819 already popular in HF circuits but it can also applied to LF circuits.

BF245

You can mute these signals using S1 and S2. The circuit should mute sufficiently any audio from 20 Hz to 20 kHz.

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741 Stereo PreAmplifier Circuit Diagram

This preamp circuit provides better than 20dB gain in each channel.

A better op-amp type will give a better noise figure and bandpass.

In this circuit the roll-off is acute at 20000 Hertz.

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Carbon Microphone Replacement To Magnetic Mike Converter Circuit

A good  magnetic mic is more better then a carbon mic and easy to find nowadays.

Good pitching beats good hitting and a good magnetic mic beats a good carbon mic. This one transistor carbon mic converter takes a carbon mic input and converts it to the magnetic variety. Note that no ground connection is used, even if the circuit is built in a metal cabinet. MIC is replacement type magnetic element that is substituted for the original carbon element.

Using miniature components the entire converter amplifier can also be housed in the original microphone case. To avoid destruction of Q1, the unit must be connected properly the first time. The “+” lead, which goes to Q1′s collector, connects to the carbon mic input that supplies a positive voltage.

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LM1036 Stereo Tone ( Bass, Treble, Volume, Loudness, Balance ) Controller Circuit

The LM1036 is a DC controlled tone (bass/treble), volume and balance circuit for stereo applications in car radio, TV and audio systems.

This stereo tone controller circuit uses mono controller to controlling tone.

Features

• Wide supply voltage range, 9V to 16V
• Large volume control range, 75 dB typical
• Tone control, ±15 dB typical
• Channel separation, 75 dB typical
• Low distortion, 0.06% typical for an input level of 0.3Vrms
• High signal to noise, 80 dB typical for an input level of0.3 Vrms
• Few external components required

NOTE:

• VR1 = BASS CONTROLLER
• VR2 = VOLUME CONTROLLER
• VR3 = BALANCE CONTROLLER
• VR4 = TREBLE CONTROLLER
• S1 = LOUDNESS SWITCH

Four control inputs provide control of the bass, treble, balance and volume functions through application of DC voltages from a remote control system or, alternatively, from four potentiometers which may be biased from a zener regulated supply provided on the circuit.

Each tone response is defined by a single capacitor chosen to give the desired characteristic. By changing the values of capacitor connected to tone controlling unit, you can control bass and treble levels. pin 3 and pin 18 of IC are for treble and pin 6 and pin 15 for bass.

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Condenser Mike Pre-Amplifier Circuit

COND MIC,
need often arises for a sensitive sound pick-up device, whether it is to be used as a simple microphone or a more exotic device as a sound operated alarm, a bugging device or a sound operated flash (for stop action photography) the list is quite unending.

The circuit given employs a condenser microphone as the transducer. Since output of the condenser microphone  is quite low, it usually has an FET amplifier built into the case.

This amp is powered by the R1, R2 resistor network.

The output of condenser microphone is fed to a two stage amplifier. Transistor T1 (BC149C) utilising current series feedback forms the first stage. The second stage comprising transistor T2 (BC147B) is connected in the voltage shunt feedback configuration. These two stage provide sufficient gain to pick up even the slightest whisper.

The circuit requires a 4.2 volt supply.This may be obtained, as in the prototype, with a 1kΩ (R9) resistor as shown in the diagram. The value of this  resistor may be altered. to suit a supply voltage other than 6 volts.

Condenser Mike Pin Configuration

Output of the microphone amplifier can be made variable by connecting a 10kΩ potentiometer / Preset as shown.

Circuit’s gain can be increased by reducing the value of R6 to 47Ω or 22Ω, depending on the input sensitivity of the main amplifier system. Increase in gain was also observed by using 3V supply and eliminating R9 altogether. The microphone should be housed in a small round enclosure.

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Voltage Controlled Volume circuit

his volume control circuit offers an unusual approach to the well-known problem of distortion in active-device attenuators. The zero-output in this case is obtained by allowing equal signals of opposite phase to cancel each other.

The input transistor operates as a ‘concertina’ phase-splitter, producing equal-amplitude opposite-phase voltages at its collector and emitter. The two signals can be brought into complete cancellation, at the summing point, by means of preset p1. The harmonic content of the two signals is very small, but not quite identical.

There will therefore actually be an even smaller distortion output at the nominally ‘zero’ point.

If something now happens to the amplitude ratio of the signals at the summing point, there will be output passed to the buffer-stage. The necessary unbalance is achieved by means of the JFET and capacitor C2. The gate bias on the FET is set by the DC control voltage applied to point A. With this voltage close to zero the FET will be cut off, so that the above mentioned cancellation takes place. As this voltage is increased there will come a point at which the channel starts to ‘bleed off’ AC collector current from the splitter; this will upset the  balance and so cause an output signal to appear. The more conductive the FET, The more output. Unfortunately, the more channel current there flows the lower will be negative gate bias and so the greater will be the distortion of the ‘regulated’ summing component.

The trick is now to employ only a moderate degree of unbalance – so that the FET operates at low distortion percentages. The process is helped also by the always present ‘clean’ summing component. The buffer stage provides gain, so that a sufficient output level is obtained. The Circuit’s frequency response extends from 50Hz to 35kHz (-3dB points). The input voltage should be limited to 100mV p-p: the output can be varied from ’0′ to 1V p-p (by using the appropriate range for the control voltage at A).

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Bass Treble Tone Control Circuit

Bass and treble circuits can be combined to form a two control tone adjust circuit, as shown here.

VR1 for Bass Control

VR2 for Treble Control

Bass and Treble controls of about ±10 dB boost or cut. It should be useful in a wide variety of situations.

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6 Band Graphic Equaliser Using 741 Op-Amp IC

With this circuit you can control and blend frequencies and tones as desired.

Essentially, the circuit consists of an IC 741 whose gain at various freguencies is determined by corresponding potentiometer setting.

The audiblefrequency spectrum is covered in six steps: 50Hz, 160Hz, 500Hz, 1.6kHz, 5kHz, 16kHz. All potentiometers are of 100kΩ linear type. The circuit provides adequate boost / cut for normal use.

power supply for the circuit can be derived from the amplifier / preamplifier itself. The wide rangeof supply voltage (6V-15V) makes the circuit very versatile. Power consumption is negligible.

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This microphone preamplifier incorporates automatic gain control, which keeps the output level fairly constant over a wide range of input levels. The circuit is especially suitable for driving the modulator of a radio transmitter and allows a high average modulation index to be achieved.

It may also be used in P.A. systems and intercoms to provide greater intelligibility and compensate for variations between speakers (the users of those devices).

The actual signal amplifier stage is Q2, which operates in common emitter mode, the output signal being taken from its collector. A portions of the output signal is fed through emitter follower Q3 to a peak rectifier comprising D1/D2 and C4. The voltage on C4 is used to control the base current of Q1, which forms part of  the input attenuator. At low signal levels the voltage on C4 is small and Q1 draws little current. As the input signal level increases the voltage on C4 rises and Q1 turns on mode, thus attenuating the input signal. The net result is that as the input signal increases it is subject to a greater and greater degree of attenuation and the output signal therefore remains fairly constant for a wide range of input levels. The circuit is suitable for signals with a peak input level up to 1 volt. The microphone may be  replaced by a small loud-speaker for intercom use.

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