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Microphone compressor using LM1458

Microphone compressor using LM1458

mic compressor using lm1458 Microphone compressor using LM1458

The LM1458 is a dual operational amplifier (op-amp) that finds applications in audio amplification and other circuits. Let’s explore how you can create a simple microphone preamplifier with compression using the LM1458.

To get greater transmitting performance from your ham radio transceiver, we need to keep the modulation level constant, regardless of your voice level. With the compressor described we have a variable gain for the microphone, taking the modulation close to 100% regardless of the intensity of the input signal. It is not enough to have good power at the output of a transmitter to have good transmission performance.

LM1458 Pin diagram Microphone compressor using LM1458

There is no point in its carrier arriving at full intensity at the distant station if its modulation is “weak” to the point of making it difficult to understand its message. As modulation depends on the power of the audio circuits. This varies with the level of  voice, it is not very easy to keep speech constant so that we always have maximum performance in a transmission. There are, however, circuits that can do this with some ease. These are microphone compressors or also called “gain mikes” which consist of preamplifiers with a logarithmic gain curve. These circuits have a very high amplification gain when the input signal level is low, and a small gain when the signal level is high. This way, if the signal source is a microphone, we will have a signal level at the output for constant modulation even when we speak loudly or softly.

The basis of the project is a double integrated operational amplifier of the MC1458 type. Each of the amplifiers in this integrated has the same characteristics as the 741.In this way, nothing prevents the board layout from being modified in the absence of the original integrated to use two 741s instead of the MC1458.In each of the amplifiers, the feedback network determines the gain and the operating frequency range. In this way, R4 and R5 determine the gain, and C3 and C4 determine the passing frequency range. If the reader wants a more bass sound for his circuit, simply increase the values ​​of C3 and C4.

The input is made through a potentiometer that operates as a voltage divider controlling the sensitivity of the circuit, depending on the type of microphone used.The logarithmic gain is given by the network formed by the resistors R7, R8, R9 and R11, the diodes D1 and D2 and the capacitors C7 and C8.This network operates as an automatic attenuator that cuts the most intense signals, always maintaining its output level in the desired range for the next amplifier without, however, distorting it.

Electrolytic capacitors must have a working voltage of at least 12 V. Other capacitors can be either ceramic or polyester, depending on the value. Diodes D1 and D2 must be made of germanium, and potentiometer P1 can incorporate the switch that turns the device on and off. The resistors are 1/8 or ¼ W with tolerances of 5 to 20% and for signal input and output we must use shielded cable to avoid humming. The input jack must match the available microphone plug. For the output, we can use a common jack with a cable with two connection plugs to the transmitter, or simply a shielded cable with a plug according to the transmitter’s microphone input.

For more info : Amateur Radio – Mike Preamplifier/Speech compressors/Audio Processors – Resources

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