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 compressor 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.
The Problem with Weak Modulation
A strong carrier signal means nothing if the modulation is weak. Weak modulation makes messages hard to understand. Audio circuit power controls modulation strength. Voice levels change constantly. This makes it hard to keep speech at the right level. Maximum transmission performance becomes difficult to achieve.
The Solution: Microphone Compressors
Some circuits can solve this problem easily. These are microphone compressors. People also call them “gain mikes.” They work as preamplifiers with logarithmic gain curves.
How mike Compressors Work
These circuits boost weak signals with high gain. They reduce strong signals with low gain. This keeps the output level constant. You get steady modulation whether you speak softly or loudly.
The MC1458 Circuit Design
This project uses an MC1458 dual operational amplifier. Each amplifier works like a 741 op-amp. You can use two 741s instead of one MC1458. The circuit board can be modified for this change.
Component Functions
R4 and R5 control the gain. C3 and C4 set the frequency range. Want more bass? Increase C3 and C4 values.
P1 potentiometer controls input sensitivity. It works as a voltage divider. Different microphones need different sensitivity settings.
Automatic Level Control
The logarithmic gain comes from a special network. This network includes resistors R7, R8, R9, and R11. It also uses diodes D1 and D2. Capacitors C7 and C8 complete the network.
This network acts as an automatic attenuator. It cuts the strongest signals. It keeps output levels in the right range. The signal stays clean without distortion.
Parts Requirements
Use electrolytic capacitors rated for 12V or more. Other capacitors can be ceramic or polyester. Choose based on the value needed.
Diodes D1 and D2 must be germanium types. Potentiometer P1 can include an on/off switch. Resistors should be 1/8 or 1/4 watt. Tolerance can be 5% to 20%.
Wiring Tips
Use shielded cable for input and output connections. This prevents humming and noise. Match the input jack to your microphone plug. Use a common jack for output. Connect with two plugs to the transmitter. Or use shielded cable with the right plug for your transmitter.
For more info : Amateur Radio – Mike Preamplifier/Speech compressors/Audio Processors – Resources
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