The compression and expansion are two important techniques used by audio engineers during recording and reproduction of program material.
The compression of dynamic range of program material (records, speech or musical broadcasting) permits maintaining constantly high modulating level while the expansion, when used with the reproduction of compressed material, restores the dynamic range and creates a ‘live’ music.
Creating these effects is costly and complex, and beyond the scope of amateur audiophiles. But the simple circuit expander compressor shown here provides a low-cost solution. Constructed with a few passive components, it can give surprisingly good results. The circuit expander compressor furnishes both the functions-compression and expansion (often expressed by the term compander)-easily.
The expander compressor uses an opto-isolator. The LED from the opto-isolator is connected to the speaker terminals via a current limiting resistor in conjunction with potentiometer VR1 to sample the program material from the output of power amplifier.
Diode D1 and resistor R1 provide protection to the LED from excessive current, while potentiometer VR1 is used to very sensitivity of the circuit expander compressor. The value of R1 needs adjustment; with high power audio amplifier its value should be increased. This should be decide experimentally.
The audio modulated light falls on the LDR which is light sensitive. So LDR’s resistance varies with the modulated light. The LDR should have a ‘dark resistance’ of about 5 MΩ, and when brightly illuminated it should offer a resistance of about 6 to 20 KΩ. It is therefore possible to very its resistance by feeding the current to LED from the audio output terminal of an amplifier. The LED and LDR should be enclosed in the light-proof box or a small plastic tubing.”
When SW1 is switched to ‘EXP’, the LDR gets connected across the high end of R2 and R3. When audio-modulated light from the LED strikes the LDR, which is now connected in parallel with R2, the combined resistance becomes lower and thus increases the output level.