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.
When the SW1 is switched to ‘COMP’ the LDR and resistor R4 come in parallel with R3. And when the LDR id illuminated by the LED, the composite resistance lowers and it compresses the signal. The amount of compression depends upon the values of R2 and R3, A high value of R2 means a greater expansion range is possible.
Compression depends upon the value of R4. As this value is decreased, the compression effect is increased. Thus it is possible to obtain practically the desired range of expander and compression. The LDR’s dark resistance value is also a participant and affects the performance of the unit. Its dark resistance should be between 5 and 10 MΩ.
The expander compressor can be used efficiently only if the pre-amplifier and power amplifier units are separate.
The program source should be connected to the pre-amplifier and out from the pre-amplifier should act as input to the compander. Expanded or compressed signal from the compander is fed to power amplifier. The output from the speaker terminals is connected to the input of opto-isolator (LED/LDR)
Audio compressor is much used in disk or tape recording, and can be interposed between the output of a mixer and the input of a tape recorder in order to ensure that the maximum recording level is not greatly exceeded.
The unit can be used as the volume control between the pre-amplifier and the power amplifier in an audio system. It can also be used in a musical instrument’s amplifier, to extend the signal-to-noise ratio on expansion, or to prevent speaker blowout on compression.
PARTS LIST OF EXPANDER COMPRESSOR
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 33Ω to 88Ω
R2, R3 = 100 KΩ
R4 = 10 KΩ
VR1 = 250Ω, 3W wire wound Pot, Panel Mounted
D1 = 1N4148
LED1 = Pilot Led
SW1 = SPDT switch