Compressor or limiter: The compressor is based on two series connected attenuator networks, whose attenuation is obtained by means of light dependent resistors (LDRs) that are illuminated by light emitting diodes (LEDs).
The input signal is applied to the noninverting input of opamp IC1a via R1. In junction with D3, D4, and ICib. ICia Provides full-wave rectification of the signal. The resulting direct voltage is used to charge C5 via D5. The diode allows fast charging of the capacitor, which can discharge only via R7.
Compression proper is provided by ICIc• Depending on the setting of P1 and P2, the output voltage of 1C1c drops when the potential across C5 reaches a certain value. This causes T2 and. via ICId, T1 to conduct, whereupon the LEDs light and the input signal is attenuated.
The attack time of the circuit is determined by the speed of the LDRs and the setting of P2.
The amplification of ICic and the setting of P1 determine the point at which voltage -limiting commences. The Output voltage is held constant when the input signal is above a certain level until the current through the LEDs reaches a maximum (about 40 mA).
The circuit as shown acts as a limiter;-• if the +ve input of IC la is connected to the output of the circuit, a standard compressor is obtained.
The circuit is able to process signals between about 10 mV r.m.s. and 2 V r.m.s. This range can be extended by adding one or several attenuator sections or by increasing the value of R1 and R2.
The LDR/LED combinations must be housed in a light-tight enclosure. The current draws by the circuit are determined largely by the LEDs and are 50 mA maximum.