The Really cheap audio compressor circuit diagram from www.freestompboxes.org is a simple, yet effective compressor circuit that can be built with a handful of cheap components. The circuit consists of two op-amps, two LEDs, a LDR, a few resistors, and a capacitor.
The first op-amp (U1) is configured as a unity gain amplifier. The input signal is applied to the non-inverting input of U1, and the output of U1 is fed back to the inverting input through a 15nF capacitor. This capacitor provides some frequency shaping to the input signal.
The LDR is connected in series with the feedback path of U1. The LDR’s resistance decreases as it is illuminated, so when the input signal is loud, the LDR’s resistance decreases, which reduces the gain of U1. This results in the input signal being compressed.
The two LEDs are connected in parallel to the LDR. The LEDs are biased on by a 100k resistor, and they are illuminated when the input signal is loud. The LEDs provide a visual indication of when the compressor is active.
The output of U1 is buffered by the second op-amp (U2). The output of U2 is then passed through a 100k output potentiometer, which allows the user to adjust the overall gain of the compressor. The circuit can be powered by a 9V battery or a 9V power supply. The bypass switch allows the user to turn the compressor on and off.
Here is a more detailed explanation of the circuit:
- U1: The first op-amp (U1) is configured as a unity gain amplifier. This means that the output of U1 is the same as the input signal. The input signal is applied to the non-inverting input of U1, and the output of U1 is fed back to the inverting input through a 15nF capacitor. This capacitor provides some frequency shaping to the input signal.
- LDR: The LDR is a light-dependent resistor. The LDR’s resistance decreases as it is illuminated, so when the input signal is loud, the LDR’s resistance decreases, which reduces the gain of U1. This results in the input signal being compressed.
- LEDs: The two LEDs are connected in parallel to the LDR. The LEDs are biased on by a 100k resistor, and they are illuminated when the input signal is loud. The LEDs provide a visual indication of when the compressor is active.
- U2: The second op-amp (U2) buffers the output of U1. This means that the output of U2 is a clean copy of the output of U1. The output of U2 is then passed through a 100k output potentiometer, which allows the user to adjust the overall gain of the compressor.
- Bypass switch: The bypass switch allows the user to turn the compressor on and off. When the bypass switch is engaged, the output of the compressor is shorted to ground, which effectively turns the compressor off.
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