John Hollis Flatline Compressor
Created: Nov 03, 2015
The design features the TL072CP, a low-noise JFET input operational amplifier from Texas Instruments. Basically the circuit is made of two parts, the volume detector and the volume booster. The volume detector is the part where the LED of our optocou
The circuit is an audio compressor and sustainer used for musical instruments such as electric guitars and basses. A compressor/sustainer is used to boost low signals and compress high signals to have a constant volume output. This allows a user to play on a constant volume whether he struck the strings hard or soft.
The design features the TL072CP, a low-noise JFET input operational amplifier from Texas Instruments. Basically the circuit is made of two parts, the volume detector and the volume booster. The volume detector is the part where the LED of our optocoupler is connected and where an inverting opamp is configured. The booster part is where the non-inverting opamp is configured with the LDR part of the optocoupler shunted together with the feedback resistor. When there is a low input signal to the circuit, the LED remains in “OFF", thus the LDR has its maximum resistance. With this, the feedback resistance in the first opamp would be almost equal to 220k ohms. Having this would make the opamps' gain much greater than one therefore boosting the signal. When a high signal is on the input of the signal, the LED goes “ON", making the LDR have its “on-resistance", which in this case would be 6 ohms. This makes the feedback resistance of the non-inverting opamp very small setting the gain close to one, thus closely maintains the output signal to almost the same as the input. A pot is also added in the end of the circuit to set the desired volume of the output.
The circuit is based on John Hollis' design. The circuit is very simple but effective on doing its function. The TL072CP is ideal in this audio application and portable devices because of its low total harmonic distortion, low noise, high slew rate and low power consumption.