Audio Engineering

Operational Amplifiers (OP Amp)

The Music Telegraph | Text 2019/01/31 [10:21]
Audio Engineering

Operational Amplifiers (OP Amp)

The Music Telegraph| 입력 : 2019/01/31 [10:21]

Operational Amplifiers (OP Amp)

 

 

Operational amplifiers (OP Amp) are essential devices for circuits in almost all audio systems. The Unity Gain (which means that the input signal and the output signal are equal in level, that is, there is no level change in the audio processing process) is made by the voltage control mechanism of the OP amplifier.

 

▲ OP Amp

 

 


The OP Amp has two input terminals and one output terminal, amplifying the difference between the two input terminal voltages. The input stage is called the differential amplifier because it amplifies the difference in voltage between the two input terminals. This OP Amp can be used to construct circuits that can be used to compute rules (arithmetic calculation), so it is called an operational amplifier in the sense of an operator.

 

 

▲ © The Music Telegraph

 



In the OP amplifier, Unity Gain is achieved by the Voltage Gain stage (see picture above) with the input voltage adjusted to 0 dB by the differential amplifier, and the ratio of input voltage to output voltage becomes 1 to 1. OP Amps that induce Unity Gain are typically Summing Amp and Distribution Amp. In a Summing amplifier, when a signal comes in from multiple input stages, it is integrated into one signal and output to a single output end, usually used in the master volume fader circuit of the console. On the contrary, in the Distribution amplifier, a signal enters one input and the signal is distributed out to multiple output ends. It can be seen mainly in the headphone line input circuit, and evenly distributes a constant level of output to each connected headphone regardless of the number of headphones connected.

 

 

▲ Summing Amp

 

▲ Distribution Amp



▲ Headphone Amp (Distribution Amp)

 

 


Another use of OP amplifiers is also found in Direct Box (DI Box; D.I. = Direct Injection). DI boxes allow electronic instruments with Hi-Z (high impedance),  such as guitar or bass guitar, to be connected directly to the Lo-Z (low impedance) device (e.g. the microphone input of the console). For example, no matter what the value a musical instrument connecting to a direct box input has a high impedance, the output is kept constant at 200 Ohm, allowing it to connect to a device supporting low impedance.

 

▲ A circuit of DI Box



▲ DI Box (manufactured by Avalon Design)



 

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