Class A Modular Preamplifier with Single Integrated Circuit

Figure 1 

The circuit was based on a single operational amplifier integrated circuit that is made to produce a modular preamplifier which functions in the Class A operation

• Preamplifier (pre-amp) – a device normally used together with the sound equipment to be of assistance in improving the overall quality of sound by detecting and strengthening weak signals for subsequent and more powerful amplification stages and processing
• Class A – refers to an output stage where the bias current is greater than the maximum output current thus, all output transistors are always conducting current due to its linearity and low distortion
• TL071 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• NE5534 – a high performance low noise operational amplifier having a combined excellent AC and DC characteristics and features such as capability of external compensation, capability of offset nulling, wide range of voltage supply, low harmonic distortion, with output voltage swing at peak to peak, high slew rate, 100 dB common mode rejection ratio, high gain of DC Voltage, unity gain bandwidth at 10 MHz, and noise voltage of equivalent input
• BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment

Transistors that are designed to work in Class A specification in the input stage can be said to have a very good quality of audio, especially when used in certain applications. The audio quality will still depend on the good characteristics of IC1 to produce this desired output. There are several ICs available in the market that can be chosen and among these is the NE5534 or NE5532 for stereo mode, and the series of TL071 or TL072 to obtain higher input impedance.

NE5534 is compensated internally for a gain equal to or greater than three and with a maximum limit specified for the equivalent input noise voltage. The JFET input operational amplifiers in the TL07_ series are designed as low noise and low harmonic distortion ideally suited for high-fidelity and audio preamplifier applications since each amplifier features JFET inputs coupled with bipolar output stages integrated on a single monolithic chip for high input impedance.

The use of these op-amps in this particular application produces excellent and pleasing results. By changing the values of the feedback resistors R6 & R7, the amplification of the signals during the operation can be altered and modified. The circuit may be connected to other desired circuits possible using the 10 pin connector J1. Since it operates in Class A, it has the lowest distortion compared to other classes.

Figure 2 

The use of this Class A modular preamplifier is typically seen as a driver for headphones that have impedance 47R and higher. This is also a requirement for high quality amplifiers for a good quality of audio. Some designs would come with input and tone control modules while others have discrete components, hand crafted, upgradeable, robust in design, and interchangeable.

Source:users.otenet.gr/~athsam/class_a_amp_ic.htm