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Class A-Designed Symmetrical Audio Preamplifier

2015-01-03 15:26  
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Figure 1 

The circuit was designed to produce a preamplifier with symmetrical audio input while the output operates in Class A type.

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 processingSSM2210 – a dualNPNmatched transistor pair specifically designed to meet the requirements of ultra low noise audio systems and is also an ideal choice for accurate and reliable current biasing and mirroring circuits due to its features of high gain-bandwidth product, low cost, outstanding offset voltage drift, excellent current gain match, and very low voltage noiseBC560 – aPNPepitaxial planar Silicon enclosed in a 24-pin dual in-line package or 3-pin TO-92 package typically used for switching and AF amplification due to its low noise and high voltage featuresBC337 – a small signalNPNSilicon AF medium power transistor used for general purpose switching and amplifying applications with features such as TO-18 manufactured package, suited for AF driver stages and low power output stages, and divided into three group types

The circuit is not primarily used for innovation but as part of a solution to provide to obtain a good result. The use of transistors Q1 & Q2 with matched thermic characteristics produces a high-quality outcome. If not available, these transistors can also be substituted by the simple transistors BC550C or BC560C. The only thing to consider is matching the transistors well so that they will overcome their weakness and be able to create similar characteristics.

The offset voltage that comes from the output of the circuit is regulated with the use of the trimmer TR1. The trimmer can also be replaced however, by two resistors with a combined 82 Ohms value which can be placed in the position of R6 & R7. On the other hand, the diodes D1 & D2 should not be placed far from the transistors Q3 & Q4 respectively. This is necessary for them to have a common thermic behavior so that the heat will be concentrated in one area in which a heatsink can handle. This is also applicable to transistors Q8 &Q10; and Q9 & Q11.

The current output of the circuit is being regulated by the trimmer TR2 which will provide a 20 mA current for every 200 mV voltage that passes across R25. For a result of flat low frequency, the capacitor C12 should be applied with a jumper or be removed from the circuit to achieve this response. The various voltages that pass through the main components aid in the regulation of the current in the circuit.

One of the best uses of this symmetrical Class A preamplifier circuit is in microphones wherein some designs are having features like selectable microphone source loading for omptimized cable/mic matching, variable-passive high pass filter, and dual-plated circuit boards for superior low level signal transfer. Since the circuit is used mostly in high-level amplifiers, some electronic audio devices can be connected such as CD players with integral DA converter, tape recorders, tuners, hi-fi video recorders, and cassette decks. Some effect devices can also be attached like tone controllers, surround decoders, and equalizers.


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