Modular Buffer Preamplifier in Class A Operation

Modular Buffer Preamplifier in Class A Operation


The circuit was designed to create a modular Class A buffer preamplifier in order to isolate stages in an audio circuit.

Terminology BF245 – a general purpose N-Channel Junction Field Effect Transistor (JFET) in a plastic package similar to TO-92 particularly suitable for use in DC, AF, LF & RF amplifiers, and VHF & UHF amplifiers due to its features such as frequencies up to 700 MHz and interchangeability of drain and source connectionsBC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipmentBC560 – a PNP epitaxial 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 features Circuit Explanation

A buffer may be considered as an active circuit using either tubes or transistors but without a voltage gain. Its main function is to transform a low impedance into a high impedance or vice versa. In this circuit, using Class A signifies that the transistors will conduct the same amount of current all the time, whether on standby or during operation. It will usually give very low distortion although Class A is very inefficient with electricity. The decrease of distortion of an amplifier is the main reason why in some cases, Class A is preferred.

The circuit is producing a gain of 1 while providing signal amplification. The total negative feedback is not being utilized in order to maintain the distortion at a very low level. Proper matching should be done on the transistors while observing absolute care on the types used as well as the resistors connected between the transistors. The transistors should also be observed for their thermal characteristics to avoid overheating. This thermal issue also applies to the diodes LED D1 & D2 in accordance to the corresponding transistors Q5 & Q2.


The concept of building a buffer preamplifier may be applied in the output of CD players to obtain high performance, in headphone amplifiers, in the tape loop of an integrated amplifier, for upgrading hi-fi and A/V audio equipment, and for large bandwidth amplifier. Some design of buffer preamplifier comes with equalizer that can be included in concert string systems such as the violin, cello, and bass. One main application of buffer preamplifier is with piezo equipped guitars to produce much better sound when the guitar is used with a regular amplifier.

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