electronic filter

The preamplifiers and radio heads of some higher-end amplifiers have very high power filtering requirements, and no hum can be heard from the speakers. Here we introduce an electronic filter with a simple circuit and excellent effect. It can almost completely eliminate hum sound by using a smaller capacitor. See the picture for the principle. BG1 and BG2 form a composite tube. The main difference between this circuit and other circuits is that the filter element is added to the base of the composite tube. Since the amplification coefficient of the composite tube β = β1·β2, the filtering effect obtained from the BG2 emitter is β1 times that of L1 and C6, which is equivalent to adding a large capacitor and a large inductor to the circuit, and most of the hum in the circuit is Suppression; due to the addition of L2 and C7, their filtering effect is expanded by β2 times by BG2, further filtering out residual hum. Another advantage of adding an inductor to the base of the composite tube is that the base current is small and can be wound with very thin copper wire. L1 and L2 of this circuit are made of about 0.15mm copper wire wound around a 1/4W100K resistor for 200 turns. The composite tube has a small range of voltage stabilizing effect in this circuit. During actual production, you can choose transistors and other components according to your own requirements.