Power amplifier electronic circuits-tube audio rf amplifiers

About The Circuit

Given below is an IR audio link circuit that has been designed to transmit audio signals as far as 4 meters. The signal is passed on to the base of transistor Q1 through the resistor R4. This signal, when passing the transistor Q1 makes the transmitting diodes D1 and D2 ON.
The circuit has to be neatly designed on a PCB with a 9V PP3 battery for powering the transmitter and reciever. The IR signals that are transmitted are modullated along with the audio input.

The IR signals that are transmitted will be recieved by the photo transistor Q2. On recieving it the emitter voltage of the photo transistor changes according to the strength of the IR signal, which again depends on the sound modullated. Apart from Q2, there are also ordinary transistors Q3 and Q4 which are used to amplify the signal to drive the speaker. Capacitor C1 and Resistor R3 are connected as a filter circuit so as to avoid the interference from all the stray IR signals.

A simple 8 Ohm speaker is enough for the output. The photo transistor Q2 should be an NPN type. They can be anyone like PNZ154, PNA1605F, BPW77NA or BPW85.

Circuit Schematic

About The Circuit
This is the simplest way of converting a loud speaker to a michrophone. Though the output voice is not so conventional, an average quality output will be obtained for sure. The circuit has to be assembled neatly in a PCB.
The basic working is that when the sound waves hit the diaphragm of the speaker, the coils in the diaphragm will fluctuate, causing a small considerable amount of induced voltage. Voltage gain is obtained by connecting the transistor Q1 in the common base mode. Current gain is obtained by connecting the transistor Q2 as an emitter follower. The circuit requires an input voltage from a 9 V PP3 battery. The capacitors must have a minimum rating of 15 Volts. The speaker K1 should have 3 inch thickness. Inorder to collect the produced sound in the output an 8 Ohm speaker or headphone is more than necessary.
It must be noted that while setting the circuit, keep the preset R2 at around 10 Ohms and connect the battery. Later adjust R2 until you get good sound quality.

Circuit Schematic

About The Circuit
Given below is a 100 Watts sub-woofer amplifier that has been designed only with the use of transistors. There are mainly seven transistors, in which two of them are used in the pre-amplifier stage and four of them in the output stage.
The greatest advantage of this type of a circuit is that there are no IC’s used and so is easy to assemble on a PCB. This circuit is designed to produce 100 Watts output and an input power of +35V/-35V, 5A dual supply is to be used. The size of the sub-woofer depends on the outpu and a normal specification of 12 inch is required. It must be noted that the transistors Q4, Q5, Q6 and Q7 are fitted along with heat sinks. The electrolytic capacitors used here should have a rating of 100 Volts.

Circuit Schematic