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 the circuit diagram of a simple three band graphic equalizer circuit. An IC called LF 351 IC is used here which is designed to operate as a wide bandwidth single operational amplifier. The IC also has a very high input impedancewhich helps the circuit to become more compatible to almost all the audio signal sources.

An input voltage between (6-30) Volts can be applicable to this circuit. It is to be noted that as the inpt voltage increases the gain will also increase. If an input voltage of 12 Volts is given for the circuit the electrolytic capacitors used in the circuit should be more than 12 Volts. The op-amp used in the circuit is wired as an inverting amplifier. The input signal is first passed on to the inverting input through the filter network. This filter network can produce a +20 to -20 dB enhancement or a cut on the frequency bands, 50Hz,1KHz and 10KHz.The gain of the se banda are adjusted using the POTs R1, R2 and R3. The circuit should be assembled on a PCB.

Circuit Schematic