Audio Distribution Amplifier is a circuit which transfer signal from one audio source to many output with a facility of appropriate amplification. The circuit published here is of Audio Distribution Amplifier is shown in figure 1, which is basically a low cost, low noise amplifier designed using one of the most popular operational amplifier LM324. The circuit of audio distribution amplifier is combination of low noise input transistor amplifier and an op-amp in order to insure good characteristics.
Circuit Description of Audio Distribution Amplifier using LM324
The circuit of audio distribution amplifier is designed using low noise NPN transistor BC550 and a quadruple operational amplifier LM324. Various other passive components like resistors and capacitors are also used in this circuit in order to work flawlessly. The audio signal from audio source (like from mobile or computer) are connected to audio input connector (CON1). Resistor R1 combined with capacitor C2 to form a network which by-pass interference (basically Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI)) if any available in input audio source. Input stage amplifier is built around ow noise transistor T1.
For reduction of distortions in output stage a feedback network is designed using a resistor R2 and a variable resistor VR1 and is connected between output of 1st op-amp (IC1-A) and base of transistor T1. The feedback network provides initial working point of input stage amplifier transistor (T1) by implementing AC as well as DC negative feedback. The current of collector T1 is adjusted to 0.2mV to 1mV.
From above arrangement (parts list used in this projects) the voltage gain of the circuit audio distribution amplifier is 30. We can calculate the voltage gain by applying formula.
The four-individual speaker or power amplifier is connected to four outputs derived by four internally op-amp of LLM324 shown in circuit diagram.
As from data sheet of LM324 operational amplifier operating voltage is about 3V to 32V, and for working of the circuit audio distribution amplifier posted here in operating range, power supply of 12V is preferable. Resistor R14 with capacitors C3 and C4 forms filter network of power supply for transistor T1. We can calculate of quiescent current of circuit by applying formula.
Quiescent Current of Circuit = Emitter current of transistor T1 + Quiescent Current of op-amp
Thus, quiescent current of the circuit is very low.
Procedure for adjusting variable resistor VR1
- Supply ripple free voltage (regulated power supply) to the circuit.
- Short both of the pin of connector CON1 (audio input connector).
- Adjust variable resistor VR1 in such a way that voltage available at collector of transistor T1 would be half of supply voltage (6V).
- Open the shorting of connector CON1.
Output voltage available at individual op-amp of LM324 (pin 1, 7, 8 and 14) can be calculated by formula given below,
Now, let us suppose if the input signal (audio) is of 1mVPP (Peak to Peak Voltage) is given to the connectors CON1, the output will be,
Note: The series combination of VR1 and R2 can be replaced by a single resistor of value 510 KΩ if the input supply voltage changed to 15V.
PARTS LIST OF AUDIO DISTRIBUTION AMPLIFIER CIRCUIT USING LM324
|Resistors (all ¼-watt, ± 5% Carbon)|
|R1, R6 – R9 = 100 Ω
R2 = 330 KΩ
R3 = 15 KΩ
R4 = 470 Ω
R5 = 2 KΩ
R10 – R13 = 22 KΩ
R4 = 220 Ω
VR1 = 2.2 MΩ
|C1 = 4.7 µF, 35V (Electrolytic Capacitors)
C2 = 100pF (Ceramic Disc)
C3, C5 = 100nF (Ceramic Disc)
C4, C6 = 220 µF, 35V (Electrolytic Capacitors)
C7 – C10 = 10 µF, 35V (Electrolytic Capacitors)
|IC1 = LM324 (Quad Operational Amplifier)
T1 = BC550 npn Transistor
|CCON1 – CON6 = 2-pin terminal connectors
12V DC Power Supply