This is a transistor equalizer circuit. We use it for controlling the audio frequency. Because some kinds of audio frequency response are not smooth. It helps a frequency offset in a tone control well.
Why make this circuit?
Look at the circuit, without ICs. So, low noise and easy.
A normal audio system has bandwidth is too wide.
It requires an equalizer circuit with a narrow bandwidth, and to divide into frequency bands increased to 5 channels or more.
It can control specific actions required frequency bands.
How it works
Transistors Graphic equalizer circuit
The equalizer circuit can adjust the frequency to 5 channel is 60 Hz, 100 Hz, 1 kHz, 3 kHz, and 12 kHz.
In circuit is Mono. If you want stereo. Just You add Mono another one only.
When we enter a signal to an input. Then, C1 passes a signal to coupling through pin1 each potentiometer (VR1-VR5).
If we adjust VR5 to a higher level in the middle of a potentiometer.
This signal will come to a high and low-frequency filter of each band. They include R and C network.
Next, the signal that through each band be combined together.
Then, C10-capacitor passes the signal to coupling to a base of Q1 and Q2.
Both transistors are a common emitter amplifier.
The higher signal goes out of a collector of Q1. It is out of phase with the input.
Then, the highest signal or the output signal comes out of emitter of Q2 via C13.
And, some signal will be feedback through R12 to each leg 3 of the potentiometer (VR1 to VR5).
When we adjust them to this point. A feedback signal will refute with a input. So, this is cut off point.
How to builds this project
You can see the PCB layout in Figure 2 as Mono.
And can see the components layout in Figure 3.
The wiring for circuit wiring and various parts to check the polarity of the electrolytic capacitors and Diodes and transistors correctly.
Though so easy, you need to solder it with neat.
Note: The PCB layout scale is 200/inch. You can print on the laser printer to make PCB easily.
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