An equalizer is a tool used in audio to adjust the frequency balance of a signal.
There are four basic concepts to understand when learning about EQs: Frequency Band, Center Frequency, Gain, and Bandwidth.
Gain
An equalizer allows you to adjust the gain, or level, of the selected frequency band.
The center frequency will be affected the most, but the frequencies above and below that point are also affected.
Bandwidth
Some equalizers allow you to change the width of the gain adjustment. This setting is called bandwidth, or sometimes “Q”.

Parametric equalizers provide users with significantly more control.

With a parametric EQ, you can select the center frequency for each band, the gain, and the bandwidth.

Parametric equalizers are more complex than graphic equalizers since you can make additional adjustments beyond volume. A parametric equalizer lets you control three aspects: level (boosting or cutting decibels), the exact frequency, and the bandwidth or range (also known as Q or quotient of change) of each frequency. As such, parametric equalizers offer surgical precision when it comes to affecting the overall sound.

Like the graphic equalizer, each frequency can be cut or boosted. But while graphic equalizers have fixed frequencies, parametric equalizers can choose a center or primary frequency. For example, if a graphic equalizer has a fixed control at 20 Hz, a parametric equalizer can be adjusted to control frequencies at 10 Hz, 15 Hz, 20 Hz, 25 Hz, 30 Hz, and so forth. The selection of adjustable frequencies (for example, by ones, fives, or tens) varies by make and model.

A parametric equalizer can also control the bandwidth—the sloping that affects neighboring frequencies—of each individual frequency. For example, if the center frequency is 30 Hz, a wide bandwidth would also affect frequencies as low as 15 Hz and as high as 45 Hz. A narrow bandwidth might only affect frequencies as low as 25 Hz and as high as 35 Hz.

While there is still a sloping effect, parametric equalizers are better able to zero in on and fine-tune the shape of specific frequencies without disturbing others. This detailed control of tone and sound permits finer adjustments to suit particular tastes or objectives (such as for mixing or recording audio).

In a nutshell, parametric equalizers offer:

Complex and deliberate operation
Select frequency adjustment
Precise range of effect
Performance for studio recording, mixing, and production
A more expensive package than graphic equalizers.

The stereo power amplifier is based on the single IC STK4141 II. This stereo amplifier circuit will deliver 25W audio power on each output channel.

2x25W Stereo Power Amplifier with STK4141II circuit.


Recommended voltage is 27.5V for 8 ohms speaker and 24.5V for 4 ohms speaker while the maximum voltage to supply this circuit should be about 41 VDC. This circuit operated with dual polarity (symmectrical) power supply, so center tap type of transformer should be used. Use minimum 2A transformer for better result.

Heatsink usage on the power IC is a must to prevent overheating and keep the good performance of STK4141II.

Use this STK4141II Datasheet for the circuit reference: