The proposed 10 band graphic equalizer circuit can be used in conjunction with any existing home theater system to get an enhanced 10 stage audio processing.

The Circuit Concept

The unit is quite simple to use. Al one has to do is feed the TV or PC audio input to this circuit and hook the output with the existing home theater amplifier.

Next, it would be just a matter of adjusting the given 10 band controls and enjoying the vastly improved sound quality.

You would be able to tailor the sound as per your preferred tastes.As an example, the midrange controls of the equalizer can be adjusted to highlight dialogue or in order to reduce the harshness over a particular range of voice audio.

Or perhaps you can roll off the high pitched even to further extents in case you wished, or simply heightened the bas boost to your liking.

Typically the controls would be able to provide upto 10dB of boost or cut at nominal center frequencies of 150Hz, 500Hz, 1kHz, 2kHz, 5kHz, 7kHz, 10kHz, 13kHz, 15kHz, 18kHz.

The circuit also includes a fixed 10kHz low pass filter stage for cancelling out unwanted noise such as hiss or other high frequencies disturbances.

How the 10 band graphic equalizer circuit functions

Referring to the given circuit diagram we can see that the associated opamps form the main active component responsible for the required optimizations.

You will notice that all the 10 stages are identical, it's the difference in the values of the incuded capacitors and the pot which effectively varies the processing leves across the various stages.

For analyzing the operation we may consider any one of the opamp stages since all of them are identical.

Here the opamps act as "gyrators" which refers to an opamp circuit which effectively converts a capacitive response to an inductance response.

Consider an AC voltage source Vi connected to the opamp stage. This pushes a current Ic via the capacitor (C1, C2, C3 etc), which constitutes a proportional voltage across the connected ground resistance (R11, R12, R13 etc).

This voltage across the ground resistance is conveyed at the ouput of the opamp.

Due to this the voltage across the feedback resistor (R1, R2, R3 etc) becomes equal to the difference between Vin and Vout which causes current to flow via the feedback resistor and back into the input voltage source!

A careful assessment of the phases of the above developed current would show that as Ic leads the voltage Vin (as it can expected for any capacitive circuit) the net input current that may be the vector sum of Ic and Io in fact trails the voltage Vi.

Using Capacitors as Tuned Inductors

Therefore this implies that in effect, the capacitor C has gotten transformed into a virtual inductor due to the actions of the opamp.

This transformed "inductance" may be expressed by the following equation:

L = R1xR2xC

where R1 = ground resistance, R2 = feedback resistance while C = associated capacitors.
Here C would be in Farads and the resistances in Ohms.

The pots effectively vary the input current to the opamps which results in a change in the value of the above explained "inductance", which in turn results in the required music enhancement in the form of treble cuts or bass boosts.

Circuit Diagram


Response Curve for the above 10 band graphic equalizer design



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