## Fixed and variable equalizer for Constant Directivity horns. | |

This is 2 different equalizers for Constant Directivity ( CD ) horns.

One is fixed and the other variable for experimentation purposes.

Fig.1: Constant Directivity horn and equalizer frequency response.

Lime: | Horn frequency response with a -3 dB point at 3.2 kHz. |

Blue: | Ideal frequency response for equalizer. |

Red: | Frequency response of practical equalizer. |

Cyan: | Frequency response of practical equalizer and horn. |

The blue graph is an "ideal" equalizer so the sum of the horn and equalizer will give a linear response.

The downside of this is that the gain is quite high ( 16 dB at 20 kHz ) so there is a high risk of overloading the circuit.

The red graph is a practical equalizer with a lower gain ( around 2 dB at 20 kHz ).

This is not a problem as horn-loaded compression drivers typically are 10..15 dB more sensitive than the midrange units.

The cyan graph shows the combined response of this equalizer and horn. It is around 2 dB down at 20 kHz, but can be extended further.

Fig.2: Equalizer points used for the following calculations.

Fig.3: Fixed equalizer. As far as I remember, these component values are for a JBL 2445 compression driver with a 2380A horn.

Low frequency gain:

High frequency gain:

Low frequency corner:

High frequency corner:

Fig.4: Adjustable equalizer.

The adjustable equalizer shown in fig.4 is designed for experimenting only.

When you have the frequency response you need, you should replace it with the one shown i fig.3.

R5 and R6 sets the difference between Avl and Avh.

C2, R7 and P1 sets the frequency and P2 is a gain adjustment.

R8 makes sure there is some feedback if P2 is disconnected ( do not try with a speaker connected ).

Fig.4: Adjustable equalizer frequency response for different settings of P1.

Lime: | Horn frequency response with a -3 dB point at 3.2 kHz. |

Blue: | Equalizer frequency response with different settings of P1. |

Red: | Equalizer and horn response. |

To change the circuit from the circuit in fig.4 to that in fig.3:

Remove U1 from the socket and, using a DC ohmmeter, measure the resistance from A to B ( Rab ), from C to D ( Rcd ) and from D to E ( Rde ).

Calculate:

Select C1.

Select R3

Constant Directivity horn equalizer calculations.

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