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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.

Constant Directivity horn equalizer.

Constant Directivity horn and equalizer frequency response.
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 lime graph is the frequency response of the 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.

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

Practical equalizer schematic.
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:

Variable equalizer schematic.
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 ).

Constant Directivity horn and equalizer frequency response.
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 ).


Select C1.

Select R3


Constant Directivity horn equalizer calculations.

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