Chasw98's 
H@LF Project
 (H@LF= Help @ Low Frequencies) 
 the subwoofer toolbox
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this page is under construction and so is the project.... ;^) 
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Features

1) User selectable Hi-Pass (6 - 24 Hz in 2 Hz increments) with trimmer control
2) User selectable low frequency phase control (0 - 360 degrees) with fine adjustment
3) Phase invert switch
4) +6dB of gain from the balanced line driver output

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Schematic

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Prototype Open View

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Prototype Back View

XLR output provides +6dB of gain. In all but the most extreme cases this 
will eliminate the need for line-level shifting devices like the CleanBox.   ________________________________________________________________________________________________

Prototype Front View
Controls with high-pass frequency modules

Controls from left to right

Phase In/Out, Phase Invert, Phase Adjust, Hi-Pass In/Out. Hi-Pass Trim.
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For the stock PCB, the controls will be mounted in the PCB itself. 
Users may of course move the controls to the front panel 

Frequency modules are 4 identical resistors soldered to a removable 8 pin DIP header
The value of the resistors determines the Hi-Pass frequency. 
An Excel Spreadsheet will give users the ability to determine specific XO points. 

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Not shown is the required +/- 15VDC power supply
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Components:

1) BurrBrown OPA2134ND op-amps
2) 1% metal film resistors
3) 2% polyester film caps
4) Allan Bradley trimmer pots
 

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PCBs will be available for purchase. Pricing is not known at this time. 
Other components parts can be purchased from Mouser/Digi-key using an online project list.

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 Here's a download LINK for the spreadsheet used to calculate the header resistor values

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Here's a picture of the first assembled prototype PCB

This version has the controls mounted in the PCB itself. 
Builders have the option of using chassis mounted switches and pots.