Foster WF-100k Faq

Foster WF-100k FAQ
*** Major revisions make on 5/3/09 ***
How do I remove the spectral tilt that is in the frequency response?
See the file "WF-100k FR with VR603 adjustments.pdf" posted on the website. This file
has frequency response curves with VR603 adjusted to various positions. VR603 is
located on the power amplifier PCB. This file shows how the amount of spectral tilt, and
overall gain, changes with the adjustment of VR603. As shipped, the position of VR603
gives the amplifier a frequency response shown in the file "WF-100k frequency
response.pdf". When VR603 is turned fully counter clockwise, the spectral tilt is almost
completely removed. If you want the spectral tilt 100% gone, you will need to jumper a
10k resistor across pin 1 and 3 of VR603.
Once the spectral tilt has been adjusted out of the frequency response, the amplifier has a
rd
3 order (18dB/octave) HP response with a Q of 0.707 and a corner frequency of 29Hz.
Note that the spectral tilt portion of the amplifier circuitry is NOT in the posted
schematic. If I can find a 100% accurate schematic for the amplifier, I will post it. DO
NOT ask me for one.
How do I change the high pass (HP) filter/bass boost or frequency?
Open the link on the sale site labeled "2
nd
order HP filter calculator.xls". This spreadsheet
should open in Excel. If it doesn't when you click on it, you will have to right click on
the link, select "Save As", save the file to your hard drive and open it with Excel
manually. The instructions in the spreadsheet should be self explanatory on how to
calculate the value of the new resistor values. Replacement resistors can be 1/8 or ¼ watt
sizes. It won't make any difference. The ¼ watt models may need to be stood up on end
to fit on the PCB.
Remove the four screws located near the corners of the amplifier plate. There is no need
to remove any of the other screws on the amplifier plate. Pull the bucket off slowly. You
will need to unplug the speaker wires from the connector on the PCB. Locate the PCB
with the volume control, it contains the resistors that need to be changed, R617 and R618.
Clip the body of the resistors at both ends, so they fall off the PCB, from the component
side of the PCB. Unsolder the leads from the sold side. Pull the leads through the PCB
from the component side of the PCB. This will keep you from accidently lifting one of
the solder pads off of the PCB. Clean the hole with a desoldering pump or solder wick.
Install the new resistors. Solder them in place.
Refer to any Chiltons manual for reassembly instructions☺
How do I bridge two of the WF-100k amplifiers together?
Step #1) Follow these instructions exactly or really bad things can happen