Modifying the Foster amp, Ver. 2

[jcpahman77]

There has been some discussion about this, and the general consensus was that it would be a good idea to start a separate thread. For those who aren't aware, J Hidley offers Foster (http://home.comcast.net/%7Ejhidley/) plate amps (Scroll down about 1/3 of the page). A couple of things make these stand out.

Same cutout as Dayton plate amps
Low price per watt (very low)
Customizable

The Ver. 2 amp, which is the topic of this thread, offers a customizable 2nd order HP filter/boost circuit that can be adjusted by changing some of the internal components. A spreadsheet (http://home.comcast.net/%7Ejhidley/2nd_order_HP_filter_calculator.xls) is available on Hidley's site which details how to do this, but we're going to cover it here a bit in an effort to make it easier for everyone to understand. I tried to attach the spreadsheet here and it wouldn't let me, click on my link above and it'll download. The spreadsheet should open with 3 highlighted cells, according to the instructions in the spreadsheet, the first two should be all you need to worry about. Those cells are, frequency, and boost. By simply typing in what frequency you'd like to boost or attenuate the spreadsheet will spit out values for a pair of resistors. Additionally, the graph and chart showing the frequency/db levels will change. I should note at this point that the spreadsheet lists the values for the two resistors in kW's. I can only assume this is meant to be k ohms.

The next step is to download the schematic (http://home.comcast.net/%7Ejhidley/WF-100K_schematic.zip) for the amplifier. It's a zipped folder, look for a file named "WF-100K schematic page 1.tiff" Near the top right of this image you should find resistor R618 and R619, they stand out a little as they are the two that appear to be hand written. Now, I don't have one of the plate amps yet myself, but the next step would be to find these two resistors physically on the amplifier board and replace R618 and R619 with the values from the spreadsheet.

If there are any questions, or if I have left anything out, please let me know. Ideally, in the coming weeks when I buy and modify a couple of these myself I'll post some progress pics.

[CokewithLime]

Just as a btw - I had earlier confirmed via email with Jack that the same customizable HP filter/boost circuit exists in the Ver 1 amps and works as he describes in the modification notes.

[jcpahman77]

Just as a btw - I had earlier confirmed via email with Jack that the same customizable HP filter/boost circuit exists in the Ver 1 amps and works as he describes in the modification notes.
Very good to know. So for anyone else following along, either of the Foster amps can be used in the above tutorial.

[charlielaub]

There has been some discussion about this, and the general consensus was that it would be a good idea to start a separate thread. For those who aren't aware, J Hidley offers Foster (http://home.comcast.net/%7Ejhidley/) plate amps (Scroll down about 1/3 of the page). A couple of things make these stand out.

Same cutout as Dayton plate amps
Low price per watt (very low)
Customizable

The Ver. 2 amp, which is the topic of this thread, offers a customizable 2nd order HP filter/boost circuit that can be adjusted by changing some of the internal components. A spreadsheet (http://home.comcast.net/%7Ejhidley/2nd_order_HP_filter_calculator.xls) is available on Hidley's site which details how to do this, but we're going to cover it here a bit in an effort to make it easier for everyone to understand. I tried to attach the spreadsheet here and it wouldn't let me, click on my link above and it'll download. The spreadsheet should open with 3 highlighted cells, according to the instructions in the spreadsheet, the first two should be all you need to worry about. Those cells are, frequency, and boost. By simply typing in what frequency you'd like to boost or attenuate the spreadsheet will spit out values for a pair of resistors. Additionally, the graph and chart showing the frequency/db levels will change. I should note at this point that the spreadsheet lists the values for the two resistors in kW's. I can only assume this is meant to be k ohms.

The next step is to download the schematic (http://home.comcast.net/%7Ejhidley/WF-100K_schematic.zip) for the amplifier. It's a zipped folder, look for a file named "WF-100K schematic page 1.tiff" Near the top right of this image you should find resistor R618 and R619, they stand out a little as they are the two that appear to be hand written. Now, I don't have one of the plate amps yet myself, but the next step would be to find these two resistors physically on the amplifier board and replace R618 and R619 with the values from the spreadsheet.

If there are any questions, or if I have left anything out, please let me know. Ideally, in the coming weeks when I buy and modify a couple of these myself I'll post some progress pics.

Here's a tip: Once you desolder the components, make sure to clean the area well with isopropyl alcohol since leftover residue and/or solder mask can interfere with the resoldering, making the solder refuse to stick to the pads! I had this problem on my first go-around and I had to start over and clean the area.

I am posting this for people's general info, since many are doing the mods.

-Charlie

[jcpahman77]

Thanks for the info Charlie. That is exactly the kind of input I was looking to get in this thread. Let's keep it going. There seems to be plenty of DIY info about what driver or crossover circuit or enclosure to use but doesn't seem to be a lot on electronics DIY.

[Dirttracker73]

in the coming weeks when I buy and modify a couple of these myself I'll post some progress pics.[/QUOTE]

You'd better get moving, the inventory is shrinking. You may have missed out on the version 2, at least at a good price.

[arlis_1957@yahoo.com]

in my case, it will depend on the true x-mas of the buyout 8"sub.

[jcpahman77]

Thanks for the heads-up Dirttracker, I get my tax return on the 15th. I guess I should send an e-mail and see if they will still be available at this price at that time.

[CokewithLime]

in the coming weeks when I buy and modify a couple of these myself I'll post some progress pics.
BTW - those of you "thinking" about ordering one of these amps - at the moment Jack's spreadsheet (http://home.comcast.net/%7Ejhidley/NHT_Drivers-2.xls) that shows items and remaining qtys indicates that all the Ver 2 amps are sold out and there is only 11 of the Ver 1 amps left !!!

[arlis_1957@yahoo.com]

i may have missed it but, i see nothing concerning type of resistor or wattage. also, the spreadsheet states kohm. really thousand?

[jcpahman77]

i may have missed it but, i see nothing concerning type of resistor or wattage. also, the spreadsheet states kohm. really thousand?
As I don't have any first hand experience here I was only able to give my interpretation of the info on the spreadsheet. I did go to school for industrial and computer electronics though so I'm not just guessing. Perhaps another e-mail needs to be sent to Jack for some clarification.

[arlis_1957@yahoo.com]

well johnny stated that he has moded two of them. im sure he'll pop in later.

[johnnyrichards]

1/4 Watt resistors such as the ones PE sells will be just fine, and yessir: they really are going to be values ranging in the kilo-ohm range.

One thing that will help is to have some 20 or 22awg solid copper conductor on hand in order to bypass the capacitor.

[jcpahman77]

1/4 Watt resistors such as the ones PE sells will be just fine, and yessir: they really are going to be values ranging in the kilo-ohm range.

One thing that will help is to have some 20 or 22awg solid copper conductor on hand in order to bypass the capacitor.

For anyone who doesn't happen to have 20 or 22 awg wire just lying around I recommend telephone or network cable. Both are usually 20-24 gauge and are solid copper wire.

[thekorvers]

For what it is worth:

I have recently picked up some of the Foster amps, but have not modified them yet. I intend to bring wires out to a small screw terminal strip, mounted on the outside of the amp case for easy access. I will mount the appropriate resistors there. This way I can easily make changes at a later time if I desire to do so without having to open the amp again and having to de-solder and re-solder, putting stress on the circuit board.

[jcpahman77]

This isn't a bad idea. When you did get it the way you wanted you could solder the components in and remove the screw strip for a more finished appearance.