Bass Amplifier Made From Cheap Aura InteractorAmplifier

This is my Firs Instrucion, so the next ones will be better ;-)
I have bought an Cheap (5Euros) AuraInteractorAmplifier from a german Shop
http://www.pollin.de/shop/shop.php?cf=detail.php&pg=NQ==&a=NTk4OTYzOTk=
It delivers approx. 16W RMS.
The Powesupply primary is 220V...240V at 50Hz.
They wrote you cold use it as Subwoofer Amplifier, but when I tried it, the sound was horrible.
So I modded it with good results.
Sorry, not much fotos since i have lost the data cable for my mobile phone :-(

Step 1: Overview

Here is the original plan before my changes. You see a lot of things needed for a buttkicker, but not for a (Sub)woofer driver, like the flipflops U3 and the multiplexer U4, they are ripped off by me.
The cutoff frequency of the lowpass filter, it is a sallen-key filter of 2nd order, is too high, so it has to be changed. All coupling condensators are too small to get a good bass response so they has tobe changed.
There are some other useless parts, but they can stay on the board since they do not disturb the sound.

Step 2: Things Needed

you will need
Tools:
a cross screwdriver to open the box and get the electronics out .
a soldering iron suitable for electronics, i used a 15W with a needletip
a file or dremel to cut out the unused IC´s.
some soldering wire

Components:
Capacitors: 3 * 100nF 16V or higer Voltage you can use mica, ceramic or foilcaps for these
2 * 470nF same as above
1 * 220nF same as above
1 * 2.2µF/16V electrolytyc Cap
i used salvaged components so it didn´t cost anything

Step 3: Ripoff Unused Parts

Unscrew the black screw in the center to get the circuit board off.
Cut off the legs of U3 and U4 and desolder the rest of the legs from the circuit board.
U3 and U4 is printed beneath them on the circuit board.
after this you will have to connect solderpads 2, 3 and 4 of the formerly U4. This will work best if you do it on the solderside of the circuit board.

Step 4: Change the Copling Caps

next you desolder the coupling caps C7, C10, C20.
then fit in the replacement caps
for replacement of C10 and C20 use values between 470nF and 2200nF at 16V or bigger voltage.
now desolder C7 and replace with an electrolytic Cap between 2.2mcF and 4.7mcF equal or greater than 16V. butbe careful to pt it in the right way because this caps are polarized, if yo look at the coponent side of the board there is a circle printed for C7. One side has a little black sqare on it this is the negative (-) side of the electrolytic cap.
the bigger the capacitance (more nF) of all the above changed caps the lower the frequencies can go. The Voltage rating of the caps you use hs no effect on the sound but must have a minimum of 16V.

Step 5: The Low Pass Fiter

Now we can change the cutoff frequency of the low pass filter around U2d, since 2kHz is much too high for a subwoofer. The filter is a sallen-key-lowpass of 2nd order.
I decided to use 200Hz as cutoff frequency because i did not have to calculate much since the frequency fall factor 10 i only had to multiply the capacitances of C13 and C14 by 10 :-)
if you want or need another cutoff frequency you can use a program like >filterlab< from arizona microchip to calculate your values for the resistors and caps arond U2d.
So if you use my values you only have to exchange C13 from 22nF to 220nF and C14 from 10nF to 100nF.

Step 6: Modify Frequency Response of the Power Stage

this is done very easy, you only have to desolder and extract C9 to get the lower frequencies better amplified.

Step 7: Youre Done

now put the board into the case, screw all together again.
thats it.

To use it connect a woofer (4Ohm gives most power) via the cynch (RCA phono) plug. be sure the speaker can withstand a minimum of 20Watts.
The Powesupply does only fit to the right plug.
Connect the soundsource, low level, not highly amplified, headphone output is ok, via the headphone plug, switch it on using the volume pot, have fun.