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equaleyez 4th July 2012 11:44 AM

modifying a PCB for a simple low pass
Greetings all!

I will first introduce myself since this is my first post. I'm a 22 year old student from Brussels, doing my bachelor in Audiovisual Techniques - Sound at the moment. I'm also an amateur electronic music composer (samples, trip hip/ hiphop). My native languages are French and Dutch (Flemish Dutch) and I've seen some electronics physics as well as analog/digital techniques, and now I would like to put my theory in action.

As this is not an easy "métier", I was thinking about starting off with an easy, very simple filter design: RC low pass filter of the 1st order. I came up with this idea because I actually need it.

I have the following setup in my mini home project studio:

- Laptop
- Yamaha MG10/2 mixer
- A pair of KRK rokit 6 studio monitors (active).


- 5 string active yamaha bass
- 300W bass amp + 300W cabinet with 4 x 10" speakers.

Now to get to the point: I would like to use my bass cab as a subwoofer when using my KRK monitors. It's not made as a subwoofer, it is a bass cab of course, but this seems like a nice and cheap (since I already have everything) solution to give me that extra bass for enjoying finished tracks, not to monitor of course. My mixer has two pair of OUTs, one is already taken by the KRK monitors, the second pair of OUT's seems good for the subwoofer.

Now, as told before, I have my basic theory in filters and electricity so I'm half way there. To make a low pass filter, I know I have to put a resistor in series with my audio signal, and a capacitator in parallel. The values of the R and C depends on how I want my cutoff frequency to be.

Yesterday I found my awful old computer speakers. When seeing the so called subwoofer that comes with the speakers, I came up with the idea of using the parts for my low pass filter. I smashed the subwoofer to get the PCB and voila, I have something that could be helpful.

Oh, before I forget to mention: I work with a mac wich means I'm really restricted in software for calculations. But in this example it isn't necessary, correct me if I'm wrong..

Here's the Printed Circuit Board I'm talking about:

The blue mini jack plug is the INPUT, the orange one is the OUTPUT.
I will explain the signal flow a bit since Logitech had the magnificent idea to use as much cables as possible... :rolleyes:
So if I connect the speakers to my laptop, the audio signal first goes to the left speaker, wich has a volume adjuster. It is then sent to the PCB in the subwoofer which, I think, amplifies the sound and then splits it in high and low frequencies, which are then sent to either the little speakers or the subwoofer.

What I now want to do is: modify the PCB so that the signal is only affected by an RC filter. I have all the soldering equipment I need.
Why not use a breadboard and buy all the components? A breadboard is temporarily and very bulky. This PCB, if modified, is more compact and if everything is soldered, it will last longer and eventually I could build a little box. Also, it's been years since I've soldered. This all seems like a good beginning exercise for future projects.

Also, I'm not quite sure about THIS: The PCB consists of amplification. I was thinking about deleting that part since I have a bass amp. I don't know but if any of you think that it is better to keep the amplification (less modification) and just connect the filter straight to my bass cabinet, then let me know. But the bass amp I have is quite alright, and Logitech's is.. cheap

- Is this a good idea?
- I'm I right if I think that I can desolder parts of the PCB to make a first order RC filter? (respecting the signal flow)
- Where do I start? I can not seem to find the low pass filter that should already be on the PCB...

I appreciate your time for reading this and thanks in advance!


equaleyez 4th July 2012 01:44 PM

I forgot about the fact that the component leads are already gone due to soldering wich will make it hard to desolder en solder again...

sreten 4th July 2012 03:23 PM


You can make up a simple lead that mixes L+R and low pass filters them.
The components should fit in a standard 1/4" jack plug. That is all you need.

rgds, sreten.

equaleyez 6th July 2012 04:45 PM

Man I couldn't see it that simple...
What goes where? on the Tip, Ring?


sreten 6th July 2012 06:47 PM


L+R resistors to the tip, resistor and capacitor in parallel tip to ground.
Say 4.7K for the resistor and calculate the capacitor based on it seeing
about 1.5K (depending on the amplifiers input impedance).

rgds, sreten.

equaleyez 6th July 2012 07:55 PM

Hi Sreten,

my amp has 22kOhms Zin for LINE levels so 4,5KOhms seems good.
Still not everything is clear to me. What exactly do you mean with

L+R resistors to the tip, resistor and capacitor in parallel tip to ground.
You are talking about using more than one resistor?
I have this cable laying around so I was thinking about using it:

sreten 7th July 2012 01:04 PM


3 resistors in standard stereo to mono, one extra filter cap.

rgds, sreten.

equaleyez 10th July 2012 02:10 PM


but I am still not following. If you have some time, a picture is always welcome :)

Thanks alot!

equaleyez 12th July 2012 07:12 PM


had the urge to dump this one.. Very frustrating how the most simple filter ever looks so hard to accomplish. Anyone any tips on how to modify my cable like SRETEN said?
It just doesn't make sense to me I'm new to this.


sasha70 12th July 2012 08:04 PM

Hi, check this link:
Low Pass Filter - Java Experiment

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