Expert: Cleggsan - 3/21/2009

Question
First, I am getting some sort of feedback through my subwoofers, (4-12 inch speakers) a thudding sound when the volume is low and when it is turned all the way down. But when I turn the volume up, the loud bass pretty much drowns out the thudding. "It's a rapid kind of deep thud". And when I change the song on my deck it does it too! Then, I tried to hook up a capacitor to my amp and the capacitor sparked when I put the fuse back in the power wire to the battery! Any Ideas?

Answer
It sounds like what you have is "motorboating" or a very low frequency pulsed feedback.  It can be from bad grounding or electrical feedback.

The power cap "may" be of some help.  But, the cap must be installed very carefully as the instructions will guide you on.  The cap must be charged up first before installing it and the connection should be as close to the amp power terminals as possible.

But, before you try installing a cap, try this to see if you can get an idea of where the cause is originating.

If you are driving them all with one amplifier, remove 3 of the 4 speakers and listen to just one at a time. If you have multiple amplifiers, turn off all but one and power only one speaker at a time.  See if this makes a difference.

Check the ground connection carefully; making sure it has a really good bare metal to bare metal contact.

It is possible a shorted speaker or wrong load on an amp could cause this phenomenon as well. And, try switching the rca cables around.

If you still can't get rid of it let me know more.  Give me the amplifier, headend and speaker make and model numbers, and the wiring scheme you are using so I can get into the details of the system you have.

Good Luck,
c

PS:  Also, read through these threads.

http://www.canadiancaraudio.com/online/general-discussion/18676-subwoofer-pulsin...

http://www.electro-tech-online.com/general-electronics-chat/39016-how-can-i-stop...

PPS:  Another thought, pull the inputs out of the sub amps, or still better, put shorted rca plugs into the inputs so you know the amps are getting zero signal at the inputs.  If you still get the thumping then it may be amp troubles or magnetic feed from speaker wire running to near the amp input circuitry.  Does the motorboating occur in all the speakers?  More in one than the other?  Let me know.