Car audio project

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Some friends of mine recently completed a DIY car audio system. They built a tuned-port box for a twelve inch Image Dynamics Sub (model: IDQ10), powered by a Zapco amp running 200W X 1 at 2 ohms, and connected to a Clarion cd/head unit.

Somewhere in this setup a very low level signal is being fed into the sub and when it is powered it moves in and out several times per second. When an audio signal hits it is often at the outer point of the cycle and the speaker maxes out and distorts.

Can anyone tell me why it is doing this and how to fix it?

I agree with Rollins- you can get that weird LF oscillation if the gain of the amp is too high. Should be simple to fix.

I don't thimk you're getting an induced signal at the input to the woofer amp. Does the LF oscillation stop if you unplug the input leads at the head-unit? Try using "shorting plugs" on the input leads at the head end (buy a pair of Radio Sack RCA phono plugs solder the center connector the outer shield lead) and see if it stops- then you migh be getting induced signal in the leads, but I doubt it. If you are, you probably have a bad ground somewhere. Check all grounds at all equipment with a good ohmmter- all should be less than around 0.1 ohms.

If all this messin' around doesn't help, pull out the amp and try it in another vehicle. If it does it there, the amp has somethong wrong with it.

Image dynamics are great woofers, eh?
OK, so if the shorting plugs stop the LF signal is it safe to continue using them?

The problem has been isolated to the head now due to the amp being tested in another system. But when we connected a different subwoofer to this system it did the same thing.
How do we fix a bad ground if it turns out to be that?
I wouldn't bet on a really low frequency oscillation being caused by a bad ground. It would be more of a growl.
Don't assume that your front end is bad. For whatever reason, car audio equipment is just not as foolproof as home gear.
Try a resistor in series with the signal lead going to the amp--say 1k ohm, but be prepared to experiment. An alternate possibility is to wire a pot into the line and try twisting it this way and that to see if the oscillation goes away. (We're assuming that the amp doesn't have pots built in.)

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