how can i stop my car subwoofer amp from "motorboating"?

I installed a "Buttkicker" subwoofer in my car. It's a kind of heavy-duty piston that vibrates the entire metal surface of the car; you bolt it to the car floor. It's the same device they use in theaters - to provide very low vibrations that are FELT more than heard.

I bought an inexpensive, high-wattage subwoofer amp, and if the volume on it is all the way up, even without any input at all, the "buttkicker" gets to pulsing at about 120 cycles per MINUTE (or 2-3 per second), when there is no input or the input volume is low.

If input level increases, then this autonomous pulsing is replaced by proper response to the input signal, and the bass notes vibrate nicely. But that's only at high volumes.

Is there some kind of filter or some other measure I can take to eliminate this?

Is it perhaps a function of the long distance the input has to travel - along a good length of shieded RCA plug stereo cable - before it gets to the amp?

Thanks. I hope the explanation is clear enough.

Bob H.

If the amp is without something wrong with it then I think the problem is caused by having too much resistance in the wires that supply 12V to the amplifier. Maybe a huge supply bypass capacitor at the amplifier will fix it.

Maybe the impedance of the "speaker" is too low for the amplifier to drive and it strains its power supply and wiring.

That is intriguing. What mfd capacitor, and what voltage, do you recommend? And how would you wire it up? Just in parallel with the car power supply (with the battery)?

Also, do you think it might be a ground loop problem?

Thanks.

bh

Haskellbob, sounds like a ground loop problem to me.
Look up ground loop isolator

This is the sort of capacitor you need but it's very expensive.
https://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=3818961

They make a 20F capacitor for car audio amps. It has a blue voltage and amperage display and costs only $499.99US EACH.

The capacitor must be connected at the amplifier especially if the battery is at the other end of the car.

You could also try adding another wire or increasing the size of the wire from the battery to the amplifier. What size wire are you using?

24 gauge telephone wires??