# Sub moves when music isn't playing...



## stevja1 (Jan 25, 2004)

Hey all....

I spent about 2 minutes searching the forums for an answer, but its kind of hard to search for. If you've seen this problem discussed before, could you please give me a link to the thread? Thanks in advance....

Okay... here is the problem. Me and my friend just installed an 800W (Peak) Legacy in his Ford Focus. He is driving a 500W RMS 10" MTX sub with it, in a box that we built. It is a sloted box.

When the car is on and we are driving around, with or without the stereo on, the sub will start thumping by itself.... thump, thump, thump, thump, in a regular beat. It is way loud and really annoying and we don't want to break the thing. When you unplug the RCA signal lines from the head unit it quits.

When it is not thumping, you can look at the speaker and it is going back and forth quite a bit, but not making noise.

When we turn the gain down, the sub still moves, but it quits thumping. Does anybody have any idea what this could be? My guess, is that there is some interference in the RCA lines from the head unit. We have no idea where it is coming from though because we ran them on the oposite side of the car. The only wire remotely close to the RCA wire is the remote wire for the amp.

Any ideas are welcome.


----------



## chimmike (Oct 17, 2002)

well if you can't hear the sub and it's moving, it's making sound at a frequency too low for you to hear.

legacy subs=crap. for all I know it could be faulty.

also.....how shielded are the RCAs.....what head unit is he using...how well is the amp grounded?


----------



## stevja1 (Jan 25, 2004)

chimmike said:


> well if you can't hear the sub and it's moving, it's making sound at a frequency too low for you to hear.
> 
> legacy subs=crap. for all I know it could be faulty.
> 
> also.....how shielded are the RCAs.....what head unit is he using...how well is the amp grounded?


Thanks for your speedy reply.

The Subwoofer is a 10" MTX8000. The amp is a Legacy. I'm not sure if what you said above is a typo or not. (I imagine if the Legacy subs suck the amps probably do too.)

He has it grounded with a 4 gauge wire to the sheetmetal in his trunk. He is using a blaupunkt headunit that comes stock with the ZTS focus. 

The RCA cables have plastic around them, I'm not sure how to determine if they are shielded well or not. They have what looks to be gold plated connectors if that means anything. The plastic is see-through. I could send a picture if that would help. (I have to find the guy and take the picture first.)


----------



## SentraStyleEMW (Aug 15, 2002)

Definately sounds like a grounding issue. But it being a legacy amp...it could also be a defective amp issue...but I'd lean to the bad ground.


----------



## Dwntyme (Dec 18, 2002)

Replace the RCA's . you have a bad set, Try not to twist them or pinch on the body, Make sure your amp is not mounted (metal on metal)....


----------



## Dwntyme (Dec 18, 2002)

stevja1 said:


> Thanks for your speedy reply.
> 
> The Subwoofer is a 10" MTX8000. The amp is a Legacy. I'm not sure if what you said above is a typo or not. (I imagine if the Legacy subs suck the amps probably do too.)
> 
> ...


The problem is where the connectors meet the cable on the RCA's you lose your shield and it causes feed back in the amp....


----------



## chimmike (Oct 17, 2002)

ahh an MTX sub, that's a different story 

6 years ago I had a small alpine amp pushing an MTX Thunder 10" in a cheapie box...man, for what it was, that pounded! lol.


----------



## stevja1 (Jan 25, 2004)

chimmike said:


> ahh an MTX sub, that's a different story
> 
> 6 years ago I had a small alpine amp pushing an MTX Thunder 10" in a cheapie box...man, for what it was, that pounded! lol.


Yeah... my friend's 10" is in a slotted box. The speaker has a 102 oz. magnet. The thing is insane. I wish I had one.

Just to sum up... it sounds like it could be any of four things....

Bad RCAs
Bad shielding around the RCA connectors near the AMP (or deck).
The Amp is toast
The Amp is grounded poorly

Lets start with the free stuff... typically on a car, is the sheetmetal in the back a good place to ground, or are there better places? I've got a four gauge ground wire. I figure that should be plenty for a 400W RMS/800W Peak Amp. We sanded off the paint around the ground, so the connector is touching... metal on metal.

Is there a recommended brand of RCA cables I should look for? What things about the cables should I look for?

How do I get rid of feedback from the amp around the connectors? Buy a new amp? 

Thanks for your help!


----------



## Punkrocka436 (Jul 8, 2003)

question....do you have a remote bass control knob installed? A while ago, i had a remote bass control knob installed, and just used the wire that i had already run, and i had the same problem. Turns out the problem was in the wire (phone cord, i figured they were all the same). Used the cord hifonics provided and the noise went away


----------



## stevja1 (Jan 25, 2004)

Punkrocka436 said:


> question....do you have a remote bass control knob installed? A while ago, i had a remote bass control knob installed, and just used the wire that i had already run, and i had the same problem. Turns out the problem was in the wire (phone cord, i figured they were all the same). Used the cord hifonics provided and the noise went away


Actually there is a remote bass controler thingy. We used the cord that it came with, but we'll try it with a different cord and let you know how it went. Interesting....


----------



## captain_shrapnel (Sep 10, 2004)

Some other things to check... 
Constant thump might be interference from pulsed electrical line, ie spark plug wires. Check for shorts between the RCA cables and power lines (sometimes the manage to touch. The speaker moving and no sound is a characteristic of a ported box playing below its cutoff frequency. This will kill a speaker. Was the box designed to spec? Ported enclosures are exceedingly touchy, and most designers use software to manage it. Hope this helps.


----------



## Ninety-Nine SE-L (May 5, 2002)

Back when I was using my infinity sub, I had something like this happen. I opened up my trunk and I saw my sub was going all the way in and all the way out by itself, very slowly. It was slow kind of like a person breathing in and out. Amp was on, radio was off, never seen it before, but it's prolly some interference being picked up by the RCA or amp.


----------



## stevja1 (Jan 25, 2004)

captain_shrapnel said:


> Some other things to check...
> Constant thump might be interference from pulsed electrical line, ie spark plug wires. Check for shorts between the RCA cables and power lines (sometimes the manage to touch. The speaker moving and no sound is a characteristic of a ported box playing below its cutoff frequency. This will kill a speaker. Was the box designed to spec? Ported enclosures are exceedingly touchy, and most designers use software to manage it. Hope this helps.


We built the box to perfectly to spec. It has exactly the right amount of airspace, and the slot is exactly the right size and length. We had a program calculate the dimensions and everything. It was kind of fun. 

The amplifier actually died, so we sent it back. We think the wierd thumping was a bad amplifier. Hopefully, we'll get a replacement soon and let you know if it was the amp or not. Just so you know, we tried powering the sub with a smaller amp, but the same wires, and everything worked fine.


----------



## Ninety-Nine SE-L (May 5, 2002)

Nea, as I was saying with the infinity, it's prolly interference picked up by a badly shielded signal wire. When mine was going in and out, it was a sealed box built to spec, and the engine was off.


----------



## stevja1 (Jan 25, 2004)

Ninety-Nine SE-L said:


> Nea, as I was saying with the infinity, it's prolly interference picked up by a badly shielded signal wire. When mine was going in and out, it was a sealed box built to spec, and the engine was off.


Hmm... did you ever get it to quit? (New RCA wires?)


----------



## Ninety-Nine SE-L (May 5, 2002)

stevja1 said:


> Hmm... did you ever get it to quit? (New RCA wires?)


Actully, I only saw it do it on 2 occasions, maybe there was interference in the area...It was doing it once when I was putting groceries in the trunk (I can turn on my radio by remote), and again on the same day when I showed my friend.

I have monster RCA cables going front to back (6-channels), and the amp was an MTX thunder 8300(?)


----------

