Car Subwoofers Rumbling

[ThatM1key]

I noticed that when I'm idle with no music playing, I hear a rumble from the back. I thought it was my exhaust but it was actually my subwoofers. At my normal volume, they play and eventually the sub amp (Monoblock with dual stereo RCA input) shuts off, and turns back on, like its overheating.

My current hypothesis: Ground issue or sub amp issue?

What I observed is that there always a rumble even at the lowest possible volume (0 just cuts off any audio output). Increasing volume increases the rumble. It used to be when I would unplug one of the sub's RCA wires, the rumble would get a lot louder. I hooked up the sub's RCA wires to my 4-channel amp and I don't get any rumble. I hooked up the 4-channel amp's rear RCA wires to my sub amp and same issue.

I switched grounds and I got the same result. Weirdly, it kind of fixed it self when I switched back the grounds. When I got both sub's RCA wires plugged in, I get no rumble but if I unplug one of them the rumble comes back.

Added information is that when the sub amp has no RCA wires plugged in, I get no rumble. No rca-line converters, no remote-relays setups, no shared grounds/ground block. Both amps share the same remote wire but the 2nd wire is very short, to not cause too much milliamp pull.

Sub amp still outputs a great amount of audio regardless if the rumble is there, so its got plenty of life.

Edit: Sub amp does not have a thump (Separate ground wire) when turning on but the 4 channel amp does (Because its got an old factory ground).

 

[ThatM1key]

I wanted to give a bit of an update. The testing above, I did it while the car was off.

I messed around with the wiring some more and concluded, that both amps are fine. RCA wires are picking up noise from the power cables. I don't know why they just started to pickup noise when it been setup like this for a couple years now.

The 4 channel handles alternative noise extremely well, its not even there. When I switched the sub's power, RCA and sub wires to the 4-channel amp (Yes full range set on), no alternator noise.

I still don't understand how the sub amp just started picking up the noise but whatever, at least I got an answer. I hoping it was a bad ground. In my setup I ran the RCA wires next to the power lines (Yeah I know, its a big no no) but I didn't have any power noise beforehand and my headunit is 5V. I guess my plan is separate the RCAs from the power lines.

 

[DVDdoug]

Electrical noise in a car can be very difficult to diagnose and correct. It can be worse with high-gain, high-power, setups.

You can try an isolation transformer.

...I kinda' don't like transformers in the signal path but if it can solve a problem it's a good thing.

 

[Zinda]

Since the noise increases with the volume it has to be coming from your source or along the path to the amp. Typically engine noise comes from head units and it will get amplified with volume setting but I've never heard a low frequency sound coming from just subs. Have you tried using 2 channels of your 4 channel amp to push the subs as a test to see if it's in the amp or in the lines into the amp? I'll bet it's s amp issue. Try tapping the heatsink to see if the noise changes or stops.

You said the noise continues even when the car is not running? Have you checked your battery voltage and voltage at the amps power connections? It could be under volting the amp and it's powering down due to low voltage not thermal cycling. What gauge wire, I only run 4 or 2ga wire to amps I'll double up on them if needed. I also use big fused distribution blocks in the rear near the amps for easy installation. But I'm one who runs an extra set of RCAs from the front to back in case I have an issue over years. It's so much easier just to run an extra one while the cars apart for the initial install. Wire is cheap compared to the time it takes to fix things. A fuse block by the amps are a must but need to be fused for the wires feeding the amp not the amps power draw. If your amp power wire shorts it needs a fuse at the front of your car near the battery rated for that wire as well.

Wires have internal resistance and they have a power loss according to the length, a loss of a volt or more is possible as well as max current handling it can carry safely but it has a max current it can carry at any temp. Meaning it can limit the amperage the amp gets and a 1000w amp plus a 400w amp will require a wire rated to carry 100 amps over the length of that wire (length decreases power rating) im basing this on using your stereo only while your car is running since your alternator will put out 14v.

Your alternator needs to be at least 160A rated minimum to safely keep your electrical system working. The chance of a stock Alternator putting out 160A at idle continuously is unlikely . Once they get hot and idle at a stop light you'll be lucky to get 60A. Add in a under sized wire to amps drawing 1400w that's 100A plus your cars normal power draw let's say 40A. I always start at the alternator output before considering the build of the system.

If you can't supply the power the amps will eventually burn up from undervolting them. I've told this to countless people right at the moment they installed their system. Bought amps from me that I used for 4 or 5 years without any issue and warned them to upgrade the alternator and all wires with way bigger amp wires than they ran and sure enough a month later they're asking to buy another amp that's even bigger! They don't listen and history repeats it's self. Typically after that 2nd amp dies they ask me about upgrading electrical systems. As I said it easiest to spend the money the first time and do it right. You're under volting the amps and probably have too small wire and poor grounds. Possibly even a speaker wire is grounding out on the sub amp. They can get cuts in the jacket while running wires to speakers. Any output wire that grounds to chassis will cause weird noises like that .

 

[ThatM1key]

I've gotten around to install that isolator I bought, the "RECOIL MGLI". I didn't want a cheap one because I heard they mess with bass signals.

Since the noise increases with the volume it has to be coming from your source or along the path to the amp. Typically engine noise comes from head units and it will get amplified with volume setting but I've never heard a low frequency sound coming from just subs. Have you tried using 2 channels of your 4 channel amp to push the subs as a test to see if it's in the amp or in the lines into the amp? I'll bet it's s amp issue. Try tapping the heatsink to see if the noise changes or stops.

I hooked up my subs, subwoofer rca cables and sub's power lines to the 4 channel amp and didn't have any problems. I think that mono sub amp just doesn't have much filtering.

You said the noise continues even when the car is not running? Have you checked your battery voltage and voltage at the amps power connections? It could be under volting the amp and it's powering down due to low voltage not thermal cycling. What gauge wire, I only run 4 or 2ga wire to amps I'll double up on them if needed. I also use big fused distribution blocks in the rear near the amps for easy installation. But I'm one who runs an extra set of RCAs from the front to back in case I have an issue over years. It's so much easier just to run an extra one while the cars apart for the initial install. Wire is cheap compared to the time it takes to fix things. A fuse block by the amps are a must but need to be fused for the wires feeding the amp not the amps power draw. If your amp power wire shorts it needs a fuse at the front of your car near the battery rated for that wire as well.

The noise only appears when the car is running, so I think it's just alternator noise.

My setup is pretty amateur. I use double 12-gauge twisted together via a short 20A fuse-tap (Yes the tapped fuse is 20A already) for the mono amp and I use the stock amp wiring for my 4-channel amp. This setup provides enough bass for me honestly. Its a 1000w amp but I think it really only uses ~300w because of the wiring. Once in a while I pop the fuse because the amp and the wiring want more than 20A's. That 1000w amp puts out so much bass still on 1/3 power and it doesn't crap out like other amps.

Wires have internal resistance and they have a power loss according to the length, a loss of a volt or more is possible as well as max current handling it can carry safely but it has a max current it can carry at any temp. Meaning it can limit the amperage the amp gets and a 1000w amp plus a 400w amp will require a wire rated to carry 100 amps over the length of that wire (length decreases power rating) im basing this on using your stereo only while your car is running since your alternator will put out 14v.

I've seen my alternator go as low as 14.0 and as high as 15.3. I'm fully aware of the wire current length, as I keep my grounds very short.

Your alternator needs to be at least 160A rated minimum to safely keep your electrical system working. The chance of a stock Alternator putting out 160A at idle continuously is unlikely . Once they get hot and idle at a stop light you'll be lucky to get 60A. Add in a under sized wire to amps drawing 1400w that's 100A plus your cars normal power draw let's say 40A. I always start at the alternator output before considering the build of the system.

I think my alternator is only 100A but I did replace most of my bulbs with LEDs to bring down the Amp usage.

If you can't supply the power the amps will eventually burn up from undervolting them. I've told this to countless people right at the moment they installed their system. Bought amps from me that I used for 4 or 5 years without any issue and warned them to upgrade the alternator and all wires with way bigger amp wires than they ran and sure enough a month later they're asking to buy another amp that's even bigger! They don't listen and history repeats it's self. Typically after that 2nd amp dies they ask me about upgrading electrical systems. As I said it easiest to spend the money the first time and do it right. You're under volting the amps and probably have too small wire and poor grounds. Possibly even a speaker wire is grounding out on the sub amp. They can get cuts in the jacket while running wires to speakers. Any output wire that grounds to chassis will cause weird noises like that .

I'm not too worried about my subs and my amps dying, its a cheap setup. My subs are 20 years old and have been re-foamed at least once. For me, its good enough power overall, when its distorting its too loud anyways. 500w total is fine.