Amp plays static when rca's connected to sub amp

[DouglasABaker]

Hi all,

I have a strange problem and I'm looking for any help to try and diagnose.

I have a pair of MTX Blue Thunder amps - a 504 (4 channel) and a 3002 (2 channel).

I have been using the 504 by itself for a couple months with absolutely 0 issues (no alt whine, etc) - 8 ga power / ground, relayed remote, rca's from the stock head unit.

I recently tried to wire the 3002 to power the subs - 4 ga power / ground, relayed remote, and rca's from the passthrough of the 504.

When I turned it all on the 504 stopped playing music and instead played static (and alt whine if the car was running). The static was loud and changing the radio volume had no impact.

I also found that the 504 started playing music again if the rca's were unplugged after it was turned on. Also, plugging the rca's into the 3002 after the 504 is powered on works ok. Interestingly, there is no alt whine or other noise if the rca's are connected to the 3002 after the 504 is on - it works exactly as expected. Last point is that it makes no difference if the 3002 is powered on or not - if the rca's are plugged into it when the 504 is turned on then I get static.

I've done as much troubleshooting as I can, and I simply can't find the issue so I'm looking for thoughts. I suspect a short, but don't know if it is in the rca passthrough portion of the 504 or the rca input section of the 3002.

Any help is greatly appreciated, and sorry if I missed any relevant facts.

Also, if anyone knows a good amp repair/refurb place in New England I would greatly appreciate it - MTX no longer services Blue Thunders...

Doug

[William from NOLA]

What if you connect the RCA's straight to the 3002 and not plug in the 504? You may be able to eliminate the inputs on the 3002 by doing so.

What about your grounds? How are they connected?

[GlassWolf]

run RCAs from the head unit's sub outs to the sub amp.
don't use the pass through of the amp.
see what happens

[DouglasABaker]

run RCAs from the head unit's sub outs to the sub amp.
don't use the pass through of the amp.
see what happens

What if you connect the RCA's straight to the 3002 and not plug in the 504? You may be able to eliminate the inputs on the 3002 by doing so.

What about your grounds? How are they connected?

<smacks head with hand>

Yeah, that would have been a smart thought, huh? I'll try that this afternoon.

As for the grounds, they are spade terminals that are attached to a sanded portion of the body (no paint, no primer) that measures 0.1Ohms resistance to battery negative.

Thanks guys - I'll be back later...

Doug

[cargodz]

Might want to look into a better grounding source while your at it... body grounds aren't the best, try finding something on the frame.

[DouglasABaker]

Might want to look into a better grounding source while your at it... body grounds aren't the best, try finding something on the frame.

Thanks for the advice :) I'm not sure how much better I can make it since it is a unibody vehicle (no frame) and the battery negative is attached to the same piece of sheetmetal
Doug

[GlassWolf]

actually body grounds are fine and equiv. to a 4AWG connection to the battery.
The frame isn't the best option since most cars are unibody today anyway, and the frame only runs half the length of the car.

if you need more than a 4AWG ground point, just run 1/0 straight to the battery for a ground, or mount a dedicated battery on an isolator in the trunk with the amp(s)

[DouglasABaker]

run RCAs from the head unit's sub outs to the sub amp.
don't use the pass through of the amp.
see what happens

I tried it this afternoon.

Results:

When either set of RCA's from the head unit is connected to the sub (I have F/R and L/R, but no sub out) it works fine.

If either set of RCA's is connected to the 504 and passed through to the 3002 everything works fine.

If both sets of RCA's are connected to the 504 alone, everything works fine.

If both sets of RCA's are connected to the 504 and the 504 is connected to the 3002 then I get static and very deep pulsing (like a heartbeat).

If either set of RCA's are connected to the 504 and the other to the 3002 then I get static and very deep pulsing (like a heartbeat).

I'm honestly baffled: I can't imagine why the RCA's work when connected to a single amp but don't function when connected to different ones.

Oh, I also tried using a different source (Ipod). All 3 sets of inputs (F & R on the 504 + the only set on the 3002) function just fine, and so does the pass through function of the 504 to the 3002.

Help!!!!

d-

[DouglasABaker]

Just to be sure, I did what you suggested. It turns out my connection to ground was unstable so I sanded off some additional paint.

On the upside, the tiny bit of alternator whine I have with just the 504 is not gone - thanks! :)

On the downside, the original problem with the RCA's is still present...

d-

[William from NOLA]

That's really strange.

Sounds like the headunit might be having an issue with two amps like that. I'm not sure what to tell ya.