QuestionI have an alpine v12 600rms digital amp and only 2 working preouts on my current deck (old kenwood) but 3 amps. my 2nd amp mtx 400.2 3rd kicker200.4. the alpine amp has a L&R pre out so i connect alpine to deck and kicker to alpine. But for some reason I'm getting a humming in 6x9's connected to the kicker, not if I connect it directly to deck, and only in the it's spkrs(6x9s)when connected to alpine preouts. alpine and sub are both fine.(pioneer premier champion pro 2500w 800rms) this is a new issue too.... 3rd amp mtx400.2 with MA 15" 1000peak not running due to next issue and lack of preouts...
Also I have a pioneer DEH-P3000IB and with my system all connected working fine, I re arranged my subs and ruined my rca cables in the process maybe shorting them? not sure. now the deck hums from all preouts any ideas why? this may be related to the alpine preout humming??? I have a good ground and good rca's now I did my best to check the wiring all looks good. It all worked fine prior to my rearranging the subs, I'm at a loss. Can shorting the rca's cause permanent damage like that? what happened is the end of the rca cables were loose I guess and when I pulled them from the amp the gold male end came right off both ends so if that was shorting before, it was loose??? The regular speaker wires all work fine no humming. And yes when I start the engine it gets worse for both issues! unplug rca's all is good. I think that issue is the deck but as for the noise from the alpine pre out? I hope I didn't ruin it too. I don't hear any humming from the 10" connected to the alpine wheather or not i have the kicker connected to the preout... Did I screw up my preouts? and what do u do if u have multiple amps with limited preouts? I don't really want to split the rca's I'll lose voltage to my amps and only have 2v I think so??? Thanx in advance!!!
AnswerHi Scott,
Let me start with a little background on how line-level RCA cables work.
The RCA plugs on your amplifier are known as "single-ended" inputs. That means that the audio signal is carried on the center conductor and pin, in the form of AC voltage. The outside shield is known as "signal ground" or "reference ground". If the audio signal is alternating between +2V and -2V, there has to be a stable 0V reference, and that's the function of the shield conductor.
If the signal ground is unstable or not present, then the result is noise. In a car audio system, it's very important that there's only one signal ground--if the ground is different between the head unit and amplifier, then it shows up as noise in the audio signal. This is what is known as a ground loop. It's actually very uncommon these days, because amplifiers are usually designed with the audio signal ground isolated from the amplifier's chassis ground. In the old days, each component used its own chassis ground as the signal ground; this often resulted in ground loops, because it's almost impossible for two components, placed far apart, to have exactly the same chassis ground potential in a vehicle.
In modern car audio, there's only one component where the signal ground is linked to the chassis ground, and that's the head unit. The head unit's ground becomes the reference signal ground for the entire system. In all the other components, the signal ground is kept isolated from the chassis ground, so the only signal ground connection is the one at the head unit. This usually works fine, except under two conditions: if the signal ground path is broken at some point in the signal chain, or if there's a problem with the signal ground at the head unit. In your system with the Pioneer deck, I think you're experiencing both of these conditions.
Let's start with the Kicker amplifier, with its RCA inputs connected to the Alpine amplifier's outputs. The signal ground on the Alpine RCA outputs should be isolated from the amplifier's chassis ground, and connected directly to the signal ground on the inputs. Apparently, that's not happening. I think there's an internal disconnect in the Alpine amplifier, so you've got a "floating" signal ground on the Kicker's RCA inputs. That's why you're getting noise. You can try this: take a couple short lengths of wire, of any gauge. Strip the ends and wrap them around the shields of the RCA cables plugged into the inputs and outputs of the Alpine amp. In other words, you're wiring the RCA shield of the input cable directly to the RCA shield of the output cable at the amplifier. Take care that the wire doesn't touch the center pin of the RCA, or the case of the amplifier. I can't promise this will work, but it's worth a try. Otherwise, you can fix the problem (as you've found) by using RCA splitters for the signal to the Kicker amp, instead of the Alpine's RCA outputs. (I'll get to your reservations about RCA splitters in a bit).
With the Pioneer head unit, you have a very similar problem, and it's one that's very common in Pioneer decks. As I said before, in a head unit the signal ground is connected to the head unit chassis ground. Pioneer does this with a very thin, very fragile internal trace. Just about any current through this conductor can burn it out; and this often happens when you're working with the RCA cables on the other end. I think your Pioneer deck has internal damage that has "disconnected" the RCA signal ground from the chassis ground. You're ending up with a "floating" signal ground throughout the system, which is resulting in noise on all the components.
As with the Alpine amp, the ultimate fix is to have the unit serviced; as with the Alpine amp, you can try correcting the problem by restoring the ground connection. To do this, wrap some wire around the RCA shield conductor on each of the RCA cables plugged into the back of the head unit. Combine the other end of each of the wires, and crimp on a ring terminal. Fasten the ring terminal down to the screw hole in the middle of the back plate of the head unit. You might have to add a second wire, from the back plate to chassis metal in the vehicle. Again, this isn't guaranteed to work, but it's worth a try.
Finally, a word on RCA splitters: when you use a splitter on the RCA cable, you are not dividing the signal voltage. An RCA splitter is basically a parallel connection, and a parallel connection does not reduce voltage. (For example, look at the fuse box in your car. Each of those fuses is ultimately connected to the 12-volt battery terminals. The main connection at the battery terminal has been split dozens of times; each fuse represents a different split, but each fuse still has 12 volts present). It's true that each additional component added to the RCA outputs will increase the current draw from the RCA outputs. The extra current will interact with the head unit's output impedance to create a slight voltage drop; but if you're just splitting the signal one or two times, the voltage drop won't be significant.
Hope this helps!
Brian
