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running RCAs by power


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dwarren 
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Posted: October 26, 2005 at 9:53 PM / IP Logged  
Right, so here is the other side... "In a vehicle, the ground plane is the ground plane. Floorpan, chassis/frame, negative battery terminal...all the same (for all intents and purposes) electrically, the path of the positive current is different.
An alternator produces "dirty power." It's not filtered or smoothed like say, a laboratory-grade power supply. Running a low-level signal (RCAs) near a cable that's essentially connected directly to the alternator CAN result in a plethora of different noises being picked up. "Static", "ticking noises" etc."
I've experienced this myself. I have even run all the cables down the same side, experienced the noise, then simply moved the RCAs over a foot or so just to experiment and the noise went awa
KarTuneMan 
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Posted: October 26, 2005 at 10:12 PM / IP Logged  

Its not that technical, audio cables WILL pick up electrical noise from ANY sorce, including the little bit of factory wiring it might come in near proximity too.

As dwarren stated....move the RCA's and the noise problem is solved. Its really that simple. no need for "o-scopes" or scientific mumbo jumbo chat.

DYohn 
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Posted: October 26, 2005 at 10:15 PM / IP Logged  
OK, who told you that DC does not induce a magnetic field?  They are wrong.  Who told you that DC-produced magnetic fields cannot induce EMI into other wires around them?  They were wrong.  EMI can be just as bad in a DC system as it can be in an AC system.  It's called physics.  Now it is true that most good quality RCA cables use sufficient shielding to be relatively immune to EMI and that noise from ground loops is generally a far more significant problem than EMI, but thinking that DC power, because it's not AC, does not induce voltages into adjacent cables is just plain wrong.
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haemphyst 
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Posted: October 26, 2005 at 10:55 PM / IP Logged  
OK... here comes Dave.
The DC COMPONENT, IN AND OF ITSELF, WILL NOT, AND CAN NOT, CAUSE NOISE. That is the physics, DYohn. The AC ripple ON the DC, caused by the alternator, CAN cause noise, so everybody that has posted so far is correct, to an extent.
Imagine a transformer. All a tranformer is is two (or possibly more, but lets keep this simple) wires wrapped around a core, right? If you put an AC current into the one side, you get a corresponding AC current output on the other side. This is simply putting EMI to work for us. If you feed DC into that same transformer, do you get a corresponding DC on the other side? No, because the DC sets UP a magnetic field, but because there is no collapse of magnetic field, there is no EMI coupling - there can't be.
Additionally, the DC component cannot cause noise UNLESS THERE IS A CURRENT FLOWING THROUGH THE POWER CABLE. The current is what makes the magnetic field, and causes the noise. Since this is the case, there will not be, and can not be, noise induced when the system is idling - there is simply not enough current in the power cable to induce the noise onto the RCA cables. A static voltage, cannot ever produce a magnetic field - current is required. Take the same transformer discussed above, and disconnect the AC neutral from the one side. There is still POTENTIAL (the voltage component of power) in the coil, but because there is no CURRENT in the coil, there is no alternating magnetic field, therefore no EMI, and no primary magnetic field or secondary output...
As you turn the system up, the current demand will increase, causing a stronger magnetic field to be produced around the power cable, potentially inducing a voltage on the RCA signal wire.
I have RARELY, if ever, had luck in separating the power cable from the RCA, and in the few times I DID have a little bit of luck - i.e. a slight reduction in noise level - it was discovered that the battery (a big ol' noise filter, people) needed replacing or the alternator was in some way defective.
There's MY two cents again...
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
dwarren 
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Posted: October 26, 2005 at 11:11 PM / IP Logged  

haemphyst wrote:
it was discovered that the battery (a big ol' noise filter, people) needed replacing or the alternator was in some way defective.

Yes, so I have heard the battery acts as a filter, and can be biggest problem in a system. This is where most of the nosie is created.

boulderguy 
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Posted: October 26, 2005 at 11:15 PM / IP Logged  

haemphyst wrote:
Take the same transformer discussed above, and disconnect the AC neutral from the one side. There is still POTENTIAL (the voltage component of power) in the coil, but because there is no CURRENT in the coil, there is no alternating magnetic field, therefore no EMI, and no primary magnetic field or secondary output...

I was following everything fine until we got here.  Think I'll politely excuse myself now...

DYohn 
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Posted: October 26, 2005 at 11:31 PM / IP Logged  

As the current flow through a cable in a DC circuit changes, the magnetic field it generates changes.  It is the changing magnetic field that can cause EMI in an adjacent circuit, just like in an AC circuit.  Of course there is no changing magnetic field as a system is "idling," as there is no changing current flow (or no current flow at all.)  But in an active car audio power system, the DC current draw is changing constantly.  This causes constantly variable (expanding and collapsing) magnetic fields around the DC power cables, which can induce noise (EMI) into other circuits.  It's been more than 20 years since college but I believe I still remember applications of Faraday's Law.....

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KarTuneMan 
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Posted: October 26, 2005 at 11:38 PM / IP Logged  
Hey when you guys figure it out......make sure you let the rest of us know.....Krunning RCAs by power - Page 2 -- posted image.
haemphyst 
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Posted: October 27, 2005 at 12:16 AM / IP Logged  
The changing of the magnetic field in the power cable is always a positive - sometimes a little more, sometimes a little less - but ALWAYS positive. There is never a SWING from positive THROUGH zero, to negative.
DYohn... Of course you are right, but why is it, then, that when there is the LEAST amount of (desired) sound, the noise (undesired sound - the alternator whine) is at it's highest? Yes, the undesired sound WILL be eventually covered up by the desired sound, but while this is happening, the current through the DC power cable is increasing, but the undesired noise is NOT increasing at a constant level (and this I HAVE demonstrated with an o-scope) - if it were, the noise would be as loud as the music, or potentially louder, at some frequencies...
There is simply not enough change in magnetic field with MUSIC signals to produce the necessary magnetic fluctuations required to induce a voltage high enough to go to the effort of tearing apart the entire car to run power down one side, and the signal down the other. There ARE fluctuations, but they ARE always POSITIVE. There is NEVER a negative swing - required to cause a total collapse of the magnetic field, inducing the voltage on the secondary - in this case the positive signal wire contained in the RCA wire.
In my experience, ALMOST ALL noise induced into signal wires does not come from amplifier power cables, but from other electronics in the car - ECUs, fuel pumps; things like that.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
dwarren 
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Posted: October 27, 2005 at 1:02 AM / IP Logged  

haemphyst wrote:
In my experience, ALMOST ALL noise induced into signal wires does not come from amplifier power cables, but from other electronics in the car - ECUs, fuel pumps; things like that.

Perhaps I missed the explanation for this, but why then would it matter if the electrical component is not an amp, rather an ECU or fuel pump, how does it vary?

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