zoomer

iTrader: (3)

I have to comment on what you say. In single ended system, there is no "cancelling" of noise on the signal and ground cables. Ground is ground it is supposed to be zero. The noise protection comes from the ground shielding the signal because it is surrounding it.

Also confusing this is the term coaxial. This is a configuration of conductors that alows for specific characteristic impedance of the cable. This is only important in Radio Frequency systems where the wavelength is the same or smaller than the length of the cable. This is not relevant in audio where the wavelength at 20Khz is very very long compared to the 20ft wire lenght in cars. What is relevant is the lumped capacitance or inductance of the cable, which is usually quite low but not insignificant and can result is some audio effects, especially in longer speaker wire. So in the coaxial cables used in satelite and cable TV, the dimensional accuracy and stability is important. But for a car audio coax shielded cable the dimensions don't have to be that accurate.

Car audio systems are unbalanced systems. A single ended preamp in the head unit drives a single ended input in the power amp. The signal line is the center conductor of the RCA inputs and output connectors and the ground is the outer conductor. When using coaxial cables the ground conductor also acts as a shield to protect against noise. The ground tries to carry any induced current away and keeps it from the center conductor. However, as many have experienced, if the ground of the HU is not the same as the ground of the PA because of poor chassis grounding, then the 2 ends of the cable are at different potentials and a current will flow. Since the ground shield is not perfect conductor you will have a voltage induced and that causes audible noise. Some car amps have isolation circuits to reduce this noise. Note that the outer shield of a coax cable does not protect against MAGNETIC fields. Shields are made of copper or aluminum braid, or aluminum foil. Those metals are not good magnetic shields. You need materials such as Mumetal to shield from magnetic fields.


In a balanced system, found on pro audio (the famous XLR connector!) both conductors are driven opposite polarity and the receiving amp is also balanced. Any noise signal that is finds its way onto both wires (common mode noise) get canclelled out by the amplifyer input circuits. A ground shield helps also but would be connected to one or both chassis grounds and none of the conductors is connected to ground. Twisting the wire ensures that both conductors are exposed to the same EM field making the noise the same on both, thus again *common mode noise*

So why unshielded twisted pair in unballanced car systems? I dont know!

For a more detailed talk see: http://www.bluejeanscable.com/articles/balanced.htm

zoomer

iTrader: (3)

You can understand why people are confused over twisted pair:

Compare the previoius link I gave on balanced vs unbalance to this link from MTX

http://www.mtx.com/caraudio/archive/...es/twisted.cfm

It is complete nonsense as the ground wire in both the coax and the twisted pair should be ZERO! Even if it did have a signal on it from noise, the unbalanced amp input circuitry would not do anything with it!

The fact that the shield of the coax has more strands than the center conductor is totally irrelevant as it does not carry signal!

You see this sort of BS in so many audio ads:
Look here! http://www.pearcable.com/sub_products_comice.htm

goalie 35

good read.. but we all know there is only one really great football team "GO RIDERS"!!! LOL

Haunz

iTrader: (3)

JL audio uses differential inputs.. some JBL and Zapco amps use balanced inputs..

I don't know of any more off the top of my head... but in most cases its probably advertised as it is a 'premium feature'...

Dukk

Zoomer - the explanation in the MTX link is exactly why twisted pair cables work on an amp with differential inputs. It may not be overly technical but it is dead on. Differential inputs were developed because it's a single ended system. Balanced would be nice but there have been only a few awkward examples over the years so it's not happening for car.

Exactly. So when the differential inputs see something, ie radiated noise, on the 'ground' wire, they cancel it against the signal wire, eliminating it, and leaving only the original signal on the signal wire.

Ground loops are a different discussion as neither a twisted pair or coaxial cable are effective against them.

How is coaxial a confusing term? By definition the cables share an axis, thus one is inside the other..

MTX and Rockford both have differential inputs on their amplifiers.

Junkinthetrunk

iTrader: (2)

I build my own RCA's using RCA ends (doh!!) and TV co-axial cable. A bit stiff but I noticed I get better stage depth and width with this type of DIY-er cable design.

I've made a few sets for friends and they luv them. Even done a few for home audio gurus. If anyone is interested, i'm selling custom made sets with solid Platinum nickel RCA ends for $1000 a foot. Once you try these, you will NEVER use anything else ever again. If you PM me, I have a 1-900 number you can call me directly at. $3 a second charges apply but it's well worth it to talk to me. Dukk actually has one of my sets and we're thinking of partnering up and going to CES with it next year. So you better get in now before the price goes up even more.

Binkey

Will you give me a discount if I buy 3 sets?

Dukk

^ 2% but you have to get a forehead tatoo.

I've actually sold TV coaxial cable for audio use.. Easy retail option for that 50foot home theatre subwoofer run. They even make RCA ends that screw right onto the standard F ends DIY FTW!!

crazy_mtb_2002

i've always bought the best rca's future shop carries, haha

Dukk

^ Then you've wasted money as they are the largest Monster Cable dealer in NA.