37 replies to this topic

[trism]

I may have this the wrong way, but speaker wires are high volt ,low current, where as rca cables are high current, low voltage...

or is the the opposite?

or am i tripping out?

[Big_Valven]

Low level (RCA) signals are low voltage and low current, whereas speaker signals are high current and high voltage.

You're right in saying that even low level signals are in fact amplified. However the amplification they use is quite different to speaker amplifiers.

The preamplifier in your headunit has a class A or "single ended" amplifier in it, which means that one set of transistors amplifies the wave as it alternates from positive to negative. This is referenced to ground, meaning that at any one time the signal may be positive or negative when compared to ground. In an RCA cable the inner wire is the signal and it is compared to the ground which is the shield. Because the shield is grounded to the rest of the audio system, any interference is dissipated not into the signal inputs, but through the system's ground and hence through the rest of the car. You should also be able to see now how coaxial "antenna" cable can in fact be used as audio signal cable as well.

But for speaker level signals:

Most car amplifiers are class AB "push-pull" configuration, where one set of transistors take care of amplifying the positive part of the wave, and another set amplifies the negative part of the wave. Each set have their own output, and they are not referenced to ground, but are referenced to each other. So by grounding one of them, you effectively short out half of the amplifier, causing damage.

Because speaker level signals are high voltage and high current, any interference induced into them is absolutely negligible because it is typically high voltage and low current, and does not have enough energy to change the signal going to the speaker. If it did, you would hear noise from your speaker even if the amplifier was off.

As for line out converters that convert speaker level to signal level, it's harder to explain but they essentially "cheat" when it comes to reducing the signal. What they end up with is a signal line, and a shield line which, as you guessed, are essentially reduced speaker signals, so instead of referencing to 0 volts as ground, the signal references to it's opposite, which still creates a difference that represents the original signal however it's also the same reason that LOCs are considered a low quality way of converting signals.

I've tried to keep it all as simple as I could while still explaining it properly, feel free to ask for more detail if you want.

It's great to see people who will engage in proper technical conversation about this sort of stuff. At the end of the day it's all about helping people understand all of this and it's great that people are willing to listen.

[bmerigan]

Here's a cool link on making your own high quality RCA cables:
http://www.tnt-audio.com/clinica/diycables.html

and in particular:
http://www.tnt-audio.com/clinica/diyce.html

Edited by bmerigan, 17 September 2007 - 05:48 PM.

[Big_Valven]

Haha after all is said and done...

... ahhhh, the world of cables.

At the end of the day I go back to my $2/mtr shielded cable and 2400w mains flex for speakers, and laugh at the people who characterise cables by their exotic construction and supposedly "different" sound. It's honestly pathetic.

But I do hope the above has helped people understand things better.

[davesmith]

man i checked out that tnt website and the amount of people who are slightly crazy about there RCA's is crazy.

Has anyone heard or uses a XLO-Geometry RCA is it worth the hastle and if i wanted to hide my (slightly tacky) Altronics cable does anyone know where i can purchase coax cable cloth or some kind of cover for my coax.

[slow as]

which of these 2, fits the d.i.y. rca best?

Belden Brand Quad Shield RG-6 75 Ohm Coax

Nom Imp (ohm): 75
Nominal Cap.: 16.2pf/ft, 53.1pf/mt



Single Core OFC Screened Audio Cable

Capacitance: 66.8pf/1M
Impedance: 65ohms@1m

both of these taken from jaycar.com.au.

i'm presuming that those stats determine which one suits better, any chance you could explain?

[RMA]

Hey guys this is excellent reading without any BS, congratulations I feel I am actually learning something worth while.



Regarding Balanced lines there are a couple of companies using this technology, Eclipse uses it on their very top end where it leaves the head unit in a balanced format and is converted back to a normal signal in a unit installed a short distance from the amp, works in theory but in some cars that have a noise issue at the rear due to computers etc the noise is picked up and then transfered to the amp.



Audison released a balanced system some years ago that is quite ingenious, they developed an RCA with two collars and one hot pin so it looks on the surface to be a normal RCA, the beautiful part is that the female receiving socket accepts these "Double Ring" RCA's as well as standard RCA connectors so there is no need for extra connectors. The system they use takes a normal signal and converts it to balanced, this can be done right up front or 20cm from the amp, no difference, but where the Elettromedia system works better is that the amplifiers are designed to accept and process this balanced input.

In short we have had some noisy cars that were running VRx amps, switch them to balanced and hey presto nice clean signal.

[~thematt~]

pf stands for pica-farads. Or 1.0x10^-12 Farads (0.000000000001 Farads). Its fairly small as you can see. Ultimately, you would want the cable to have the least amount of influence on the signal, so Capacitance would ultimately need to be quite low. The lower the better.

However, given the numbers are already quite small, compared to the systems they are attached to, they will not contribute very highly to the overall quality of the signal.