cfox10
maylar
10+ year member
Electrical Weenie
I've been wanting to post a FYI blog on this for a while... so this may be more than you wanted to know...
RCA outputs are single ended, ground referenced. The signal travels down the center wire of the cable and returns through chassis ground. The second conductor in an RCA cable is a grounded shield.
Radiated electrical noise is also ground referenced. Any noise that gets past the RCA shield becomes part of the signal. The circuits at the receiving end can't tell the difference.
Balanced lines are double ended, differential. The 2 sides of the signal are on separate wires, neither of which is ground. The circuits on the receiving end are designed to amplify the difference between the wires and reject any ground referenced noise that's common to them. For this rejection scheme to work, the noise picked up by both wires must be exactly the same. Any difference in noise becomes part of the signal. This is why most balanced line cables are twisted shielded pairs - twisting the wires close together helps insure that they're in the same noise field.
Having the plus and minus sides of the signal on separate wires requires a preout driver for each line. In pro audio it's done with dedicated driver circuits. In car audio it's done by redefining 2 of the RCA outputs (usually the rears) as the "minus" side of the signal. So the front RCAs are the plus side, and the rears are the minus side of the channel. You need 2 RCA cables per channel.
Combining 2 channels into 1 is exactly the same as bridging amp channels. You get double the voltage and no ground reference. The big advantage though is that the balanced line inputs will reject noise that's common to the 2 wires. It's important then to keep the + and - cabes close together so they pick up the same noise.
Now here's the geek part... //content.invisioncic.com/y282845/emoticons/nerd.gif.c6fa51ddf7ff75f1c0371fbc648f70ae.gif
What makes a balanced line "balanced" is that the impedance of the 2 wires to ground is exactly the same. A high impedance circuit will pick up more noise than a low impedance circuit (trust me on that). The 2 wires in a balanced line will pick up the same noise if and only if their impedances are equal. The IEEE definition of a balanced line is, "A system in which all signal carrying lines have the same impedance to a common point (usually ground)". This definition holds true whether we're talking audio, RF, digital data or telecom balanced lines.
HTH.
RCA outputs are single ended, ground referenced. The signal travels down the center wire of the cable and returns through chassis ground. The second conductor in an RCA cable is a grounded shield.
Radiated electrical noise is also ground referenced. Any noise that gets past the RCA shield becomes part of the signal. The circuits at the receiving end can't tell the difference.
Balanced lines are double ended, differential. The 2 sides of the signal are on separate wires, neither of which is ground. The circuits on the receiving end are designed to amplify the difference between the wires and reject any ground referenced noise that's common to them. For this rejection scheme to work, the noise picked up by both wires must be exactly the same. Any difference in noise becomes part of the signal. This is why most balanced line cables are twisted shielded pairs - twisting the wires close together helps insure that they're in the same noise field.
Having the plus and minus sides of the signal on separate wires requires a preout driver for each line. In pro audio it's done with dedicated driver circuits. In car audio it's done by redefining 2 of the RCA outputs (usually the rears) as the "minus" side of the signal. So the front RCAs are the plus side, and the rears are the minus side of the channel. You need 2 RCA cables per channel.
Combining 2 channels into 1 is exactly the same as bridging amp channels. You get double the voltage and no ground reference. The big advantage though is that the balanced line inputs will reject noise that's common to the 2 wires. It's important then to keep the + and - cabes close together so they pick up the same noise.
Now here's the geek part... //content.invisioncic.com/y282845/emoticons/nerd.gif.c6fa51ddf7ff75f1c0371fbc648f70ae.gif
What makes a balanced line "balanced" is that the impedance of the 2 wires to ground is exactly the same. A high impedance circuit will pick up more noise than a low impedance circuit (trust me on that). The 2 wires in a balanced line will pick up the same noise if and only if their impedances are equal. The IEEE definition of a balanced line is, "A system in which all signal carrying lines have the same impedance to a common point (usually ground)". This definition holds true whether we're talking audio, RF, digital data or telecom balanced lines.
HTH.
calebkhill
10+ year member
CarAudio.com Elite
Nice.
I believe this video describes it. Interesting and the dude taking seems to know what he's saying.
Look for the "balanced imput" video here.
Audio basics: videos
@maylar
Let me knew if the video its inaccurate as most of the reference is in the context of recording studio wiring, though I think the same concepts apply.
I believe this video describes it. Interesting and the dude taking seems to know what he's saying.
Look for the "balanced imput" video here.
Audio basics: videos
@maylar
Let me knew if the video its inaccurate as most of the reference is in the context of recording studio wiring, though I think the same concepts apply.
maylar
10+ year member
Electrical Weenie
Video:
Ugh. Totally wrong. Balanced lines have NOTHING to do with phase. That's a very common misconception.
Yes, the + and - sides of the signal are 180* out of phase. But that's not the "magic" of a balanced line. A balanced line input is differential. It will subtract ground referenced noise that's common to both wires (if the difference in noise is zero, there will be none on the output). The key factor is that the impedance of the two wires to ground are equal (balanced). This concept seems to escape even the pros.
calebkhill
10+ year member
CarAudio.com Elite
Explain this.
I don't understand how impedance its related to wires, and what it means on the context of balanced signal.
maylar
10+ year member
Electrical Weenie
It's not the wires, it's the source. Every signal source has an impedance to ground. In pro audio they take care to control that impedance at 600 ohms. A balanced line has the same source impedance on both the plus and minus sides of the signal. That's what makes them balanced. Only if their impedances are the same will the 2 wires pick up the same noise so the diff amp can subtract it.
In car audio the source impedance of preouts is typically a couple hundred ohms, but it's not controlled. It used to be part of HU specs but I don't see it listed any more. The CA community came to understand that "lower is better" when it comes to source impedance because lower resistnce picks up less noise and can drive long cables without HF losses. The head units that support balanced line configuration are high end decks and the source impedance of the preouts is really low - like 50 ohms.
maylar
10+ year member
Electrical Weenie
Just to bring home the concept of balanced lines in car audio -
Suppose you have a deck and amp that both support balanced line configuration. You set the HU to balanced mode and the rear preouts become the minus sides of each channel. You run 4 RCA cables, 2 per channel, to the amp's 4 inputs.
Now you want to connect a second amp, not balanced inputs, to the HU. You tap into the plus cables with Y adapters. Will this work? Yes. Will you notice a difference? Probably not.
However...
The system is no longer "balanced". The extra load on the plus side preouts changes the line's impedance. It's now physically impossible for both + and - cables to pick up the same noise. They have different impedances.
From a practical standpoint this is insignificant, because an extra amp input won't load the preout enough to make much difference. I just wanted to make the point clear.
There is one important detail about this scenario though - any noise picked up by the extra amp wires will not be rejected by the balanced line inputs because it only appears on one side of the line.
Suppose you have a deck and amp that both support balanced line configuration. You set the HU to balanced mode and the rear preouts become the minus sides of each channel. You run 4 RCA cables, 2 per channel, to the amp's 4 inputs.
Now you want to connect a second amp, not balanced inputs, to the HU. You tap into the plus cables with Y adapters. Will this work? Yes. Will you notice a difference? Probably not.
However...
The system is no longer "balanced". The extra load on the plus side preouts changes the line's impedance. It's now physically impossible for both + and - cables to pick up the same noise. They have different impedances.
From a practical standpoint this is insignificant, because an extra amp input won't load the preout enough to make much difference. I just wanted to make the point clear.
There is one important detail about this scenario though - any noise picked up by the extra amp wires will not be rejected by the balanced line inputs because it only appears on one side of the line.
calebkhill
10+ year member
CarAudio.com Elite
OK Maylar. I think I took a few things from this.
i'm guessing its in a cables design of what the impedance of the line will be. Correct?
Now, a balanced line has the same impedance on +-, therefore the noise is rejected equally, canceling it out. Is this what's referred to in the video? The process of noise rejection happens at the amp correct? And the flipping of phase on over side of the line cancels the extra noise, given the impedances are equal. Correct me if i'm wrong.
now. You mentioned that if either side isn't equal, or you split the signal, the impedance is unequal therefore cancellation of noise won't take place.
Let me ask, when the signal is unequal, will it just output a difference in noise, out will the total sum of noise still pass through.
i'm guessing its in a cables design of what the impedance of the line will be. Correct?
Now, a balanced line has the same impedance on +-, therefore the noise is rejected equally, canceling it out. Is this what's referred to in the video? The process of noise rejection happens at the amp correct? And the flipping of phase on over side of the line cancels the extra noise, given the impedances are equal. Correct me if i'm wrong.
now. You mentioned that if either side isn't equal, or you split the signal, the impedance is unequal therefore cancellation of noise won't take place.
Let me ask, when the signal is unequal, will it just output a difference in noise, out will the total sum of noise still pass through.
maylar
10+ year member
Electrical Weenie
No, it's the design of the preout circuits that determine the line impedances. Wires don't matter.
The video claims that it's the phase difference between plus and minus wires that causes noise cancellation. That's totally wrong. The input circuit in a balanced line is a differential amplifier. They are designed to amplify the difference between the 2 wires. If the noise on both wires is exactly the same then the difference in noise will be zero, and none will appear at the amp's output. The noise is subtracted out, or essentially cancelled. Again, that's why we use twisted pairs with balanced lines so that both wires are in the same noise field and pick up the same noise.
The 2 wires having the same impedance is a requirement for picking up the same noise.
The noise rejection happens in the amp's input circuits, yes. The phase flipping is needed in order to make a minus side signal that's not ground. It adds to the plus signal (algebra: subtract a minus and it becomes a plus). But that does NOT cancel noise. (You need to stop watching that lame ass video).
The difference.
calebkhill
10+ year member
CarAudio.com Elite
I think I got it.
But, when we go back to the pre amp circuit, all a pre amp circuit will do in this conversation is to determine a wires impedance. Which leads to a question,
hu are designed to give equal impedance in pre out circuita correct? Or are not all designed to do this.
.... Since that is what is needed to create a balanced system to begin with. Equal line impedance.
Now. You already said that a signal is balanced at the amp, not the hu, which clears up alot of confusion.
it happening at the amp supports the fact that twisted wires are used.
So overall, we need a "balanced capable" amp, and a hu that gives each line equal impedance, to have a balanced system.
With that being said.
To get it together... a single RCA has a +-. Given the line impedance is equal from the hu, the signal reaches the amp. The amp then flips either +-, amplifying the difference, in this case, since the entire signal is opposite, flipped, canceled...., the difference in signal is then amplified, which means the +- is now amplified as a whole, and since the noise is foreign and is ++ on both lines, it is now --, canceled, since the balanced amp is amplifying the difference.
That last part was probably explained by me real weird lol.
But correct me in anything I said
. I'll be running an all zapco setup, so balanced systems are on my mind at the moment.
But, when we go back to the pre amp circuit, all a pre amp circuit will do in this conversation is to determine a wires impedance. Which leads to a question,
hu are designed to give equal impedance in pre out circuita correct? Or are not all designed to do this.
.... Since that is what is needed to create a balanced system to begin with. Equal line impedance.
Now. You already said that a signal is balanced at the amp, not the hu, which clears up alot of confusion.
it happening at the amp supports the fact that twisted wires are used.
So overall, we need a "balanced capable" amp, and a hu that gives each line equal impedance, to have a balanced system.
With that being said.
To get it together... a single RCA has a +-. Given the line impedance is equal from the hu, the signal reaches the amp. The amp then flips either +-, amplifying the difference, in this case, since the entire signal is opposite, flipped, canceled...., the difference in signal is then amplified, which means the +- is now amplified as a whole, and since the noise is foreign and is ++ on both lines, it is now --, canceled, since the balanced amp is amplifying the difference.
That last part was probably explained by me real weird lol.
But correct me in anything I said
. I'll be running an all zapco setup, so balanced systems are on my mind at the moment.
Very good explanation in this thread, but I'll throw in some fake math in case it helps someone.
Regular RCA's are just a power and ground wire rolled up into a single wire, positive uses the plug, negative runs outside that wire and grounds to outside edge of the rca plug. Lets call the positive signal X and the negative Y. Any extra noise from the outside, lets call that B. If any noise makes it into the X signal, so X+B, it's going to be ampified.
A balanced line uses 2x the rcas. Each RCA is a positive and a negative seperately. So each plug now has a X in the center and a -X on the outside. The other plug would be Y and -Y. The noise however is still just going to be B. Since the balanced plugs reject anything that isn't the inverse of the original signal, Y and -Y are amplified, but the B component that will be in both, will be the same polarity and will NOT be amplified, it's +B in both the -Y,Y and -X,X fields. Impedence comes into play because without equal impedence, the MAGNITUDE of the signals won't be the same in each wire. Same voltage going down each wire without equal impedence obviously isn't the same thing.. You won't have a x and -x nor will the B's be exactly the same.
Regular RCA's are just a power and ground wire rolled up into a single wire, positive uses the plug, negative runs outside that wire and grounds to outside edge of the rca plug. Lets call the positive signal X and the negative Y. Any extra noise from the outside, lets call that B. If any noise makes it into the X signal, so X+B, it's going to be ampified.
A balanced line uses 2x the rcas. Each RCA is a positive and a negative seperately. So each plug now has a X in the center and a -X on the outside. The other plug would be Y and -Y. The noise however is still just going to be B. Since the balanced plugs reject anything that isn't the inverse of the original signal, Y and -Y are amplified, but the B component that will be in both, will be the same polarity and will NOT be amplified, it's +B in both the -Y,Y and -X,X fields. Impedence comes into play because without equal impedence, the MAGNITUDE of the signals won't be the same in each wire. Same voltage going down each wire without equal impedence obviously isn't the same thing.. You won't have a x and -x nor will the B's be exactly the same.
calebkhill
10+ year member
CarAudio.com Elite
And another thing. Say you don't have a balanced hu, but a balanced amp.
Even though you won't reject ALL noise, you can still atleast cut down on noise but getting the difference.
Lastly for now. What is noise?!
Even though you won't reject ALL noise, you can still atleast cut down on noise but getting the difference.
Lastly for now. What is noise?!
maylar
10+ year member
Electrical Weenie
No. HU RCA outputs are ground referenced. The minus side of the signal (the RCA shield) is ground. A ground wire can't pick up radiated electrical noise. So if the noise is only on the plus side of the signal, it won't be rejected. That's why using twisted shielded pair RCA cables in car audio is a joke. One of those twisted wires is just a ground wrapped around the signal wire.
AC voltages that get induced into the cables. Static, hum, alt whine, engine ignition noise, electric motor sparks.. lots of noise sources in a car.
calebkhill
10+ year member
CarAudio.com Elite
So the hu has to have balanced outputs.
What about the Zapco amps, and the RCA to symbilink cables. Those adapter cables won't create a balanced input?
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