posted
Hey I was jsut wondering if anyone knows why it is that balanced rejects more noise than regular unbalanced. Is it just because it has really high common-mode rejection? Or some othe reason? Thanks Ash
-------------------- ****************************** ****************************** Posts: 1130 | From: Los Angeles, CA | Registered: Aug 2001
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posted
Before we can really get into balanced lines and it's advantages, we need an example. Network cards, good old rj-45 nic's. The have both a receiver and amplifier which share the same impedance. This is what makes it balanced. Due to this equal impedance and the fact we know of it, we can use simple twisted pairs of wire to carry a signal a very long way.
For example, a 6 foot cable will not necessarily be better than a 100 foot one. Most buildings have mazes of this cabling running throughout and they can still achieve amazing network speeds.
A balanced line simply rejects noise by comparing 2 sources.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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In a balanced line, if noise is introduced into the cable, it's introduced equally into both wires. The receiver sees the same signal on both wires, and sends it to ground.
Since audio is AC, and never shows the same voltage on both wires at the same time (there 180degrees out of phase), the receiver sees it, and passes it along.
Twisted pair is used because it has natural noise rejection properties, due to the twists.
The number of twists per inch determine it's noise immunity, but, too many per inch cause loss of signal...
posted
Thanks or the replies. Audiophile, we are talking about the same thing. Noise induced into BOTH lines is referred to as common-mode noise. Yesterday in one of my classes we went over a few balanced line amps, and I remember the prof. saying that they have VERY high common-mode noise rejection..so this makes sense.
So, if you want to run a completely balanced system, you would have to have a balanced pre-amp right after the HU, and also a specialized receiver on the other end, near the amps right? Doesn't balanced use special cable, with three conductors(2 signal and a ground)?
Emu, I don't think what you are referring to is balanced. Just because a source, transmision line, and receiver have the same impedance doesn't mean it's a balanced system. For instance, the phone system is 600 ohms all around(the output imp. of your phone is 600ohms and the characteristic imp. of the lines are also 600ohms. This isn't a balanced system though....they do tht because it minimizes reflections of signal in the line.
posted
One thing I neglected in Emu's post, if all the impedances are equal, nothing will come through, due to loading.
I've done a little investigation into this...
Ash- I didn't want to get too far into all that, I dont know peoples experience at first glance. Yes, they exhibit very high CMRR (common mode rejection ratio), and do require a preamp and "receiver" at the other end. Yes, generally, 3 wire per channel, 2 hot, one neutral (I think that's how they're referenced). It's actually pretty easy to build the preamp and receiver, if you wanna try. One thing I'd recommend is high voltage/low impedance out of the "receiver" unit, which could be tough to get in a car, unless you want to build a very large switching supply in the 40 or so volt range...
If you dont want to build the receiver part, some amplifiers have balanced inputs, you'd only need those amplifiers, and the pinout for the input, and a mate for the input plug.
I have several good schematics if you want a couple....
posted
Cool man, thans a lot, I'll take you up on your offer of schematics. Can you send them to my e-mail address? No, I don't feel like building a power supply like that, but I am interested...thanks for the offer Ash
-------------------- ****************************** ****************************** Posts: 1130 | From: Los Angeles, CA | Registered: Aug 2001
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posted
in a balanced there is a positive sig and a negitive sig. the receiver just subtracts one sig from the other and the rest is noise and send it to ground
Posts: 137 | From: wilmington, DE | Registered: Mar 2002
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posted
In balanced line the signal is split in to two identical signals, except they are 180 degrees out of phase, on two conductors. Any noise the sysem is exposed to will then be in phase on both conductors. On the other end, the out of phase element is reconstituted, and the in phase element is dropped. Very simple and elegent. You need a transmitter and receiveer.
punch
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