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Topic: Differential Circuits and Balanced Lines (Read 1428 times) previous topic - next topic
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Differential Circuits and Balanced Lines


I was searching where come this information. I read this circuits and lines reject the noise by the following equation and duplicate the signal output, but I can't find where read it.

Phase voltage: V+
Contrary phase: V
Noise: N
Output voltage: V

(V+ + N) – (V + N) = V

Example 1:

Phase voltage: 5
Contrary phase: –5
Noise: 3

(5 + 3) – (–5 + 3) = 10

Example 2:

Phase voltage: –5
Contrary phase: 5
Noise: 3

(–5 + 3) – (5 + 3) = –10

You can use any value of noise and change polarity (+/–), and you get same results.

I know for that is necessary start quad cable for improve equal noise in phase and contrary phase. And I know that is impossible to get perfect geometry by production process.

Re: Differential Circuits and Balanced Lines

Reply #1
I know for that is necessary start quad cable for improve equal noise in phase and contrary phase. And I know that is impossible to get perfect geometry by production process.
It only takes two wires which normally carry complementary (balanced) AC audio signals...  When one signal is positive the other is equal and negative, they both cross zero at the same time and the voltages are both inverted during the opposite half-cycle. 

Common-mode signals and common mode nose are canceled out.   Since (approximately) the same electromagnetic noise is picked-up by both wires it is canceled.

In the real world a 3rd ground shield is used and XLR connectors have 3 terminals,  The XLR cables running all-over recording studios and stages everywhere are 2 signal -conductor s plus a grounded shield.  No "special construction" is needed to keep noise pick-up below audible levels.

Microphone cables are the usually most critical because a mic signal is 1/100th to 1/1000th of a line-level signal and any noise will be amplified (along with the signal) 100-1000 times.    The real noise problems are acoustic noise and preamp noise, not the cables.

With line-level signals, the balanced connection prevents hum caused by ground loops which can happen with the signal refenced to ground with an unbalanced connection.    Direct-electromagnetic noise pick-up is rarely an issue with unbalanced line-level signals as long as the cable is shielded.

Ethernet cables have 4 balanced twisted pairs and no shield.   Of course, digital is highly-immune to noise already.

Re: Differential Circuits and Balanced Lines

Reply #2
You are right!

The distance between conductors in balanced cable is so small that the difference is almost equal to 0.

But if by design someone wants completely cancel noises, you need a star-quad cable or pair twisted. Always it has two interleaved conductors with same color. It don't worry what pair to use for phase and complementary phase, but same color cables must be used for the same. You can see how wire these.

Some studios use one straight pair cables for lower capacitance than star-quad. But it's to other topic. Cables always works like RLC low pass filter. But it is really difficult to understand if someone isn't engineer or you need an engineer for help you (my case). These equations are important when you want really long cables, I help me to calculate with Wolfram Alfa and this tool.

Best regards!