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Old 29th July 2008, 06:01 PM   #1
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Default Differential sensing RCA

Some amps have input like this. While the input is RCA (not balanced), the receiver is configured to sense differential between + and - (like balance input receiver).

Both the + and - are passed via capacitor (E001-E002-E003-E004). The - (shield) is not connected directly to ground, but by capacitors (C021-C022)

What is the merit of doing this? Better sound? Breaking ground loop?

Why not just connect the shield of the RCA directly to ground?
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Old 29th July 2008, 07:10 PM   #2
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The input circuit is a balanced circuit. It amplifies the difference between the two inputs. It just so happens that these two inputs are the 'hot' and 'cold' of the RCA, but it could as well be pin 2 and pin 3 of an XLR.

The advantage is that there is no direct connection between the two grounds of the source and the amp, so no possibility for ground loops that, which could set up a voltage across the ground lead, which could lead to hum etc.

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Old 30th July 2008, 02:19 AM   #3
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Is there any other advantage, like for sonic performance?
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Old 30th July 2008, 03:03 AM   #4
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There are no other advantages in sonic performance than the ground issue as said by janneman. The circuit behaves like any other circuits, what ever it is connected as single-ended or balanced.
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Old 30th July 2008, 03:43 AM   #5
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This reminds me of what a member said (JC if I'm not mistaken) that signal return ground has the same importance as the hot lead. In attachment post#1, the ground is treated the same way as the hot lead. Many overlooked the importance of signal ground return path.
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Old 30th July 2008, 03:50 AM   #6
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Yes the return Ground is very important
I always layout my amp PCBs with no connections between signal and power Ground, so you can try different signal to power Ground connections to find the best (no hum) connection (which in most cases will be the start Ground)
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Old 30th July 2008, 05:41 AM   #7
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Another example. It's easy to short RCA shield to ground (R530=0ohm), but it is not done that way.

Janneman, if the "signal" is actually the difference between + and - , then any degradation in - (ground return) has the same effect with degradation in + ?
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Old 30th July 2008, 05:54 AM   #8
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this is one discrete
differential opamp I designed.

Compares the input voltage from R7 & R8
The gain is set to 10:
- R9/R7 = 10
- R12/R8 = 10
The input differencex10 appears across R14 which refers to GROUND.

As you can see this application of my own
is one Temperature stabil 1.25 Volt Discrete Bandgap Reference.
Works very well.
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Old 30th July 2008, 06:03 AM   #9
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the problem with both of these circuits is that they present different impedances to ground for both +ve and -ve inputs. I think this is what lumanauw was saying. Consequently, common mode signals aren't cancelled effectively.

The imbalance added by the resistor/cap to ground is an attempt to resolve the problem caused by using a single ended, screened cable. When the source is single ended, more 50/60Hz leakage current flows in the screen than the center conductor and so the use of balanced input is broken. These are both attempts to re-direct the leakage current away from amplifier input.

Let's be honest here. A differential input only really works with a balanced output and twisted pair/screened interconnect cable.

http://www.dself.dsl.pipex.com/ampin...d/balanced.htm
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Old 30th July 2008, 06:12 AM   #10
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And so, Iain:
what circuit or opamp or amplfier you use your self?
for you balanced application?

and that link to Self site,
what chapter do you refer to?
Maybe you even can refere to one of D. Self 's figures ..


My circuit published has no problems. It is for a DC output, this time.

So, if connecting reference to same point = star ground.
One leg has got 1.000.000 Ohm
One leg has got 1.003.300 Ohm
Has got no practical significations for my application.

I am not into Space Tech & I hope no other Diy Audio hobbyist is either.

Because 'our un-scientific space tech' may not meet NASA standard.
At least not yet

hehe
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