XLR, RCA and CMRR - a few questions - diyAudio
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Old 7th May 2012, 01:08 AM   #1
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Default XLR, RCA and CMRR - a few questions

Recently built the John Broskie "Unbalancer" PCB:

The Unbalancer

I'm using a DAC which has two AD1865 chips, and true balanced output - the i/v stage is passive via a resistor so the dac output is only about 200mV. Hence I thought it would be great to kill two birds with one stone and get good voltage gain and CMRR using Broskie's linestage design.

Here comes the hard part - controlling the volume with remote control.

I bought a "LITE MV04" kit:
4 Way Motorized Remote Control KIT set MV04

It has RCA and XLR inputs, and also does either RCA or an XLR output.

HOWEVER, upon receiving the kit & manual (see attached) pdf, turns out the XLR output (which I intended to run to the Broskie PCB) is not true balanced - as you can see in the schematic the circuit accepts a true balance input - 3 wire signal but the output (BN7) is only 2 wire signal (L & R) so it is not a true balanced output.

What would the best plan be to maintain good CMRR in this situation?
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Old 7th May 2012, 01:24 AM   #2
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Common mode rejection occurs in the *receiving* device. It can be degenerated by impedance imbalance but is independent of signal balance.

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Old 7th May 2012, 01:38 AM   #3
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Quote:
Originally Posted by Chris Hornbeck View Post
Common mode rejection occurs in the *receiving* device. It can be degenerated by impedance imbalance but is independent of signal balance.
Not sure I follow - isn't the receiving device in this case the MV04?

In any case, would you recommend:

(1) DAC with XLR output -> Lite MV04 (true XLR input, RCA output) -> Broskie Unbalancer PCB

OR

(2) DAC with XLR output -> Lite MV04 (true XLR input, 'not true' XLR output) -> Broskie Unbalancer PCB
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Old 7th May 2012, 03:15 AM   #4
gootee is offline gootee  United States
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I didn't look at the schematic but "not XLR" does not mean "not balanced".

Balanced just means that there are two signal wires for one channel, with the signal on one wire being an inverted copy of the signal on the other. When they are "recombined" through a differential amplifier, anything that was the same on both wires (i.e. "common mode") gets cancelled out.
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Old 7th May 2012, 03:55 AM   #5
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OK - how do we determine whether the "false" xlr output of this volume controller is, in fact, balanced or not?
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Old 7th May 2012, 08:17 AM   #6
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BN7 can provide a ballanced output.......pos and neg for left and the same for right plus a common ground .......provided the correct connections are made at XP2 and XP3.......seee the schematic notes .

Last edited by epicyclic; 7th May 2012 at 08:37 AM.
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Old 7th May 2012, 12:21 PM   #7
AndrewT is offline AndrewT  Scotland
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Balanced refers to the impedances not the signals.

A balanced connection works just as well with
a.) two in phase signals of different amplitude
b.) a single amplitude signal and a zero amplitude signal
c.) a pair of opposite phase signals of the same amplitude
d.) a pair of opposite phase signals of different amplitude

The common factor of all of those different signal arrangements is that the impedances of the two signal connections are close to identical. The closer to identical (including resistance and inductance and capacitance) the better that the balanced impedance connection performs it's job, i.e. to reject interference.
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Old 8th May 2012, 02:08 AM   #8
gootee is offline gootee  United States
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Quote:
Originally Posted by AndrewT View Post
Balanced refers to the impedances not the signals.

A balanced connection works just as well with
a.) two in phase signals of different amplitude
b.) a single amplitude signal and a zero amplitude signal
c.) a pair of opposite phase signals of the same amplitude
d.) a pair of opposite phase signals of different amplitude

The common factor of all of those different signal arrangements is that the impedances of the two signal connections are close to identical. The closer to identical (including resistance and inductance and capacitance) the better that the balanced impedance connection performs it's job, i.e. to reject interference.
Really? I thought that for the standard balanced audio outputs and interconnects it had to be c. Otherwise, standard "balanced inputs" audio equipment wouldn't handle it correctly.
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Old 8th May 2012, 02:35 AM   #9
SY is offline SY  United States
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If the source impedances are the same, what happens when you inject a common mode current? Even if one side has a signal and the other side doesn't? You get a common mode voltage, which will then be rejected. Now, suppose the source impedances were different and the signal voltages were exactly equal in magnitude and opposite in polarity. Then the common mode current will be partially converted to a differential voltage. All the balanced input cares about is the differential voltage, not how that differential voltage was obtained.
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Old 8th May 2012, 02:44 AM   #10
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There's lots of confusion on this topic, and I think that most comes from the use of the word "balanced". It's the kind of thing that started with a fairly well understood meaning, and grew, and grew, until it means almost nothing specific now.

In classic pro sound environments there's floating and non-floating "balanced" and in prosumer environments there's impedance "balanced", etc. But the magic doesn't happen at the sending end of things, it happens at the receiving end, and the important part is that the receiving end be differential.

All inputs, even single-ended ones, are differential to some extent, but those made to reject common mode signal/noise are called "balanced". A poor choice of terms but we're stuck with it.

For a differential input to work properly it's only necessary for source impedances to be equal, and signal imbalances don't matter, as several good folks have said better.

All good fortune,
Chris
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