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What are the effects of direct balanced to single ended connection?

Fred Jacquot

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#1
While measuring the Motu M2 in loopback, I inadvertently used a TS-XLR cable on the M2 input.
The connection was TRS -> XLR -> TS.
TS connectors have only 2 contacts, one among them being the sleeve. They are the equivalent of an RCA connector with different mechanical interface.
TRS have 3 contacts. They are the equivalent of an XLR connector with different mechanical interface.
Basically I was connecting the balanced output to an unbalanced input, shorting the 'cold' wire to ground.

While it's accepted to connect unbalanced to balanced, the reverse is not a good idea.
- first it's shorting one output to ground, never a good idea, even if properly designed hardware should survive this.
- second its distorting the signal. A lot. in the case of the Motu M2, harmonic 2 goes from -120 dB to -90 dB, harmonic 3 from -108 dB to -80 dB, ...

Since a picture always explains more than a lot of text, here you are:
1588512509269.jpeg

1588512521983.jpeg
 

sergeauckland

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#3
Unbalancing a balanced differential output by shorting XLR pin3 (jack ring) to ground is fine or not, depending on the type of balanced output.

If fully floating transformer balanced, that perfectly fine.
If centre-tapped transformer balanced that's not fine at all as half the transformer gets shorted out. Pin 3 needs to be left unconnected in this case.

However, most outputs these days are not transformer balanced, but use electronics. These can also be OK or not.

If the output is electronically servoed, then it's effectively 'fully floating' i.e. not ground referenced, so shorting pin 3 (jack ring) to ground is fine.
If the output is just plain electronically balanced, i.e. with an opamp on each leg, then it's not OK as one leg will be shorted to ground. Usually each opamp has a series resistor to protect the output against just such an occurrence, but that raises the output impedance from essentially zero ohms to 75-100 ohms so sometimes isn't done.
If the output isn't differential, just impedance balanced, then shorting pin 3 (jack ring) to ground should be fine assuming the sensible case that it's pin 2 that actually carries the signal.

In summary, it's impossible to know what's the right way unless you know what sort of output you have. In many cases, the manual will tell you how to unbalance the output. If they don't then the safest is to leave pin 3 (jack ring) floating, and if that works, then great. If the sound is weak, with only high frequencies, then pin 3 needs to be tied to pin 1 (ground).

S.
 
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