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I have an AV receiver providing amplified outputs and preamp outputs.

The pre-amp outputs are in the form of (unbalanced) RCA jacks. My speakers are amplified and have a balanced input.

I was looking for an audio transformer to convert the unbalanced into balanced signal, but I read here that

If everything in your facility is unbalanced and the wires are fairly short, there's no point to using balancing adapters. But you can still reduce hum, if you're willing to do some soldering:

Use the same kind of two-conductor shielded cable as you'd use for balanced wiring.

Connect the "hot" conductor -- usually white -- to the center pin of the phono or phone plug at each end.

Connect the other conductor to the sleeve of each plug.

Now here's the trick" connect the cable's braided or foil shield to the sleeve at one end only.

Use shrink wrap or tape to make sure the shield doesn't touch anything at the other end.

Be consistent about which end of the shield gets connected... one way is to always connect the shield at your patchbay or mixer, and always leave the other disconnected.

Obviously this solution will result in a signal that is 6dB weaker than a real balanced one, but what are the implications concerning noise and hum immunity?

What is the length that should not be exceeded with this solution?

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  • \$\begingroup\$ This is fine. Length depends on interfering sources, in a home, 5m is probably OK (as long as the preamp can drive the cable capacitance). In a TV studio with kilowatts of thyristor dimmed lighting, not so much. You'll be able to tell, when you can hear hum - or not. \$\endgroup\$ – Brian Drummond Feb 2 '18 at 10:22
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    \$\begingroup\$ @BrianDrummond I have about 10 m, but I guess the conclusion is the same \$\endgroup\$ – FarO Feb 2 '18 at 10:58
  • \$\begingroup\$ Until we have room temperture super-conductors , all shields and wires will have resistance and inductance and pick up stray magnetic fields by mutual induction somewhere on a 100 dB log scale of your environment unless perfectly balanced. Physics Rules are hard. Just ask Wheeler. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 2 '18 at 14:29
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The "have the signal and gnd wires inside the shield but connect the shield only at the speaking end" -trick actually extends the metal shield of the speaking device. It's extended to only few millimeters apart from the metallic shielding of the listening device.

This surely reduces the effect of capacitively coupled AC fields. For example normal room lightning lamps spread strong 50Hz electric fields capacitively.

Magnetic fields which have high enough frequency, for example those of the radio waves, get killed by the eddy currents in the shield and stay away from the inductive loop between the signal and GND lines. 50Hz magnetic field isn't affected. It penetrates centimeters to copper. Fortunately in normal living house rooms there rarely are so strong 50Hz magnetic fields that the small gap between the wires catch substantial hum voltage inductively.

But the system isn't balanced. There are still all those common mode noises left which are catched by big loops. Those loops are caused by multiple connections between the signal grounds of the devices. Most harmful unwanted connections are via the mains electricity supply. In many devices the signal ground has at least some connection, often zero ohm galvanic connection to the protective earth wire or at least a capacitive connection to the mains supply wires.

The 2 different connections between the signal grounds of the devices easily collect hum and other noise voltages. It's like an unwanted noise signal source were in series with the caused loop. Because the signal grounds no more are directly together, but through a noise voltage source, that noise is directly added to the signal voltage.

Often people try to insert tape or otherwise break at least the galvanic contact via the protective earth. That often reduces the hum substantially, but compromises the electric safety. In addition microphone eaters get easily half of the mains supply voltage to their lips through the rf interference filter capacitors in the power supplies and the devices have a tendency to get damaged if they are connected or disconnected without removing the mains supply from the whole system before.

The proper solution is to have balanced audio signals if the environment is noisy and loops cannot be avoided. One trick to reduce the loops without safety compromises is to use isolation transformers to supply the mains AC.

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I was looking for an audio transformer to convert the unbalanced into balanced signal

Why bother - if your inputs are balanced then there is no problem using them directly connected to your unbalanced pre-amp output. Use a balanced cable and make sure the shield is connected at the speaker end. Don't connect the shield at the pre-amp or both ends. This is to avoid ground hum or possibly ground currents unassociated with the audio.

A signal that is 6 dB weaker may not result in a signal-to-noise ratio degrading of 6 dB. It all depends on the noise and whether it is mainly differential or common-mode.

What is the length that should not be exceeded with this solution?

This cannot be answered without knowledge of the prevailing noise and interference levels at your installation.

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