1. ## More on balancing

A question about a Behringer EQ in another thread prompts this post.

The point has been made that when you're connecting an electronically balanced output to an unbalanced input, you have to leave pin 3 of the output (or the ring, if a 1/4" plug) open. This is because an electronically balanced output is really two opposite-polarity unbalanced outputs, and connecting pin 3 to pin 1 simply shorts one side.

This is different from a transformer-balanced output, which really is a balanced output with the signal between pins 2 and 3. For a transformer balanced output you need to connect pin 3 to pin 1.

Enter Behringer. According to their website, their inputs and outputs are "servo balanced." This is a special technique that imitates a transformer. They state very clearly that when connecting to an unbalanced source or termination, pins 1 and 3 must be connected together. Since a lot of people use Behringer, this is something to keep in mind. Also, this is not a new circuit - Tapco used to use it - and I wouldn't be surprised if we start seeing more outputs like this.

Then there is the "impedance balanced" output, which is unbalanced from a signal standpoint but balanced from an impedance standpoint, which is what matters when you're trying to eliminate hum. Peavey among others use this for some outputs. With this kind of output, there's a resistor from pin 3 to ground that's equal to the output impedance of the amp that's connected to pin 2. Since it's just a resistor, you won't get burned if you connect pin 3 to pin 1, or if you don't - either way.

Summary: there are essentially four kinds of "balanced" outputs. When connecting a balanced output to an unbalanced input, here are the rules:

Floating, isolated transformer output: connect pins 1 and 3.
Normal electronically balanced output: leave pin 3 open.
"Pseudo balanced" or "impedance balanced:" either way.
Servo-balanced: connect pins 1 and 3.

You can always solve a problem by sticking a transformer between the balanced output and unbalanced input, and you'll eliminate ground loops to boot. A sound tech should have a couple of transformers in stock.

2. Well put Karl - I thank-you for your time in explaining and point out things that are commonly overlooked solutions. It's these fundamentals, that help troubleshoot problems that arise and eats up time trying to figure it all out.

As a matter of fact, I will print this out and stick in my engineering notes (because I can't/don't remember it all)

3. Then there's the issues of zero-levels when connecting unbalanced signals to balanced inputs...

4. Yup. The output is taken across pins 2 and 3; when you do it unbalanced and take the output across 2 and 1, you get half the voltage or a 6dB drop. On the other hand, when you're feeding an unbalanced input, you're likely feeding a consumer-type piece of gear that expects a lower level anyway. This can actually be a blessing.

5. I was more thinking of the issues when connecting -10 unbal to a +4 bal input...

6. Yeah, there often isn't a dang thing you can do about that except stick in a preamp. The last studio I had anything to do with, we had a simple board with minimal controls (on purpose). There were two inputs where we built up a box with external preamps with a huge variety of connectors and various balanced/unbalanced/phantom arrangements.

7. ## Plese help me to make balanced 1/4 trs cable.

Hi, I want to make several BALANCED 1/4 TRS cables to connect MONO instruments like guitar, bass, and was wondering if itīs just CONSTRUCTED like a normal STEREO cable, but instead of plugging it to a stereo signal input must be in a balanced TRS input (MONO). is that it?

I know that in order to use a balanced cable i have to use balanced gear too in the whole connections, and that i have to use a mic cable with two VIVOS with only one shield. The TIP for the left channel, the RING for the right channel and only one SHIELD to the sleeve, and repeat the exactly the same in the end-plug.

Believe me I've checked a lot of websites and could not find this analogy, please let me know which is the way step by step to make balanced cables!

8. Unbalanced stereo == 3 pins: left/right/ground
Balanced mono == 2 or 3 pins: +/-/(ground or reference)

9. Quote: karl eilers
Yeah, there often isn't a dang thing you can do about that except stick in a preamp. The last studio I had anything to do with, we had a simple board with minimal controls (on purpose). There were two inputs where we built up a box with external preamps with a huge variety of connectors and various balanced/unbalanced/phantom arrangements.
I actually had to work around this issue once for a few sources by using direct-outs (post-trim, before everything else) of a separate mixer.

10. jua, not sure precisely what you're doing, but the rule is, when connecting a balanced input to an unbalanced source, short the ring to ground. (Only with a line-level input. A mic input may have phantom power on it and this causes big problems.) Same with an XLR input; short pin 3 to pin 1.

A guitar is an unbalanced signal and the usual cord is a simple tip/sleeve 1/4 plug. When you patch such a plug into a balanced input, the long sleeve (long because there's no ring) shorts the low side of the balanced input to ground, so you get what you want. A tip/ring/sleeve cord is not as good, because then the input's low side is connected to nothing except the cord itself.

It is with balanced outputs that the problem I occurs which my original post addressed.

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