What is a Balanced Combiner? - AVS Forum
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Old 07-06-2012, 10:35 AM - Thread Starter
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I was talking to a salesman about a preamp in which I was interested. It has both XLR and RCA inputs and outputs. The circuitry is single ended but he told me a balanced signal is converted (not just pin 3 to ground) to single ended at the input and a balanced input signal is 6dB higher at the XLR output than the RCA.

I asked him if a transformer or op amp was used at the balanced input. He told me neither, a "balanced combiner" is used. me. I did some googling but could come up with nothing satisfactory for "balanced combiner" for this application. I forgot to ask him how a single ended signal is converted to balanced (if required) at the output.

So what is a "balanced combiner" in this instance?

As a second question I have read that transformers and differential op amps are most commonly used at the input of a balanced circuit. Are there any other choices?
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Old 07-06-2012, 10:52 AM
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I never heard of a balanced combiner either when used for audio. I'll guess it's an op-amp. The only advantage to balanced connections with consumer gear is to avoid hum from ground loops. This is important in professional facilities where the connected gear might be across the room or even in different rooms, but it's not usually needed in hi-fi and home theater equipment. A balanced connection is not a bad thing! But it's probably not work paying extra for.

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Old 07-06-2012, 10:55 AM
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Quote:
Originally Posted by GGA View Post

I was talking to a salesman about a preamp in which I was interested. It has both XLR and RCA inputs and outputs. The circuitry is single ended but he told me a balanced signal is converted (not just pin 3 to ground) to single ended at the input

Very common. This discussion of balanced versus single ended can get messy because of the circuitry inside. Most op amps have push-pull outputs and may also have differential inputs.
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and a balanced input signal is 6dB higher at the XLR output than the RCA.

Yes and no. The signal at pin 3 or 2 of a XLR output is usually about the same as the signal on the RCA, but the signals on pin 2 and 3 have opposite polarity, so the difference is about double the signal on the RCA.
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I asked him if a transformer or op amp was used at the balanced input. He told me neither, a "balanced combiner" is used. me. I did some googling but could come up with nothing satisfactory for "balanced combiner" for this application.

"Balanced combiner" is a new one on me, and I've only been doing audio for more than 50 years.
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I forgot to ask him how a single ended signal is converted to balanced (if required) at the output.

One easy way to convert single-ended to balanced is with an inverting amplifier whose gain is 1. It is often done this way. Their are also special chips that package this function up and add some nifty refinements. Precision of the gain of 1 is not as important here as it is at the input.
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As a second question I have read that transformers and differential op amps are most commonly used at the input of a balanced circuit. Are there any other choices?

That inverting amplifier with a gain of 1 works well at both the input and the output. Getting its gain close enough to 1.0000 for good common mode rejection can be an issue. But there are chips that do it well, too.
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Old 07-06-2012, 05:00 PM
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It sounds to me like the device accommodates a balanced input, but does not really treat it as balanced. And that the output is similarly unbalanced, but can drive a balanced line, due to the signal being 6db up. By "balanced-combiner", I suspect it means simply taking one side of the balanced line and combining it with the single-ended line.

It sounds like a cheap way to add XLR connectors to a receiver so that it looks like more than it is. But as Ethan said, balanced inputs isn't something you need.
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Old 07-06-2012, 06:41 PM
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Sounds like the sales pitch of an ignorant sellrat.

Balanced to SE can only be done one of two ways, using a transformer or active devices. A transformer would be useful to a sales pitch to many customers and as it wasn't mentioned, one can assume it is active. Active falls into two categories; opamp and discrete. The latter is more difficult and expensive to do and has a cache amongst 'phools, I'd be surprised if the marketing literature from the manufacturer didn't mention it prominently, like Marantz with the HDAM. So my guess is it uses an opamp, probably a single device.

To do a balanced output is similar, either transformer or active. As almost every audio device (line level) would have an opamp as an output buffer, simply add an inverting stage to make the opposite polarity, voila, single ended and balanced outputs.
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Old 07-06-2012, 08:12 PM
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Quote:
Originally Posted by A9X-308 View Post

Balanced to SE can only be done one of two ways, using a transformer or active devices.
Balanced can only be done correctly one of those two ways. However, you can take the positive side of the balanced input, wire it directly or through a resistor to the RCA input, and it will work un-balanced, with no common-mode rejection. You leave the negative pin of the XLR unconnected.

I suspect that is what is going on in that preamp. A "balanced-combiner" made with a few resistors.
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Old 07-07-2012, 12:33 AM
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Quote:
Originally Posted by MarkHotchkiss View Post

Balanced can only be done correctly one of those two ways. However, you can take the positive side of the balanced input, wire it directly or through a resistor to the RCA input, and it will work un-balanced, with no common-mode rejection. You leave the negative pin of the XLR unconnected.
But then it's not balanced, just using XLR connectors. With a non centre trapped transformer as source it wouldn't work, and it would leave the unterminated polarity to act as an antenna with whatever it picked up able to get back to the inputs of the opamps. Silly design technique - the not used polarity should at least be terminated via a resistor.
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Old 07-07-2012, 12:49 AM
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Originally Posted by A9X-308 View Post

Sounds like the sales pitch of an ignorant sellrat.
.
'

That is my best guess.
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Old 07-07-2012, 03:05 AM
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Quote:
Originally Posted by A9X-308 View Post

But then it's not balanced, just using XLR connectors.
Yes. No common-mode rejection. No better than a single-ended interface.

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With a non centre trapped transformer as source it wouldn't work, and it would leave the unterminated polarity to act as an antenna with whatever it picked up able to get back to the inputs of the opamps. Silly design technique - the not used polarity should at least be terminated via a resistor.
Oops, Your right. You would need to at least ground the negative side. A resistor would do, as you said.

Even though it's stupid, I bet that is what's been done.
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Old 07-07-2012, 11:46 AM
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Originally Posted by arnyk View Post

"Balanced combiner" is a new one on me, and I've only been doing audio for more than 50 years.

Same here. When I asked Google, most of the hits were related to radio circuits.

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Old 07-07-2012, 12:59 PM
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Hmm--can't speak to "balanced combiner"

Here is an article on balanced:
http://www.jensen-transformers.com/an/ingenaes.pdf

And the semiconductor folks who specialize in it:
http://www.thatcorp.com/THAT_IC_Products.shtml

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Old 07-07-2012, 02:06 PM
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^^^ For those reading and looking to understand more on balanced and unbalanced system interconnection, Jensen Transformers AN003 and AN004, and Rane Note 110 are both good and easy reads.

That also reminds me I need to get in another order to Profusion for a 5173 to go with the 1570. I have a couple of the 1200 receivers, but not found an application for them yet.
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