There are a few different answers to that.
In an unbalanced connection there is a single "signal line".
In a balanced connection there are two signal lines, each carrying an equal and out-of-phase signal, which are summed to produce the output signal.
In many modern devices the two balanced output lines each carry the same signal level as the single line in the unbalanced output.
And, at the receiving end, you can either simply sum them, which produces a higher signal level, or "sum and divide by two" to match the level.
However there is no wide agreement about which is "the right way to do it" - so implementations vary.
Completely aside from this...
Until recently balanced connections were common in "pro gear" but rarely found in "home gear".
And pro gear also often uses a higher signal level for better tolerance against noise.
So, until recently, there were more or less defined "home levels" and "pro levels".
But the levels often used on the balanced outputs of pro gear are often high enough to overload the inputs on home gear.
(And, as a result, the balanced outputs on a lot of "pro/home gear" are set to a level somewhere in-between.)
The upshot of all this is that there is not clearly defined standard... so different manufacturers use different levels.
SOMETHING ALSO WORTH KNOWING....
Back in the days of purely analog equipment...
Most preamps had an active section with more or less fixed gain... which was fed by a passive volume control potentiometer located directly at the input.
As a result, because analog potentiometers are very difficult to overload, if the input signal was at a very high level you could simply turn the volume down.
In contrast, most modern processors take the incoming analog signal, and convert it directly into a digital signal before further routing or processing it.
Therefore the incoming analog signal goes "straight to the converters"...
And most converters actually have a fixed, and relatively low, overload level...
So, if the signal coming from your source is excessively loud, it can overload the A/D converters...
And, since the volume control is at the end of the signal chain, you CANNOT prevent this overload by turning down the volume on the preamp or processor.
(Instead, in order to prevent it, you must reduce the output level of your source... either by turning it down, if you have the option to do so, or by using a fixed attenuator in-between.)
Note that none of this is an issue with incoming DIGITAL signals....
Because the "output level" of a digital signal depends, not on the level of the signal, but merely on the numbers it contains.
So, for example, the signal level coming out of a decoder, which outputs a digital signal, will never exceed "0 dB", and will never overload anything.
And the digital output from a CD will never exceed "digital FF"...
(And, when processing is applied that might cause these numbers to increase, "normalization" is applied to ensure that the final numbers remain "within range".)
Gary?
How come than that most balanced are +6db against single ended?
