Balanced vs. Unbalanced

[electrotyler]

I'd like to know whether or not I've got the full details on all of the differences between balanced and unbalanced. Balanced: +4dbu, 3-conductor (signal+, signal-, ground), either TRS or XLR usually. Unbalanced: -10dbV, 2-conductor (signal+, signal-/ground). I understand the importance of shielding signal by using balanced ____, and that ground loops can cause hum, but what else is there to know? What's with dbu vs dbV? ...and why do they call it "balanced" instead of "shielded?" I want some background and technical detail if anyone has any. Thanks.

Tyler

[brianroth]

I will try to answer in a nutshell, and then elaborate if needed! <g>

In Ye Olde Daze, audio signals were distributed via balanced lines that were transformer isolated in and out. The "+" and "-" legs of the signal were not referenced to ground...each "floated" above ground.

Unbalanced signals were more of an off-shoot of consumer "hi fi" gear as well as things like instrument amps. In this case, one leg of the signal is "hot" and the other is grounded.

An unbalanced signal does NOT have to be "hifi '-10' levels. Many inexpensive mixers, etc. provide unbalanced in/outs at levels well above "-10"

As for all the dBV/u/M/a/b/c...x/y/z <g>, here are some general rules.

Going back again, 0 dBm for audio was defined as 1 milliwatt into 600 Ohms, which happens to be approx. 0.775 Volts across a 600 Ohm load. Note that folks doing RF design also use dBm numbers, but with a 75 or 50 Ohm load...but skip that for now!

In audio, that 0.775 Volts signal level became a standard VOLTAGE reference, and since most equipment no longer has the 600 Ohm input/output impedance design, the 0.775 Volt reference became 0 dBu to clearly differentiate it from the signals in a 600 Ohm system.

0 dBV is a "hifi" reference level that uses 1 Volt as the reference instead of 0.775 Volts.

Very simplified, in a nut-shell explanation.

Bri

[gog]

... if i'm going from a mic cable into an outboard pre (with balanced xlr ins and balanced/unbalanced 1/4 outs) to a compressor (with balanced/unbalanced 1/4 ins and outs) to a 24/96 soundcard (with unbalanced rca ins), should i use balanced cables from the pre to the compressor even though i have to use unbalanced cable to go into my soundcard. is there any advantage to staying balanced as long as possible in the signal chain, or is it moot because i end up eventually going into the soundcard unbalanced?
whew... i probably overexplained that, but would very much appreciate any help.
thank you in advance.

[Brian Brock]

balancing will help protect your signal while it is travelling in the balanced cable. It is essentially a means of dealing with electromagnetic interference, by isolating and removing it. So sure, it will do its thing between the balanced boxes, and then the unbalanced boxes will only get simple shielding to help them. But balancing is really only necessary when/where there is enough interference, namely near sources of it or over any large lengths of cable, to cause noticeable noise..

[brianroth]

As a general rule, if you can maintain a balanced signal path between "point A and B", then DO it! One advantage of a balanced signal path (as long as you can hold it together) is that it should provide better immunity to noise picked up from the "great outdoors", so-to-speak.

I am speaking in very general terms...much of the new gear that claims to be "balanced" I/O are from gear-makers that are telling lies.

Bri

[Family Hoof]

Another advantage to balanced is that it's often a hotter signal due to the balancing circuitry and a device taking advtange of the fact that it has two copies of the audio signal (+ and-) to work with. If transformer balanced, then you've also got the advantages of isolation between the internal circuitry (and it's noise + grounding) and output signal, as well as the possibility that the transformer is stepping up the voltage (more gain = yay!).

Something to keep in mind however, is that a lot of analog audio circuitry is unbalanced throughout the device, with balancing stages at output and/or input. Some may argue that the added components are a disadvantage (more noise, more altering of the signal), but I feel that when properly implemented+designed there is no harm done and the advtanges would be worth it anyway. Obviously most pro audio equipment designers agree.

I don't like to name drop but the classic Neve consoles we all love are a good exmaple of unbalanced signal path with transformer balanced i/o.

[gog]

thanks guys... just what i thought pretty much, but always nice to hear from folks in the know. much appreciated.

[brianroth]

"A lot of analog circuitry is unbalanced throughout the device" is an understatement! <g> Try...the vast majority. I can think of a few vintage Langevin and perhaps RCA and/or Altec preamp modules that were balanced from input to output, but they are by far the exception. Of course, variable mu limiters (like Fairchild or Gates) had to be push-pull internally to "null out" thumping and other artifacts caused by jerking around the gain of the input tubes.

Once "inside the box", balanced/unbalanced is pretty much a non-issue, except perhaps for mixing buses. Even then there are better ideas for larger (read: desks with physically long bus traces) such as breaking down the desk into "chunks" and then mixing those "chunks".

It's the outside world interface that most everyone has to confront, so the workings of the innards is generally not of interest to the user.

The QUALITY of the balanced I/O is far more important as far as a system is concerned, and there is some real crap out there in that department.

Bri

[wedge]

So, a balanced XLR stereo out, like the one coming out of my board, will be +4 dbu. If I take that into an unbalanced piece o'equipment, then the signal is automatically notched down to -10 dbv, or a 14 db difference? Boy, is this ever Greek to me...

[Family Hoof]

So, a balanced XLR stereo out, like the one coming out of my board, will be +4 dbu. If I take that into an unbalanced piece o'equipment, then the signal is automatically notched down to -10 dbv, or a 14 db difference? Boy, is this ever Greek to me...

No, it's not stereo. If the XLR cable is sending a stereo signal then both channels are unbalanced ( left wire and right wire sharing the same ground - 3 wires). It's a balanced mono signal that's going through the XLR cable at +4dBu. If you connect this to a -10dBV unbalanced input two things happen. 1) The negative wire gets connected to ground (usually) so now you've only got the + and ground and your balanced signal has become unbalanced. 2) The +4 signal IS NOT "notched down". It's still +4 which is a much higher voltage than that -10 input wants to see and will therefore be clipped. If you want to lower this +4 signal then you need a dedicated device to do so (if the option isn't already built into one of the two devices you're connecting).

[I'm Painting Again]

and the actual db difference is around 6db not 14db..BTW..correct me if im wrong..

[McQ]

and the actual db difference is around 6db not 14db..BTW..correct me if im wrong..

-10dBV (relative to 1Vrms) => 0.316 Vrms
+4dBu (relative to 0.775Vrms) => 1.228 Vrms
Difference = 20*Log(1.228/0.316) = 11.8 dB

Cheers!
McQ

[wedge]

If the XLR cable is sending a stereo signal then both channels are unbalanced ( left wire and right wire sharing the same ground - 3 wires).

So, then -- please bear with me -- the mix output on my board (XLR) is unbalanced, in a fashion, then going into a piece o'gear that's has two unbalanced ins is fine?

[I'm Painting Again]

and the actual db difference is around 6db not 14db..BTW..correct me if im wrong..

-10dBV (relative to 1Vrms) => 0.316 Vrms
+4dBu (relative to 0.775Vrms) => 1.228 Vrms
Difference = 20*Log(1.228/0.316) = 11.8 dB

Cheers!
McQ

im thinking of something else..mono summing into a stereo bus..i think..hence the ~6db..about 1/2 the 11.8db figure..

but thanks for the correction i thought i might be mistaken..im kind of new to this tech stuff..

[Family Hoof]

If the XLR cable is sending a stereo signal then both channels are unbalanced ( left wire and right wire sharing the same ground - 3 wires).

So, then -- please bear with me -- the mix output on my board (XLR) is unbalanced, in a fashion, then going into a piece o'gear that's has two unbalanced ins is fine?

If indeed the output of you mixbus (stereo bus, master fader) terminates in a single XLR jack (I've never seen this before myself), then yes it is unbalanced stereo and you want to plug it into (2) unbalanced mono inputs, left and right.