Recording balanced signals

ibenko · Post by **ibenko** » Mon May 24, 2004 8:28 am

Hi group!

I'm really confused about balanced and unbalanced signals. I have been reading that its better to record signal from let's say guitar using di-box, but as far as I understand di-box turns unbalanced signals into balanced, but how does this help in any way, when there are only unbalanced inputs on my luna 2496 box (or am I wrong?).

Tnx for any help!!!

Best regards, Igor

BingoTheClowno · Post by **BingoTheClowno** » Mon May 24, 2004 9:57 am

Read this:
http://www.mediacollege.com/audio/balan ... anced.html

It's all about noise cancelation, in short: the balanced audio signal is made of two lines of the SAME identical signal but with one line being out of phase. If noise is introduced in the 2 lines, the phase of the noise signal is the same on both lines and when it gets combined at the input it gets canceled while the good audio signal is not because it is out of phase.

deejaysly · Post by **deejaysly** » Mon May 24, 2004 9:59 am

From what I understand, the di-box gives you a balanced output to combat any induced noise that may occur particuarly with long cable runs (from the di-box to mixer/amp or else).

You can use the balanced output from your di-box by simply using an unbalanced lead to make the connection to your luna box.

NOTE that by this I mean that the lead would take only the hot(+) and ground connections from the balanced output of your di-box to your luna unbalanced input.

Another variation that may, in some cases, give you a stronger (louder) signal would be an unbalanced lead taking only the hot(+) and cold(-) signals from your di-box balanced output to your luna unbalanced input (cold(-) to ground on input).

I am assuming the connection from your di-box would be relatively short (~3meters)?? If so, you wouldn't really notice any difference even if the luna inputs were balanced.

Apply my comments if you wish but at your own risk.

I can say that this is what came of a similar dilemma I had when connecting the balanced output of my ART MIC pre-amp to a Pulsar phono/RCA input for which I made up my own cables so that I knew what was going on!

Maybe more info than you called for.....

[ This Message was edited by: deejaysly on 2004-05-24 11:10 ]

Liquid Len · Post by **Liquid Len** » Mon May 24, 2004 10:45 am

Disclaimer : I'm by no stretch of the imagination an audio engineer. Maybe I've misinterpreted what I think I'm hearing, etc, but my two cents :

DI boxes in my experience also color the signal. I know they're not supposed to do this, but after A/B'ing keyboard gear, I definitely like the DI sound better, and have not been able to duplicate the effect simply by EQ'ing and compressing. A brass sound that comes off as plastic and blatty, somehow transforms into a more convincing patch when run through a DI. Piano sounds more like a real piano, the attack is a bit more sustained and it sounds fuller. I always used to go direct into whatever mixer was used, now I have DI boxes for all my keyboard gear. As for guitars and basses, it depends on what kind of sound you're looking for, but I've never been able to get a good bass or guitar sound going direct (I always used to mike a guitar cabinet but now I'm hooked on the Celmo amp modellers in combination with a guitar that's been recorded DI).

[ This Message was edited by: Liquid Len on 2004-05-24 11:46 ]

ibenko · Post by **ibenko** » Mon May 24, 2004 7:21 pm

Tnx for all of your info, this is really great group.

As far as I see the only benefit I get is the coloration of DI-box. I guess I just have to use short and quality cables, to get less noise, am I correct?

Tnx all!

Best regards, Igor

Counterparts · Post by **Counterparts** » Tue May 25, 2004 3:21 am

Liquid Len wrote:

DI boxes in my experience also color the signal. I know they're not supposed to do this, but after A/B'ing keyboard gear, I definitely like the DI sound better, and have not been able to duplicate the effect simply by EQ'ing and compressing.

I agree. My understanding is that a using a DI box replaces the need to mike up an amp when e.g. recording guitar, so I think that you get this cabinet effect as well as a balanced signal.

Royston

Kenf · Post by **Kenf** » Tue May 25, 2004 4:55 am

Hi
The most important job a DI box does is impedence matching. A guitar typically has a high output impedence and a balanced mixer input was traditionally 600ohms so if you connected you guitar straight on the input to your mixer just about all of the signal will be lost. Most synths can be connected straight into a balanced mixer although they are unbalanced they have a low output impedence, therefore their signal level is high enough.
You can connect unbalanced to balanced and viceversa, really depends on the impedences. Most modern unbalanced equipment can be connected to balanced inputs and viceversa

ibenko you could connect the output of your DI box to the input of your Luna, it's not ideal but would work OK.
DI boxes 'colour' the sound normally because they use a transformer. Many folks say that is what made the vinage Neve equipment sound as it does.... Its all in the iron.

Regards
Kenf

[ This Message was edited by: Kenf on 2004-05-25 05:55 ]

blazesboylan · Post by **blazesboylan** » Tue May 25, 2004 5:09 am

On 2004-05-24 20:21, ibenko wrote:
Tnx for all of your info, this is really great group.

As far as I see the only benefit I get is the coloration of DI-box. I guess I just have to use short and quality cables, to get less noise, am I correct?

Tnx all!

Best regards, Igor

Hi Igor.

OK, sorry for beating a dead horse here, but I love this stuff...

Apologies in advance if I bore you to tears!

The short answer is: you can probably get away with using all unbalanced lines.

The long answer...

1) As BingoTheClownO mentioned, balanced lines carry hot and cold signals. The cold signal is 180 degrees out of phase from the hot one. For example:

Code: Select all

Sound wave:
   __      __
  /      /      /
      __/    __/

"hot" channel:

   __      __        __ +.775 volts
  /      /      /  __     0 volts
      __/    __/   __ -.775 volts

"cold" channel:
       __      __    __ +.775 volts
      /      /     __     0 volts
  __/    __/      __ -.775 volts

When you take the difference of these two "balanced" inputs, you get the amplitude of the signal that has no noise offsets in it. A radio frequency spike would offset both voltages by the same amount. Taking the differences cancels out that noise.

Code: Select all

   Vdiff = (Vhot + Vnoise) - (Vcold + Vnoise)
         = Vhot + Vnoise - Vcold - Vnoise
         = Vhot - Vcold

Noise can be introduced in many ways (radio interference, electrical problems. and so on). Balanced cables only take care of the interference end of the spectrum; electrical hum and what-not will not be resolved by using balanced cables.

2) Balanced lines are most useful when doing long cable runs. By long, I mean more than 25 feet / 7 metres. (That's an empirical number I've heard from various sources.)

3) Cables are affected by more than just length. There are a few parts to a cable:

The wire(s): hot signal for an
unbalanced cable; hot and cold
signals for a balanced cable. The
guage of wire used is extremely
important for sound quality! Over
here in N. America cables are rated
by the American Wire Guage (AWG).
Not sure if Europe has some
(more sensible other standard
but... The lower the AWG, the
thicker the wire. Generally
I think 22 AWG is the standard
for guitar cables. The higher
the AWG, the thinner the wire,
the more resistance you get, the
less length of cable you can
use. At 1000' of cable, AWG 26
resists current 3 x as much as
AWG 22 cable of the same length.
This makes signals more
susceptible to noise fluctuation.
Always use thick wire!
The shield of the cable. There
are braid shields of varying quality
(the higher the coverage the better
the shielding, e.g. 95% is better
than 90% for preventing noise).
I think the other option, foil
shielding, is never used in audio
cables, but you'll see it in
computer cables (such as serial
cables). Always use cables that
have good shielding!
The insulation (PVC / etc),
which doesn't affect sound
quality unless you're recording
outside or stepping on your
cables a lot.

4) It also really depends on what your signal sources are. You need high impedance inputs for most instruments, but low impedance inputs for microphones. (By "low" I mean in the kOhm range -- not low like speaker inputs, in the 2-16 Ohm range.) The inputs to the Luna box (and just about every other input device in the known universe) are high impedance, though presumably not high enough for a raw guitar signal.

In fact, microphones and guitars both need preamplifiers to crank up the amount of electricity flowing into the Luna input. A guitar pedal will do the trick for a guitar preamp. A mic will require either a preamp-in-a-box, or a mixer (which has multiple preamps built into it). The preamp boosts the signal level to be high enough for the Luna box inputs.

Synthesizers and CD players, on the other hand, generally have the correct output ratings to plug straight into the Luna.

You do have to watch for "Pro" gear though. In the specs of any effects device, mixing console, etc., you'll find that the output is rated relative to either -10 dBv or +4 dBu. The Luna accepts -10 dBv signals. High end audio gear often outputs +4 dBu (or both). In a nutshell, this means that some high end audio gear will cause clipping at the Luna inputs. You can get around this by reducing the level of Pro gear, but often you won't be able to reduce it sufficiently to get -10 dBv - acceptable levels. The line-out from a Mesa Boogie guitar amp will cause you problems, for example.

5) Also consider whether your signal sources are balanced or unbalanced.

Microphones are almost always balanced.
Guitars and guitar effects, synthesizers, and so on, are almost always unbalanced.
Rackmount effects and mixers vary widely.

The specs for your gear will tell you whether the output for a particular device is balanced or unbalanced. In general -10dBv gear is unbalanced and +4dBu gear is balanced, though this isn't a strict rule.

If your gear has visible connectors you can also tell by looking at them. RCA cables, such as are used to connect to the Luna box, are always unbalanced -- they have 1 signal wire and 1 ground. XLR mic cables are balanced -- 2 signals and 1 ground. 1/4" cables come in "TS" (tip-sleeve) and "TRS" (tip-ring-sleeve) varieties. TS, or mono, connectors are used for unbalanced cables (1 signal, 1 ground). TRS, or stereo, connectors are used for balanced cables (2 signal, 1 ground).

Usually you can plug a balanced device into an unbalanced input by simply strapping the cold output to ground.

Code: Select all

   balanced hot output ----> unbalanced in

   balanced cold output 
                         |-> unbalanced gnd
   balanced ground -----/

So you could, for example, create XLR - to RCA cables this way; or TRS - to - RCA cables. If you have a soldering iron this is fairly easy to do; otherwise most music stores will custom-make them for you.

6) Ground loops. This can be a problem for noise. Ideally you should plug all your equipment into the same AC power line. Also all your equipment should be grounded. Unfortunately some gear (notably guitar pedalboards) provide ungrounded power supplies.

Warning about plugging all your gear into one AC main: if you plug microwaves or fans or toasters into the same line as your music gear, you will break the circuit every time. I've found that 10 amps of current is enough to power *lots* of amps and stuff. But when all that gear gets really hot and you turn the fan on... hey, what happened to the lights?!?!?

Big commercial studios usually have their electricity modified so that they get 15 amps of current per outlet.

Oh, I'm speaking N. American again -- i.e. 110 volt power. I guess standard power in most European countries would be 5 amps, no? (And pro studios would generally get 7.5 amps.)

7) Lights. Don't use dimmer switches! They are absolutely terrible noise sources. Those tall lamps you can buy at Ikea with the dimmer knobs are really awful. (The ones with on/off switches are fine.)

Now having said all that, you are unlikely to get noticeable noise even with cheap unbalanced cables. Unless:

your cables are near a power generating station or a radio tower or something
you have long runs of cables
you have snakes / multicore cables (which, if they are cheaply made, introduce crosstalk between channels).

You can spend a lot of time trying to set up the perfect noise-free environment, but in my experience you'll end up troubleshooting no matter how many precautions you take. It's better just to get going and make music, and solve the problems as they hit you. And boy oh boy, the problems will hit you!

But that's half the fun of it, right?

Now regarding the DI boxes and coloration: there are actually two types of DI boxes:

passive: these boxes use a transformer.
active: these boxes are transformerless.

(An "active" circuit is one with a chip in it; a "passive" one is a circuit with no silicon at all.)

Active DI boxes should not give you any coloration (unless you drive them way beyond their maximum voltages, which is really hard to do unless you're sadistic).

Passive DI boxes can introduce coloration. A cheap transformer is rarely ever transparent -- i.e. it doesn't have a very good frequency response graph compared to most expensive transformers.

Obviously, though, this isn't necessarily a bad thing. Tape saturation, tube distortion, EQ, compressors, and speakers all "colour" the sound, after all.

OK I'm sorry for writing so much! But I do hope that some of this is helpful.

A really good book for anyone who's interested in all this geeky stuff

is:

Sound Reinforcement Handbook
by Gary Davis and Ralph Jones.

It's very accessible and well-written.

For more comprehensive coverage (e.g. if you want to build your own mixing console from the ground up, schematics included...) there's:

Handbook for Sound Engineers
edited by Glen M. Ballou

These are both great books and you'll never finish reading them. They'll always be sources of new insights.

Ugggh, must sleep...

Good luck Igor!

Johann Tienhaara
Victoria, B.C., Canada

deejaysly · Post by **deejaysly** » Tue May 25, 2004 7:47 am

Johann, nice post man!

Only thing I was sure about was the balanced to unbalanced connectivity - connecting the cold balanced output to unbalanced ground!

When I was loooking at connecting my ART pre-amp, which has active (transformerless) outputs, to my Pulsar input, I felt a bit nervous connecting balanced cold to unbalanced ground although I know that this connection is required for units with balanced output transformers when connecting to an unbalanced input. I ended up just taking the balanced hot and ground to the unbalanced input and avoided shorting the (active) balanced cold to ground.

I am assuming, in your info, that because the balanced cold output is a low level signal (mV) with a "high" impedance, shorting it to ground wouldn't do any damage in your experience??

at0m · Post by **at0m** » Tue May 25, 2004 8:04 am

Hey blazesboylan, maybe that elaborate reply deserves its own topic in Tips 'n' Tricks forum!

ibenko · Post by **ibenko** » Tue May 25, 2004 10:37 am

Tnx all, especially blazesboylan! I have been bugging my mind with this for really long time, I pretty much understand now everything. Blazesboylans post should certainly go to tips/tricks.

Best regards, Igor

bassdude · Post by **bassdude** » Tue May 25, 2004 7:44 pm

And keep this link handy if you make your own cables and want to avoid ground loops, noise etc.

http://www.rane.com/note110.html

blazesboylan · Post by **blazesboylan** » Wed May 26, 2004 3:33 am

Hi all.

Thanks for the kind words. Is there some way of magically transferring my original post to the tips'n'tricks forum? I.e. can I "mind-meld" it over there?

A few follow-ups:

Bassdude: Thanks for the link! That's a great website. It made me realize that my statement "balanced cables do nothing for eliminating ground hum" is obviously false. The whole point of a balanced cable is to eliminate the need for a ground or "zero" reference point.

I do think that the Rane folks are idealizing a tad, but that's just my opinion. I don't think any studio anywhere in the world has all-balanced connections. Nevertheless, the bigger the installation, the more you have to worry about this stuff. Noise grows with your studio.
Another correction: I said that N. American buildings generally provide 10 amps of current to electrical outlets. This is wrong. It's 15 amps. Thank goodness I'm not an electrician, I'd be fried by now!
DeejaySly's dilemma: after much scratching of my head and wondering "can it possibly be true?!?" I discovered that you're right (of course! ). It's generally a bad idea to connect "cold" out to ground when your output is electronically (as opposed to transformer-) balanced. There are certain types of active circuits designed to prevent frying the chips when "cold" output is shorted -- electonically servo-balanced outputs / floating balanced outputs / etc. Dunno how they work. Those types *do* allow you to connect your preamp "cold" out to ground without any worries. But you'll have to check the specs of each device before wiring 'em up in this way.

Thanks for the tip! If you hadn't pointed this out I would have carried on happily connecting cold to ground without ever thinking about the consequences!

Of course you can build / buy a transformer-based balanced - to - unbalanced interface. But whether it's worth the time / money really depends on how much you care about eliminating buzz, hum, etc and getting the most out of your signal. You cut your signal in half when you throw away "cold", so you lose 6 dB. That more or less means that the input to the Luna box is half as "loud" as it could be, and loses a significant # of bits in the A-D conversion. You can compensate of course but then you increase noise.

Anyway my take is: if someone wants to pay me well then I'll happily spend time preventing every last micro-volt of noise in my rig. In the meantime I'd rather spend my precious time & money on cool CW plugins.

Thanks folks! Cheers,

Johann
Victoria, B.C., Canada