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
