This is copied more or less verbatim from the General Discussion forum, at at0mic's request. This re-post includes corrections (thanks DeejaySly!).
This is mostly a "tip" on using balanced cables. It touches a bit on noise reduction as well. This "tip" is not strictly a CreamWare tip, though the subject matter certainly affects all CW users.
You can read the original topic for various other folks' insights into balanced cables, DI boxes, cabling techniques, and so on.
Apologies in advance if I bore you to tears!
---
Question: "Should I use balanced cables when wiring up my gear? I've heard that balanced cables reduce noise."
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
Code: Select all
Vdiff = (Vhot + Vnoise) - (Vcold + Vnoise)
= Vhot + Vnoise - Vcold - Vnoise
= Vhot - Vcold
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.)
Also if you have *lots* of cables, it's best to use balanced wherever possible. 8 channels of unbalanced cables at 20' each should be fine. But 24 channels of 20' unbalanced cables will start to cause you occasional noise problems. (These numbers are empirical figures that I made up.
)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 RF 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 current 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 nominal output is rated at either -10 dBV or +4 dBu. The Luna accepts -10 dBV signals. High end audio gear often outputs +4 dBu (or both, with a switch to choose -- like the A16 Ultra). 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 (volume) of Pro gear, but often you won't be able to reduce it sufficiently to get levels acceptable for -10 dBV inputs. 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.
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).
Code: Select all
balanced hot output ----> unbalanced in
balanced cold output
|-> unbalanced ground
balanced ground -----/
However, as DeejaySly points out, it's not safe to do this if your output device is *not* transformer balanced (i.e. if it *is* "active"). Many devices these days use chips, rather than transformers, to output a "hot" and inverted "cold" signal. Chips fry very easily, and if you connect a chip's output to ground with no resistance, you may draw too much power and damage the chip.
Some chip-based ("active") devices use servo-balanced or floating balanced outputs. You can use the modified cable technique above for these types of devices. (What exactly they do to isolate the chip from short-circuiting is beyond my knowledge.)
6) Ground loops. This can be a problem for noise. Proper grounding is very difficult to achieve for a small or home studio, simply because of the time it takes to sit down and properly lay out your entire audio system. If you're a masochist though
I'd recommend reading a good audio engineering book (see bottom of this post for a couple of suggestions). It is, however, definitely worth investigating if you get into problems with hum, buzz, and so on.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 un-grounded 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 15 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?!?!?
Oh, I'm speaking N. American again -- i.e. 120 volt power. I guess standard power in most European countries would be 7.5 amps, no?
A good rule of thumb is: all of your audio gear into one circuit, and other stuff (lights, fans, coffee machines, refrigerators
) into a different circuit. This way if you draw too much current from a fan or fridge, the circuit breaker doesn't cut off power to your audio gear.7) Lights. Don't use dimmers! 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.)
Noise generators. Keep your audio cables away from power cords, fluorescent lights, TVs and CRTs (LCD monitors are fine though), radios, and cell phones. Hearing a cell phone "ring" through your headphones at high volume is very unpleasant... 
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 lots of cables
- you have snakes / multicore cables (which, if they are cheaply made, introduce crosstalk between channels).
But that's half the fun of it, right?A quick note on DI boxes and coloration: there are actually two types of DI boxes:
- passive: these boxes use a transformer.
- active: these boxes are transformerless.
Active DI boxes should not give you any coloration (unless you drive them way beyond their maximum voltages, which is 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, microphones 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:It's very accessible and well-written.Sound Reinforcement Handbook
by Gary Davis and Ralph Jones.
For more comprehensive coverage (e.g. if you want to build your own mixing console from the ground up, schematics included...) there's:
These are both great books and you'll never finish reading them. They'll always be sources of new insights.Handbook for Sound Engineers
edited by Glen M. Ballou
Cheers,
Johann Tienhaara
Victoria, B.C., Canada
