Re: GAIN STRUCTURE 101 - Easy as you like

[andy butler <akbutler@tiscali.co.uk>]

Overview:
Essentially we have 3 different types of connection,
instrument(guitar), microphone and line.
If you want the easy route, just remember never to mix those
without some kind of device to convert from one to another,
then scroll down to the section on levels.


Microphones:
Most of these output at mic level, particularly these days.
If your mic outputs on an XLR socket, then consider that
this is definitely the case.
A microphone has to be plugged into a special kind of input
called a microphone pre-amplifier (aka mic-pre) before it
goes to anything else. You can use a dedicated mic-pre, or 
plug into some other piece of gear that has a mic input, like
a mixer or some fx units.

Guitars: 
These also need their own special kind of input, such as is provided by
a stomp box, a DI box, an effect unit with "instrument" input,
a guitar pre-amp or a regular guitar amp.
Piezo pickups need special circuitry, which may be inside the guitar.


Stomp Box:
Generally, these are designed to accept a guitar being plugged into them.
You may get away with plugging a mic that has a 1/4in jack lead on it
into some stomp boxes, e.g. the DL4.
Essentially, though, plugging a mic into a stomp box is very likely to not 
work well
at all.
The output from a stomp box is good for plugging into a guitar amp, DI 
box, or
an effects unit with instrument input, and of course you can always 
connect another stomp box.
If the stomp box doesn't have "hard bypass" (or if it's always switched on)
it's possible to plug it into a line level input.
Hard bypass means that the input and output of the stomp box are directly
connected by a wire, so in bypass you need to treat that stomp box output
exactly as if it were a guitar output.
If there's no hard bypass, then the stomp box output is likely to
be good for plugging into a line level input. Beware though, stomp
boxes can be made with very cheap components and may significantly
degrade your signal (e.g. Boss, though I don't know about the most 
recent products).

It's possible to connect the aux send



Impedance:
Now we have to mention impedance, but only for completeness, you 
can skip this bit and just learn the "rules" if you like.
Impedance is a quantity measured in Ohms, usually an input or an 
output will be called low impedance, or high impedance, but in every
case it's also possible to represent that with a number with a big range
of possible values. e.g. a line level output might be just a few ohms,
a microphone output, say, 300 ohms, a guitar output 10,000 ohms and the 
input of a guitar amp with valve input can be 1,000,000 ohms.
In order to get a good signal from the output of one device to the input
of another the most efficient case is when the impedances match.
It's usually case that a lower output impedance can be plugged into
a higher input impedance.
Impedance affects level of signal, but it is a totally separate 
consideration. 

Most microphones (usually with XLR) are low impedance, but some (mostly 
older ones)
are high impedance. A high impedance mic (will have 1/4 in jack) will be
more likely to work if you plug it into a stomp box or guitar amp.


Matching Transformer:
These are adapters to convert from:- 
a) low impedance to to high, which might help you plug your
  mic into a stomp box or guitar amp.
b) high impedance to low, might help plugging a guitar into a line level
   input.
Matching transformers aren't used much, and they pick up hum.
Probably best to see them as only an emergency quick fix.

 

Guitar Amp:
Has an input especially designed for guitar which should
only be used for guitar(or similar).
May have output/input at line level.
May have an fx loop designed for line level devices ( i.e.
an output and input for you to insert a line level fx).


DI Box:
Convert from instrument(guitar) level to line level.
In studios, some DI boxes convert to mic level.
DI Boxes vary in quality and price from "a cheap box" to "sublime".



Line Level Inputs:
(Audio Interface, Mixer, FX Unit)

This is what you'll find on a mixer, an audio interface or a rack mount fx 
unit.
Things not to plug into line level inputs are microphones and guitars, 
even if
you get a good level without too much noise the frequency range will be 
affected.
Some mixer inputs are switchable from mic to line level.


Phono Inputs:
Some devices have an input designed for plugging in a turntable (eg DJ fx).
These are not good for any other kind of signal.


LEVEL SETTING:
OK, now you've got your gear connected up.
Each device in the chain should have the loudest possible signal
running through it without distortion. 
We'd also like to have a nice high level running through each cable.

Ideally, each device would have a meter to help you get the input level,
in which case you simply follow the signal along starting from source 
(mic/guitar)
at each stage turning up as far as possible without distortion. Of course 
you ought to turn
down all levels before you start that. 
Some devices have a simple overload led monitoring the input, so turn up 
till it's going
into the red, then ease back down a bit. An occasional flash into the red 
is usually ok,
and is considered to be a sign that level is correct.
Remember that most players will be quieter for soundcheck than for 
performance, in
which case it may be worth taking level down a hint to compensate.
If there's no metering on a device then have a guess on the level, 
followed by
a troubleshooting session.

Of course, what we want is zero distortion, and as little background noise 
as possible.

To get rid of distortion:
Start at the end of the chain, the opposite end to the instrument.
Go to each level control in sequence. Turn it down gradually while
listening to the distortion, if distortion clears up then you've found
the problem, and you need to make up gain later in the chain (preferably
one step further from instrument).
If the distortion stays, getting quieter with the rest of sound, then turn
that control back up and move on to the next control( one step nearer
to instrument). 

To get rid of noise:
Start at the beginning of the chain, at the instrument.
Go to each level control in turn and try turning it down,
when the noise drops as you do this, you found it's source.
To reduce noise, take that control down, and make up the gain
by turning up a control earlier in the chain (preferably
one step closer to instrument). 

It may be necessary to go round the distortion and noise removing 
procedures
a few times.


Skim read the next bit in case you own a device which is covered by it:-

Decibels:those -10dB and +4dB figures.
Easy, 
it's just another volume control.
If your device can switch between -10dB and +4dB levels then there's 2 
possibilities
1) Pressing the button doesn't change the amount of output for a given 
input. In that
   case use the +4dB setting, and if that device introduces distortion go  
to -10dB.
2) Pressing the button makes the output louder/quieter, go for +4dB as 
long as the
   next device in the chain doesn't distort.


Specially designed Inputs:
Some devices are made for a variety of uses, the inputs
are carefully made so that you can either plug in an instrument(guitar) 
or a line level device. 
(e.g. Lexicon Vortex, EDP )
A lot of people use a DL4 with a microphone plugged in direct, it seems
to work well enough as the DL4 doesn't add much noise. Likely though
a given mic won't match the DL4 input. Try some different mics
(maybe an older model, with 1/4in jack, not XLR, more likely to be high 
impedance), try
an impedance converting transformer ( lo to high ), or get a
pre-amp.


OK, that's all you actually *need* to know, feel free to skip the rest.



Unnecessary Details:
In this case, these Decibels are telling us a the signal strength that is 
expected by 
the device, or the kind of level that they can put out.
-10dB was originally chosen for domestic equipment, so that the output of 
your
cd player matches the input that your amp expects. However, volume wars 
also
exists with cd players (stupid, but true), so manufacturers push up the 
output
of their cd players to the max, and then have to reduce the sensitivity of 
their amplifiers to match. 
So, -10dB as a standard is essentially obsolete.

+4dB is the standard in the recording studio for analog gear, it's louder
than the old -10dB standard......by 14dB

When you see dB markings on your mixer faders, these are slightly 
different in that
they don't tell you how loud your signal is, they just give an indication 
of
how much the level will vary when you move the fader.


Decibels are always relative, but may be relative to an agreed standard,
in which case (-10, +4) they are absolute. 

1 decibel is considered to be the smallest noticeable volume increment.
6 decibel louder corresponds to a 2x increase in signal strength, but
will not sound twice as loud, because your ear doesn't work like that.
Your ear has what is called a logarithmic response, which means that
a doubling of power in the signal only sounds like a step up in volume.
That's why you're able to hear sound at an incredible range of levels,
and why the Decibel scale is used to represent that. 


hope that helps

andy butler



Rick Walker wrote:
> 
> still perplexed,    Rick
> 
> 
>