Tuesday, May 09, 2006

Gain Structure :|: Matching Signal Levels

Today's Topic: The Basics

Last week Tuesday we discussed the basic idea of gain structure. Gain structure is the way you adjust input and output levels on sound equipment to make sure you get the largest possible dynamic range. That dynamic range will give you a relatively noise-free system without distortion.

To set the gain structure correctly between all your components, you need to know a few details about each of your pieces of equipment:
  • Input level expected
  • Output level given
  • Input impedance
  • Output impedance
  • Unity Gain Location on the Knobs
We'll cover only the first two today. Input and output "levels" are voltages often referred to with deciBel (dB) measurements. For most sound systems, you have three options for levels:
  1. Line Level :|: used by professional audio equipment
  2. Consumer Level :|: used by consumer audio equipment, keyboards, guitar pickups, and other instruments.
  3. Microphone Level :|: used by professional microphones and direct boxes
A line level signal carries about 1V and is referred to as +4dB. A consumer level signal is between -10dB and -20dB and a mic level signal is -50dB.

To "align" your components correctly, you need to make sure that the output level of each piece of equipment is the same as the input level of the piece that follows.

Here are a couple examples...
  • The singer's microphone has a mic level output and plugs directly into your mixer which has a pre-amp which accepts a mic level input. Perfect.
  • The guitar player's instrument level output could plug directly into the mixer, except that the mixer's pre-amp doesn't accept an instrument level input. To alleviate this, you add a direct box between the instrument and the mixer. The direct box can handle an instrument level input and outputs a microphone level signal which the mixer can accept.
The biggest challenges to gain structure arise when you put a piece of consumer grade audio equipment in line with professional grade audio equipment. The system will probably work without correcting the level imbalances, but the dynamic range will suffer.

The simplest (and most expensive) way to solve this problem is to make sure that every piece of equipment between the sound mixer and the power amplifier is professional grade and uses only line level inputs and outputs. However, sometimes the money isn't available or there is an old piece of gear that I just really love and want to keep using.

To handle this, you have to put a level reducer inline between the output of the pro gear and the input of consumer gear. Then place a pre-amp of sorts between the output of the consumer gear and the input of the next piece of pro gear to bump the level back up. A good example of line balancers are the Whirlwind LM2U and LM2B.

So, here's your job before our next installment in seven days: go and check the back of all your gear, or (heaven forbid!) the manuals, to see what input and output levels each piece uses. Then see if you have any imbalances, and try to fix them!

Let me know if you have any questions.
More Posts on the Basics

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