| Author | FAQ- Ground Loops, Gain Controls & Line Drivers |
| Mr. Brownstone
Head Honcho Joined:
Nov 16, 2001
|
 Posted:
2001-11-19 20:13
This post was in response to a ? regarding line drivers, but ended up all about ground loops & gain control settings. ------------- Let's look at a system and find out what a line driver does. Your head unit sends out pre amplifier signal from 0.00 volts(nothing) to a maximum figure (4-6.5V on higher end models). From there, it travels to the line driver (or EQ or xover) where you can increase the voltage about 3-9dB for a maximum figure (usually 12V on most line drivers, 8-9V on EQs & xovers). At that point, the signal coming in will not be amplified by the line driver, rather, contribute to distortion because it is overdriven. The basic purpose of a line driver is to allow the amplifier to be set at one of it's lower (or lowest) settings to avoid system noise. You can't make a signal cleaner by adding a device. How does the device know what is desired & what is not? Adding a device always adds noise, but who knows if that noise will sum or cancel out or both? SYSTEM NOISE One of the good characteristics about system noise is that it is constant. Alternator whine, cd/turn-on thump doesn't go up in volume with the turn of the volume switch, and therefore can be overpowered. It may change frequency with engine speed, but not get louder. The pre amplifier gain control of an amplifier does 1 basic thing--tell the amplifier's main device how much signal to ignore. When the gain is set at the loudest setting(ex: 0.1V) the amplifier will amplify nearly ALL of the signal that it recieves. When the gain is set at the quietest (ex: 6-8V) the amplifier will ignore much signal below a certain threshold--let's just say 0.5 volts Remember that noise is constant, so if you have 0.2V of alternator whine, and you have the gain setting set at it's loudest setting(0.5V), the amp will pick up the alternator whine AND music. When you have the amp gain set at the quietest setting (6-8V), the amp will IGNORE anything below 0.5V. This basically eliminates the noise. Line drivers also assure that you are able to maximize your amplifier output before clipping. When you have a 0.5V head unit (CD) you may not be able to get the most out of the amp. If you have a head unit with a 6.5V preout or higher (and it is legitimate), you WILL NOT LIKELY need a line driver. Noise can usually be isolated once you set the amp to the 4V setting; which should allow for both noise rejection, AND gain overlap.
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