Owner`s manual

Minimizing Noise in Autosound Systems

Figure 5. RF bypassing the power supply lines.

Level matching

Last but not lease is level matching. I’m presenting this last because all the other aspects of the

system must be right first. Level matching helps minimize hiss: the residual noise output of the

head unit. Level matching can help make other noise sources (alternator noise, etc.) less evident by

making the useful signal much louder than the noise. This is the reason for keeping the volume

setting high, and the amplifier sensitivity low.

In many autosound systems, the manufacturers have provided level controls to allow the sensitivity

of the unit to be adjusted to suit the requirements of the installation. Since there isn’t much stan-

dardization within the autosound industry, such controls are both useful and necessary. As men-

tioned earlier, picking the proper operating level for a device gives you the best compromise

between noise and headroom.

In the case of a head unit and a power amplifier, you want the output of the head unit to be high

enough to still give reasonable headroom, while being as far above the noise floor as possible.

From the amplifier’s standpoint, the level control should be set so that the amplifier is just barely

at clipping (or too loud, whichever comes first) at that point. Typically, this means a fairly high

(75% rotation) volume control setting. Remember: even though the head unit’s volume control is

turned way up, you still get full power output out of the amplifiers once you match their sensitivity

to the output level of the head unit.

There is another benefit of getting the highest possible operating level for the head unit: If noise is

induced into the cable or wiring connecting the head unit and the power amplifier, the audio signal

is subjectively much louder. This helps to make it more inaudible.

Here’s a simple procedure:

1. Set the amplifier level control to minimum sensitivity. This is the setting where the volume is

lowest.

2. Using either FM radio or a tape for a signal source, advance the volume control on the head unit

until you hear distortion or reach 100% rotation. If you reach distortion before 100% rotation,

reduce the setting of the control slightly until the distortion disappears.