Owner`s manual

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3. Now adjust the sensitivity control on the power amplifier until you hear it clipping, or the
system is too loud.
Remember: you want as much signal going through the cable between the head unit and the power
amplifier as possible.
Interfacing with internal power amplifiers
Sometimes you have no choice but to use an existing powered radio/head unit as the front end for a
sound system. Although the speaker connections were originally meant to drive loudspeakers,
there’s no reason why they can’t drive components that were originally meant to be used with an
unpowered head unit. The biggest problem is simply one of signal levels.
Basically, a powered radio (I’m going to use the term “radio”, but you can translate that to mean
radio, cassette deck, or manufacturers installed stuff) is nothing more than a head unit with a lot of
output capability (voltage drive capability). Don’t confuse the fact that this thing was meant to
drive loudspeakers (power) with it’s voltage output capability. Voltage and power are two different
things.
Now there’s two different types of radios we are dealing with...the so called “normal” ones with 3
to 5 watts output on a good day and the “high-powered” ones with 10-15 watts output. Basically,
there’s three ways to get high power when you’ve only got 14.4V to work with:
1. Use an output transformer to lower the effective impedance of the load. If you reduce the load
impedance to around 1 ohm, you can get some pretty impressive output figures. Unfortunately,
output transformers are large (if you want them to pass low frequencies) and expensive (if you
want them to sound good). That precludes using them in a radio intended to be built into a
dashboard.
2. Use a DC to DC inverter to raise the 14.4V to something better suited to getting more power
into a 4 ohm load. Again, it’s bulky and expensive. This sort of technology is better suited to
being hidden in the trunk.
3. Use two amplifiers in “bridge” configuration. This is the infamous “floating output” connection.
Figure 6 shows how it works. Basically, as amplifier 1’s output is going positive, amplifier 2’s
output is going negative, by exactly the same amount. The net result is that the power supply
voltage is doubled, which means four times the power into the same impedance. For example, if
one amplifier would deliver 3.5 watts into a 4 ohm load, then two amplifiers in bridge connec-
tion would deliver 14 watts into the same 4 ohm load.
Figure 6. How bridged, floating outputs work.
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Minimizing Noise in Autosound Systems