Preamp
The Original Preamp
This is the circuit that started it all. The circuit is very easy to build, and all parts are easily obtainable from Radioshack and should be less than $20. Gain with parts shown is just over 4. I built it on perforated veroboard, and used jumpers to make all the connections. The preamp was taped down to the amplifier with a few layers of tape on the bottom of the circuit board to prevent shorting out the connections. Keep the wiring as short as possible, and twist the signal wires to prevent inducition of rf noise. values are not all that critical, but the capacitors need to be rated at least 25v. you can use 1/4 or even 1/8 watt resistors if you choose. and before people say "ewww a lm741", keep in mind that the slew rate is more than adequate for bass, and is dirt cheap.
Connections (2006 Mazdaspeed6, may be different for other cars)
12v power can be tapped on the amp harness: orange wire
Ground: tap black wire
In+: cut purple wire, connect harness side to preamp input, amp side to preamp output
In-: tap yellow wire
Also, because this is and inverting preamplifier, you will have to cut the speaker wires (green and red wires) and connect green wires to the red wires, and vice versa, or you could swap the connectors at the ends of the wires with new ones. otherwise the sub will be out of phase in relation to the other speakers. i tried the circuit non inverting, but it didnt work as well. I have used this circuit for a while, have no problems with ground loops or noise. Don't expect to have ground pounding bass but many people have enjoyed the improvement in sound.
if you have any doubts on building circuits or if the above schematic looks like greek to you, DO NOT build the circuit as you could damage your car, start a fire, fry the amplifier, etc. i assume no responsibility or liability for this information. use at your own risk!
The schematic:
