DIY Active Crossover

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I don't know how much experience you have with car audio but... You'll need some sort of power supply to run it. The basic choices are:

Run it from regulated voltage and use a voltage divider for biasing. This is the simplest supply but there are drawbacks (turn on pop is the biggest problem).

You can build a simple switching power supply using an oscillator, capacitors and diodes to generate the ±voltages. More complex than the first but the parts are relatively easy to get and there are generally no big problems to overcome.

You could build a transformer based switching power supply. The parts are a little more difficult to get and sometimes there are really strange problems (can be due to transformer winding layout or even board layout).

If you want a crossover that's easy to tune, look up some information on 'state-variable' filters. They use 3 op-amps per channel. The tuning can be accomplished over a wide range with 2 pots per channel. The pots for each channel must be on the same shaft so they track evenly.

The National Semiconductor LF147 datasheet has the basic circuit. The datasheet for the LM148 has equations if you need them. I'd suggest using an op-amp like the MC33078 or MC33079. If you want to spend more, the BB OPA4227/OPA2227 may be a little better but are not as forgiving and are much more expensive. The OPAx228s are not an option because they are not stable at unity gain.

Don't forget that you will have to have an isolation circuit for the input to avoid engine noise. The transformer based power supply helps here because you can use the transformer to produce a floating ground. For the others, you can use a standard ground loop isolator (available at car audio shops) or you can design a balanced input (additional op-amps will be required).

No matter what route you take, there will be a considerable amount of time/effort required.
I've never seen the insides or the circuitry in a purpose made car crossover or EQ. The op-amps in the preamps of most car amplifiers are typically run with supplies that are resistor and zener dropped from the main rails (like in almost every solid stage guitar amp ever made). Some amps use regulators like the 78xx/79xx or the LM317 and it's negative voltage friend.

You can power the circuitry by:
1. building (or salvaging from a cheap junk amp) a step up power supply, and then regulating the output. This will give you the advantage of galvanic isolation from the preamp's ground, and the power supply's ground.
2. using teh car's voltage. I would use a 7809regulator to make a clean and stable 9 volts, and then power the op-ampswith that, using a voltage splitter to make the pseudo-ground. There will be no isolation of grounds, so you Nee either a balanced input stage or an input transformer

Have you ever considered using those switched capacitor filter ICs? wire up the pins to set high/low pass and the desired crossover slop profile, and then use a high frequency oscilator, the break point of the filter is a function of the oscilator's frequency. then just whip up a variable osc with something like a 555
You can make a 3 way corssover from a 2 way easily enough, just use standard high and low pass filters for the high and low pass, and then cascade a low pass filter and then a high pass filter to make the bandpass. I'd still suggest looking at those switched capacitor filters. They look pretty neat.

Maybe you want to see if you can borrow a copy of "The Art of Electronics" by Horritz/Hill (big *** thick bible of electronics). i've also seen some interesting designs in the "Audiphiles Project Sourcebook" (more of a compilation of cookbook projects).
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