I'm planning on doing some simple speaker upgrades on my '97 Lexus SC400. I don't see any crossover components on the schematics I've seen. Should there be any modulation of the output at the amplifier level before it gets to the speakers, or is it common practice to just send the full range signal to everything except the tweeters and just let them deal with it? I'm having a hard time wrapping my brain around the idea that a "premium" factory sound system would omit something so obviously helpful to sound quality and so easy and cheap to implement.
Generally HPF in tweets is limited to a 50 cent electrolytic cap, all other channels get full range in everything I've seen. Not sure what cars with built in "subwoofer" use, but I suspect that is a dedicated channel from the factory amp that is Low passed from the amp or source (A full range speaker and some test tones would confirm either way).
Of course your simplest drop-in solution is coaxial which uses the same simple "crossover", components will generally have a more advanced outboard crossover and generally never put LPF on the midrange probably largely due to the cost of coils, but also 95% of people who buy them intend to use them full-range without an amp, or at low enough power that full range doesn't run into mechanical limits of the drivers.
Of course your simplest drop-in solution is coaxial which uses the same simple "crossover", components will generally have a more advanced outboard crossover and generally never put LPF on the midrange probably largely due to the cost of coils, but also 95% of people who buy them intend to use them full-range without an amp, or at low enough power that full range doesn't run into mechanical limits of the drivers.
There are two "premium" sound systems for your car and they are Pioneer and Nakamichi. If it doesn't say NAKAMICHI on the faceplate, it's the Pioneer system. On both, your amp is in the trunk under the CD changer. That amp is designed especially for that car as a 4+1 output. Everything is preset for those speakers. On the speakers themselves, you will find capacitors for a passive crossover setup, especially on the front which are only 4 inch. There is no such thing as an electronic crossover module on any factory system, why would you be looking for one on a schematic? Electronic crossovers are aftermarket products. Factory OEM equipment uses passive crossovers.
The best upgrade you can do for an SC is to yank the head unit, factory amp, changer, and speakers and start fresh. The mids are TINY and you can modify the doors to accept 5 1/4 or go to the kick panels for 6.5in. Remove the factory single sub from the center of the deck and go to a box, a big 6 channel, or a couple amps.
Whatever you do, don't get it twisted that your 13 year old "premium" system is even comparable to today's average stereo in cars. Hell I have sat in the honda minivans on the lot and it was louder than most cars around the 2k mark. Factory stereo's are REALLY good these days. After 2000 manufacturers started really bringing it up a level and by 2005 to now it has been a race to make the flashiest car with the best factory stereo and blah blah..........
So while you seem to expect too much out of a 1997 premium system wanting an electronic crossover, which no manufacturer does, it would really hurt your feelings to know how far behind it is compared to todays factory systems.
All three of my SC400's sound systems went straight to the trash the first week, and if you expect any more than you have yours should follow suit. Tiny speakers, head units that only work with the amp they provided, and old technology all around.
Thanks for the input. I'm glad to run across someone familiar with both the car and the potential audio for it. What I'm looking to do is achieve some moderate improvement with modest outlay of both cash and exertion and hopefully keep it looking as stock as possible. I wasn't really expecting to find I had an electronic crossover, but I was surprised that there might be no passive components either. One of the statements you made implied there are some capacitors on the speakers - can you give me any details about what is used stock? Also, is there a low pass filter going to the sub? What do you mean by "the amp is preset for those speakers"? If you have any suggestions on nice soft dome tweeters that will fit nicely in the stock mountings I'm all ears.
What I'd like to do is just add passive crossover components as needed, replace the tweeters and the front door drivers with higher quality units in the stock mountings, and let that be it. I might later do more, but thats all for the moment. I'm not interested in investing in a high end, high power system for this car, I just want to invest enough to make it a somewhat more enjoyable system to listen to at moderate volumes. I value my hearing too much for high SPLs, and I get enough exposure to that with my band anyway.
Mine has the Nakamichi system.
What I'd like to do is just add passive crossover components as needed, replace the tweeters and the front door drivers with higher quality units in the stock mountings, and let that be it. I might later do more, but thats all for the moment. I'm not interested in investing in a high end, high power system for this car, I just want to invest enough to make it a somewhat more enjoyable system to listen to at moderate volumes. I value my hearing too much for high SPLs, and I get enough exposure to that with my band anyway.
Mine has the Nakamichi system.
You'll find passives on the mids and tweets in the form of a capacitor, the sub output is crossed over in the amp on the 5th channel.
The volume knob is the problem on the system when wanting to start swapping things because it links back to the amp, not just the head unit. So if the Nak amp doesn't push the mids the way you want (and it won't) then you would want to replace the amp... problem.... the head unit volume knob is on the dash, not the radio so you have no volume control now.
So now you find yourself with new mids on an old amp that you don't like due to very low power. You can't replace the amp without losing volume control and that eliminates the head unit as well. Soooooo...... BOO. That's what I think of it.
The volume knob is the problem on the system when wanting to start swapping things because it links back to the amp, not just the head unit. So if the Nak amp doesn't push the mids the way you want (and it won't) then you would want to replace the amp... problem.... the head unit volume knob is on the dash, not the radio so you have no volume control now.
So now you find yourself with new mids on an old amp that you don't like due to very low power. You can't replace the amp without losing volume control and that eliminates the head unit as well. Soooooo...... BOO. That's what I think of it.
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Fortunately, you won't be the one to have to listen to it. 
I'm assuming that if I upgrade only the tweeters and 4" mids in the doors as I propose, maybe adding an air core coil to the mids to roll off the top a bit, and I replace with near the same sensitivity (or better) and impedance, AND I'm already happy with the SPLs the system can produce without any upgrades (just want a more accurate, smoother sound), then at least in theory I'll be able to achieve my objective. My total cost I'm guessing to be in the range of $180 plus my valuable time.
If you visit, we'll ride in YOUR SC, okay?
Just for discussion, suppose I decide to do a true upgrade in the way you suggest. How much am I likely to spend for a pro installer to put in a system that you would consider worthwhile?
I'm assuming that if I upgrade only the tweeters and 4" mids in the doors as I propose, maybe adding an air core coil to the mids to roll off the top a bit, and I replace with near the same sensitivity (or better) and impedance, AND I'm already happy with the SPLs the system can produce without any upgrades (just want a more accurate, smoother sound), then at least in theory I'll be able to achieve my objective. My total cost I'm guessing to be in the range of $180 plus my valuable time.
If you visit, we'll ride in YOUR SC, okay?
Just for discussion, suppose I decide to do a true upgrade in the way you suggest. How much am I likely to spend for a pro installer to put in a system that you would consider worthwhile?
Caps on the tweets are RIGHT on the driver (like you'd see with a coaxial).
Personally I've seen many people happy with the addition of a small subwoofer (easiest option would be powered) as it really adds the illusion of a lot more impact.
I have found almost invariably, as the other guy said, replacing merely the speakers can be problematic since often they use odd impedence drivers, and they are super efficient so if you ONLY replace the speakers, your improvement in SQ will be at the expense of some output and possibly they may reveal shortcomings in the factory head and amp.
Personally I'm not overly concerned with upgrading rears since unless you use multi-channel processing with adjustable time delay it just creates a sloppy mess of phase and time issues. I focus on the front stage blending to the sub stage and there's plenty of reflection in a car for rear "fill" to take care of itself or you can simply leave the factory hooked up on HU power.
You can get a simple amp wiring kit for 20$ish on eBay, a high/low level converter for 15$ish and I know Stephen Mantz (forgot his name on this forum) at Zedaudio has some outstanding small American made 4 channel amps for appx 85 shipped. Go over the partsexpress and pick up components to fit your locations in the front (nearly anything there will be an upgrade from stock). From here you can have your factory head unit and some good quality front stage with ability to adjust crossover frequency (at the amp). If you have the time and ambition, 40-50$ worth of materials plus some time to put some sound deadening in the doors will greatly improve midbass performance.
To do a subwoofer from there is a whole other can of worms but I'd budget 100$ minimum plus some time for anything decent.
If you don't want to DIY the install, often the big box stores do the cheapest installs. Most shops charge from 40-90$ an hour (depending on your area) for labor and even for a professional, replacing factory speakers is a half hour job, and an amp install is 2 hours depending on the car give or take). 45$ to replace stock speakers or install a head unit and 90-140$ for amp installs is about the range you'd expect to pay.
Personally I've seen many people happy with the addition of a small subwoofer (easiest option would be powered) as it really adds the illusion of a lot more impact.
I have found almost invariably, as the other guy said, replacing merely the speakers can be problematic since often they use odd impedence drivers, and they are super efficient so if you ONLY replace the speakers, your improvement in SQ will be at the expense of some output and possibly they may reveal shortcomings in the factory head and amp.
Personally I'm not overly concerned with upgrading rears since unless you use multi-channel processing with adjustable time delay it just creates a sloppy mess of phase and time issues. I focus on the front stage blending to the sub stage and there's plenty of reflection in a car for rear "fill" to take care of itself or you can simply leave the factory hooked up on HU power.
You can get a simple amp wiring kit for 20$ish on eBay, a high/low level converter for 15$ish and I know Stephen Mantz (forgot his name on this forum) at Zedaudio has some outstanding small American made 4 channel amps for appx 85 shipped. Go over the partsexpress and pick up components to fit your locations in the front (nearly anything there will be an upgrade from stock). From here you can have your factory head unit and some good quality front stage with ability to adjust crossover frequency (at the amp). If you have the time and ambition, 40-50$ worth of materials plus some time to put some sound deadening in the doors will greatly improve midbass performance.
To do a subwoofer from there is a whole other can of worms but I'd budget 100$ minimum plus some time for anything decent.
If you don't want to DIY the install, often the big box stores do the cheapest installs. Most shops charge from 40-90$ an hour (depending on your area) for labor and even for a professional, replacing factory speakers is a half hour job, and an amp install is 2 hours depending on the car give or take). 45$ to replace stock speakers or install a head unit and 90-140$ for amp installs is about the range you'd expect to pay.
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