crossover help

[seanski]

ok, so this is my first post in here, and i have a quick and easy question for you all about a two way passive crossover that i bought for my oversized bookshelf project. it has an IN+, T+, W+, and COMMON connections, as im sure you know. does the - from both speakers go to the common? and then does the common connect to the - on the amp? i know what the +'s are for, but have never done this before, and i need your help.

thanks in advance,
sean

[ThomasW]

. does the - from both speakers go to the common? and then does the common connect to the - on the amp?

Yes the commons are all connected together and wired to the (-) from the amp

[seanski]

thanks, thats what i thought it was, i just wanted to make sure.

[seanski]

ok, i have another question. the crossover i am using is 8ohm. my woofer is 8ohm too, but my tweeter is 4ohm. do i need to add resistance to the tweeter to make it 8ohms? or is it fine to do 4 ohms?

[Dave Bullet]

Welcome to the world of crossovers!

First rule - never buy off the shelf crossovers. It's a bit like buying a diesel engine then a can of gas and trying to get them to work together. Unless a crossover is designed to work with the drivers (speakers) you have, it will either sound awful (and you'll play around forever trying to get it sounding right) or you could damage your amplifier by wiring changes to the crossover that present a very low load and cause short circuit.

If you pad (add extra) resitance to the tweeter - you'll lower its output which you may need to do anyway since most tweeters are more sensitive than most woofers (meaning tweeters usually play louder than woofers at the same volume). Playing with the resistance in a circuit of a crossover, changes the slope and crossover frequency between drivers... A change within a crossover usually has an impact in other areas, so changes need to be considered & modelled to see the potential impact.

You can always try adding 4 ohms to see what happens... but don't expect it to sound good. Unless you know what you are doing.....

What are your goals?

[seanski]

Welcome to the world of crossovers!

First rule - never buy off the shelf crossovers. It's a bit like buying a diesel engine then a can of gas and trying to get them to work together. Unless a crossover is designed to work with the drivers (speakers) you have, it will either sound awful (and you'll play around forever trying to get it sounding right) or you could damage your amplifier by wiring changes to the crossover that present a very low load and cause short circuit.

If you pad (add extra) resitance to the tweeter - you'll lower its output which you may need to do anyway since most tweeters are more sensitive than most woofers (meaning tweeters usually play louder than woofers at the same volume). Playing with the resistance in a circuit of a crossover, changes the slope and crossover frequency between drivers... A change within a crossover usually has an impact in other areas, so changes need to be considered & modelled to see the potential impact.

You can always try adding 4 ohms to see what happens... but don't expect it to sound good. Unless you know what you are doing.....

What are your goals?

hey, thanks! i have been building automotive sound systems for a few years, but this is my first home audio project.

my goal with this project pretty much is to learn what goes into building 2-way speakers. it is a learning process for me, because i have never built any home audio gear before. i have built cabinets for subwoofers though, and i know how to determine volume, layout, and port length (if it is to be ported).

the project i had in mind was a set of oversized bookshelf speakers/studio monitors that i could use just for casual listening on my computer.

i need help choosing drivers that would work well together. i have a pair of 1" silk tweeters, and i want to find a pair of 5" or 5-1/4" drivers that sound good, but are not too expensive ($35-ish max each) and i dont really know where to start on crossovers.

[seanski]

the tweeters i have are these.

not the best by any stretch of the imagination, but i am on a low budget, and they had good reviews, and its a buyout.

[bluewizard]

Well, I'm no expert, but the high frequency section of the crossover is completely independent of the low frequency section of that crossover. If they weren't independent, then you really couldn't have biwire/bi-amped speakers.

So, you design the low section using the impedance of that driver at the crossover frequency. Then you design the high section using the impedance of that driver at the crossover frequency.

In short, design the low crossover for roughly 8 ohms then design the high crossover for roughly 4 ohms.

The only reason you need to 'pad' the high section or tweeter is to attenuate it; to bring it down to an output level equal to the woofer. Or, if by change you are trying to raise the overall impedance of the circuit, in which case, you really shouldn't be using 4 ohms tweeters in the first place.

Or at least, that's my opinion.

Steve/bluewizard

[seanski]

The only reason you need to 'pad' the high section or tweeter is to attenuate it; to bring it down to an output level equal to the woofer. Or, if by change you are trying to raise the overall impedance of the circuit, in which case, you really shouldn't be using 4 ohms tweeters in the first place.

all i'm trying to do is make it equal the impedance of the woofer. idk if that really matters though.

i know what crossovers do, but i really have no idea how they work. im a noob :P

[fbov]

Sean,
You're using a selection of off-the-shelf parts to make a speaker. Since you ask specific questions, you're getting answers, but the message should be clear - speakers sound a lot better when they combine characterized drivers with custom-designed crossovers.

As a first project, an off-the-shelf XO will provide a good learning experience, especially for someone seeking to make a career of it. No matter what combination of drivers and XO you use, it will make sound. As your technical understanding improves, you can learn to characterize drivers (or find data on yours) and how to use XO-development tools, and a whole lot more. Then you can upgrade the XO and see if it does sound better, and what flaws you now hear.

Have fun,
Frank

[bluewizard]

I think you may have missed my point. There is no need to make the tweeter impedance equal the woofer impedance. They are completely separate systems with regard to the crossover.

So, make the low frequency part of the crossover for the 8 ohm woofer, and make the high frequency part of the crossover for the 4 ohm tweeter, and just accept the overal rating of 4 ohms for the final system.

Assuming, you are just making a simple 12db/octave (2nd order) crossover, you only need two coils and two capacitors. One coil and capacitor for the low frequencies, and one coil and one capacitor for the high frequency section.

To get your basic values, you can start here -

http://www.apicsllc.com/apics/Misc/filter2.html

Notice, you enter separate values for the ohm rating of the woofer and the tweeter.

Now you need to decide on a workable crossover point. To do that, you need to look at the frequency response graphs of your speakers and decide at a common point well within the flat working range of both speakers.

Now if you can find an off-the-shelf crossover, that crosses over at the point you need it to, and is capable of handling both 4 ohm load and 8 ohms load, you are there, more or less, for a beginner.

But, it would be far better, to calculate the correct coils and capacitors needed to crossover at the specific crossover point wisely chosen for your specific 8 ohms and 4 ohms speakers. Part Express sells the necessary coils and capacitors.

To make the crossover even more accurate, don't use a generic 8 ohms and 4 ohms, look at the frequency response and impedance graphs for your specific speaker, and use their actual impedance at the wisely chosen crossover point.

Now, you have a basic crossover. But there is plenty of room to build on it from there.

For example, tweeters are typically louder than woofer, so you need to reduce the volume on the tweeter. Simply done with either a fix L-Pad, as an be seen at the bottom of the link above. Or, by using a variable L-Pad available from Part Express for roughly $10.

No, as the frequency rises, the impedance of the woofer rises, tolerable, but generally not a good thing. At least, put a Zobel Network on the Woofer. Again, you can see a calculator for the Zobel Network at the bottom of the link provided above.

Now, you've done about all you can do unless you want to start testing your speakers to see if there are any peaks or dips in the overall response.

You have an off-the-shelf crossover, and your basic question has been answered. Now, MY question is, are you sure you have the right crossover? How did you choose it? Pick a frequency at random? Pick a frequency that everyone else seems to be using? I think, you need to give the crossover point a little more thought than that.

The window or frequency range where these two speakers can effectively be crossed over with a normal off-the-shelf crossover, is very narrow. The only way to reasonably evaluate the necessary crossover point, it to look at the frequency response charts of both speakers.

So, to your basic question, yes, the 'common' or 'ground' or (-) is common to the input, the output, and to all the components.

If you want to know whether you've made the right choice, then we need to know the woofer brand and model, the tweeter, and the crossover you've chosen.

The tweeters you have, or at least linked to, have a rated low end response of 3,000hz, which means your crossover will most likely be 3,500hz or 4,000hz. Which in turn means you have to have a woofer or Midbass that comfortably reaches up into that range, and that would be a very rare midbass.

Steve/bluewizard

[alias2]

.... but are not too expensive ($35-ish max each)
and i dont really know where to start on crossovers ....

http://www.madisound.com/catalog/product_info.php?products_id=8525

spend $60 on the above and save yourself lots of grief, /alias2

Baffle step appears minimal but its great value - easily worth it for the bits if you want to mess about with the crossover and turn it into a genuine freefield mini, otherwise its more AV bookshelf. Sealed volume is as always flexible. Vented ~10L Tuned 40 to 55 Hz. (possibly not bad as the top section of a budget 3-way either .....) /sreten.

[seanski]

that looks sweet! thanks!

[alias2]

FWIW you can drop the Alesis tweeters into the above by adding series
~ 1.0R resistors to them. Its not ideal but you have no measurements.

You can then keep the Vifa tweeters for another project, Here you
do have measurements / specifications which you should save.

http://www.rjbaudio.com/Audiofiles/FRDtools.html

http://www.zaphaudio.com/

http://www.rjbaudio.com/projects.html

/alias2