A crossover question

mr greg

mr greg

Active Member
I decided to upgrade my bookshelf speakers I built a few years ago. I think it was 1966. New 12" woofer and a silk dome tweeter. The woofer is spec at 18-5KHz and the tweeter at 2k-20k. Am I right in assuming that I should use a 2k crossover or should it be more like 3k or 3.5k? I have been looking at the Dayton Audio crossovers from Parts Express.
 
That's a tricky question you're about to get a buttload of answers to. If you don't have a means to test, pick a point in the middle.
Most manufacturers will overlap a bit - it depends on the drivers. Cross the tweeter over 3000, the woofer over at 4000. Test equipment would give you a way to measure the "hump" created at the overlap, or your ear may be good enough to hear a preponderance of 3500.
I suppose another method to use if you don't have access to test equipment would be getting the graphic frequency test readouts of both drivers and cross them over where the line intersects, as long as it's within the published ranges of each.
 
Without cabinet volume or design (ported vs sealed) and T/S parameters on the woofer, the question is moot. However, something to consider is a simple DIY crossover that operates the woofer as "full range" and a utilizes a high pass filter to limit the tweeter feed within it's operational range (2K or higher). Another factor to consider is the sensitivity of each driver--a really "efficient" driver will overpower one of lower efficiency and the "system" will be way out of balance.
 
Normally don't want to cross your tweeter at the low end of frequency range. It can reproduce it, but power handling and output down there can be very limiting. Normal rule of thumb is to go double the fs and cross there, or somewhere above. What is tweeter fs?

I am just guessing a good crossover point would be between 2500 and 3500 but would want to see frequency curves for both and T/S parameters.
 
There is a huge unanswered variable here, and that is the sensitivities of the drivers in question.

If the tweeter is close to the woofer in sensitivity, you will need to run the highest possible crossover point to maintain reasonable system power handling.

If the tweeter is much more sensitive than the woofer, it will allow some flexibility/choice in the crossover point, but will also require padding down to balance the system.

If the tweeter is less sensitive than the woofer(this is possible with domes) you have chosen drivers poorly, and are facing a much bigger challenge.

Without cabinet volume or design (ported vs sealed) and T/S parameters on the woofer, the question is moot.
Click to expand...
Agree, alignment will dictate the actual usable upper limits of the woofer despite the FR spec on paper.
 
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You could run the twelve all the way up and use mechanical doping to smother the break-up, then just use a capacitor to protect the tweeter. Try a couple of different values of capacitor and judge by ear. I'd aim to have the tweeter coming in as high as possible and let the twelve do the rest. The specs of the mid/bass driver would be useful info to have.
 
As the woofer and tweeter are new, and you have the box volume - you might want to share the information and advance the conversation.
 
I'd add: Get a microphone (not necesary an expensive one) and get familiarized with ARTA software.

After succesfully tweaked several crossovers, I've found that without measurements, it's a waste of time and effort.

The "book" crossovers usually don't work as you expect, I don't know why, but crossover slopes never look like the graphs when you measure with a mic.

I've recently made a custom crossover for a customer, I've ended with a 2nd order xover for the tweeter (measured impedance: 7 ohm), using a 1uF capacitor, what in a "calculator" means more than 10KHz crossover point, but they sound awesome in this configuration, with a woofer crossed at near 2.5KHz. The "measured" tweeter crossover point looks close to 2.5KHz in a graph.

So my advice is: start with a mic, and doing measurements you'll be able to see the effect of the components in your crossover, and match the output if you need to attenuate the tweeter.
 
I agree with all of the above. At this point the question is kind like saying you're going to update your home built hot rod, you bought new tires and wheels and an engine, and you're wondering what to use for a transmission. The answer is, it's kinda complicated to get right. :D
 
Off-the-shelf crossovers and 'book' values are completely useless as drivers are not resistors and their output is often very far from flat.

We need the driver impedance measured first at the target crossover range, and the frequency response. To get the impedance we just need a multimeter, signal gen, resistor and a spreadsheet (of course there are automated tools to get this too).

To get the response curve is harder. I've had good results with the Dayton Audio iMM-6 mic and the iPad 'AudioTools' app with the LARSA module (here).

Once you have all that the crossover can be built. 12" speakers often have a big peak around 2kHz so if going 1st-order you might find an inductor value giving a 'book' crossover of 1kHz or even lower actually keeps on-axis output flat to 2kHz.

Tweeters should be easier so just some resistors to match the SPL (and the filters, of course!).

Hope that helps.
 
rmeade said:
As the woofer and tweeter are new, and you have the box volume - you might want to share the information and advance the conversation.
Click to expand...

Thank you all for your reply's.
The box is 1.5 cuft. It was originally a ported 3 way system. I made a new front for it for just the 12" and tweeter and no port. I had a couple of Electro-Voice X-36 crossovers that I hooked up but I'm not sure they are the best ones to use for this set up. If I had to I would make new ported fronts. Which would bring up the question of how big a port.
The 12" has a sensitivity of 87db and the tweeter 92 db. I did put a L-pad on the tweeter.
Thanks,
Greg
 
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Assuming the tweeter is 8-Ohms, you need roughly 3R3 in series and 10R in parallel to the tweeter (connected after the crossover). Use 10W wirewounds.
 
mr greg said:
If I had to I would make new ported fronts. Which would bring up the question of how big a port.
Click to expand...

Which, again, can only be determined by inputting the T/S parameters for the woofer into a model like Box Plot. There are free ones available on the web. The woofer you are using may or may not even be happy in a ported enclosure.
 
Can o' Worms

You may have opened a bigger can of worms here that you had intended here, Greg. If you can list the make and model numbers on your drivers, I'm sure plenty of guys here will look up the necessary parameters and calculate what you came looking for.
Without that information, the best you can do is protect your tweeter.
 
Porkloin said:
You may have opened a bigger can of worms here that you had intended here, Greg. If you can list the make and model numbers on your drivers, I'm sure plenty of guys here will look up the necessary parameters and calculate what you came looking for.
Without that information, the best you can do is protect your tweeter.
Click to expand...

The drivers are from Parts Express. A GW-1248 woofer and a GT-520 tweeter.

I did rewire one of the cabinets last night bypassing the x-over for the 12" but still running the tweeter thru the X-36. I did notice some improvement in overall balance.
Thanks,
Greg
 
Which driver sounds sweetest between 2k and 5k ?
I would put more to that driver, but stop a bit short of it's extremes.
 
Just a quick glimpse of the FR plots from those drivers, and assuming you can get the woofers in an alignment that doesn't drastically alter that response below 2500, i would want to cross below that nasty little 2500hz bump in the woofer's response. Without real world measurements or commitment to a specific box, i would aim for an XO around 2khz. The tweeter is probably going to want a few db of pad to balance the system.

That woofer's(being a 12) polar response is going to be getting pretty beamy by 2k, i don't think i'd want to try to push it any higher.
 
FWIW I'd go for 1st order at 1kHz, this should keep it flattish to 2.5kHz. We need the measured impedance to calculate the inductor value though, it is likely more like 10 ohms at 1kHz.

HF is tricky because of it's response curve. You could try a 1st order at 5kHz and bring it's level down 3dB or so and see how that goes.
 
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