Passive Crossovers/ Zobel etc..

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Scarlett Rose

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Sep 8, 2023
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Hello New Here. I am a weekend local sound provider. I have a small club system.
Anyways I am loading 4x small wedge monitors. they are 12" x 1" little guys.
So I measured my boxes and best I can figure by my "close as I can get bending a tape measure" measurements the box is approx 1 cu ft sealed. So I punched up a bunch of differant speakers into a calculator till I came up with one that suited a 1.15cu ft sealed box and comes out to be 3db down @ 79Hz. I bought cheapo PRV D280Ti-S compression drivers and ABS 11x4" 90x40deg horns.
Now not knowing the frequency graph of the woofer I just went with the horn drivers recomended crossover point of 1.8K 12Db. I purchased ACT 15 crossovers off ebay. They have a -12Db attenuation for the horns as my horns are 109Db sens. and the woofer is 97Db sens. I realize this is NOT gonna be a Phase aligned perfect crossover setup, but trying to make it best it can be and Ill massage the rest with a graphic EQ.
This is a link to the crossovers schematic.

Can any of your more technical guys tell me if I can add a zodel accross the woofers terminals without messing up the crossover? I allready calculated using an online calculator what resistor and cap I'd need, just not too savy with crossovers.
1694206128347.png
 
Unfortunately, to know if that crossover is remotely well matched to your project, or if a Zobel will help, you really need the frequency response curve and Impedance curve of the drivers in your actual box.

Room EQ Wizard (aka REW) is free software that can do both, and entry level measurement mics start around £30, so assuming you have an audio interface and necessary cabling, it's not painfully expensive to get started with a measurement rig that will at least take out much of the guesswork, even if it's not quite as accurate as proper pro setups.

Edit PS: if you're determined not to invest in measuring, there's lots of helpful info on how to make the most of the situation in this thread over at DIYAudio.
 
Yea I don't have a interface or anything. Mostly Im wondering C1 and R2 I know are the 2nd order of the woofer crossover, but they are wired much like a Zobel(just wrong values) wasn't sure what R2 is doing. I calculated that my woofers woulda needed a 9ohm resistor and 11.30uf cap(12uf) for a Zobel, But not sure how that would work being as there is all ready a Cap/Resistor setup be for it.
Pretty much the monitors are gonna "be what they are" I'm all ready planning on having to carve the EQ, just would be nice if a Zobel flattened them out a hair(that is assuming they aren't all ready).
Ive got the boxes mostly set(padded, horns/drivers installed x-overs installed) just waiting on 12"s to get here and figured maybe Id slap a Zobel accross the terminals while I was doing it.
I dont have a ton into these boxes, just trying to make um throw enough vocals back at the players without feedback.
 
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R2 will be adjusting the shape of the transition from flat to rolling off, aka the Q of the filter. Whether or not you need it (or a Zobel) depends on the driver's frequency and impedance responses, in-box.
What model of 12" are you using?
 
As shown, the woofer circuit looks like a 1st order crossover L1 with a Zobel comprised of C1 and R2. At high frequencies, C1 effectively becomes a short, putting R2 in parallel with the driver, dropping the impedance below 2 Ohms, probably burning up and needlessly wasting power, unless using a 2 ohm woofer.
Looks odd.


The head SpeakerAddict is: [email protected]
Phone 562 972-5992
Maybe he has an explanation of why R2 would be so low.
 
David Morison said:
R2 will be adjusting the shape of the transition from flat to rolling off, aka the Q of the filter. Whether or not you need it (or a Zobel) depends on the driver's frequency and impedance responses, in-box.
What model of 12" are you using?
Click to expand...
Not sure what exact model they are. They are overstock OEM speakers I found on ebay, They give all the T/S specs though, that is how I figured them to my boxes and figured the zobel components.
There is not much room to add a port on these so I keep punching up differant speaker T/S specs till I found one that matched my sealed box volume. My next choice was gonna be an Eminence Delta Pro but they woulda been -3Db down at like 103Hz. So when I found these they they showed a -3Db downpoint of 79Hz so I went with them.
I think I will email speaker addict and ask about that resistors function. I have 8ohm drivers. It almostt seems like a 1st order crossover with a zobel with way off values.
Art, how does the high section of this crossover look to you? Is the coil a bit small(half the woofer value).
 
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1694332278045.png

OK, so Green is the response of the filter* without R2, showing a bit of a peak (ie High Q) before rolling off.
Red is with R2, showing a more moderate response.

* Note that this assumes a perfectly straight 8Ω impedance - real drivers will show deviation from this.

Art, I don't think your low impedance at high frequencies will be a problem because the combined block of C1+R2+Driver is in series after L1, so the high impedance of the inductor at higher frequencies will dominate. With 100V of pink noise applied, R2 only dissipates a couple of watts (again, assuming a flat 8Ω driver):

1694355851463.png
 

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Scarlett Rose said:
Art, how does the high section of this crossover look to you? Is the coil a bit small(half the woofer value).
Click to expand...
Like David said, you really need the frequency response curve and Impedance curve of the drivers in your actual box.
Without knowing the impedance curves of your horn/driver combination, and the impedance curve of the woofer and resultant roll-offs, it just looks like parts to me.

I don't have any of the programs David has that could help figure out what would be a good starting point for your project.
 
I suspect you're really over thinking this. Without knowing how the components currently perform you don't have a starting place for designing a passive crossover network. You need to measure the speakers acoustic output using software like REW or OSM. Model the desired acoustic performance with an active crossover, ideally a DSP based crossover where you can choose characteristics easily and only then start to chose passive network values and details like Zobel networks. The Z network itself is to adjust the apparent impedance of a driver to make the passive crossover network work as desired. Have you measured the impedance of any of the drivers? I gather this is a bit of entertaining DIY as much or more than an effort to get some usable live music speakers so go the extra mile to do it effectively.


Scarlett Rose said:
Not sure what exact model they are. They are overstock OEM speakers I found on ebay, They give all the T/S specs though, that is how I figured them to my boxes and figured the zobel components.
...
Click to expand...
 
There is a freq plot for the horn drivers. When I bought the crossovers they said compare the upward slope of the crossover to the horn driver plot. The upward slope should be opposite the roll off of the horn so I picked one that was.1694390684441.png
1694390771878.pngAs far as the Zobel it was really just a "that might be good" thought, I figured flattening the impedance might atleast make it a little better even if the crossover area is not flat. Im not expecting these to be killer monitors just want to throw vocals back at players on a small stage without feedback.
For FOH I only use proven designs Ive heard in person or know of to sound good, no DIY cause Im a 100% novice to speaker building. I kinda know the jist, BUT I dont. Mainly I got 4x empty wedges off a freind for a good price and decided to load them with components that were "in the same class" with the boxes which are, rat fur covered mdf.
Guess maybe I should just throw the drivers in and fire um up and see what they do,befor I try to bench engineer with no data.
 
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Yes, better to hear/test what the crossover you bought does with the drivers before bothering to change it.

The frequency response you posted is for the PRV D280Ti-S compression driver on a WG14-25 45x45 degree round exponential horn (6.8"x6.8" x10.25"), you bought 11x4" 90x40 degree horns.
Frequency and impedance response are determined by the horn/driver combination, so don't expect a 90x40 degree horn to be very similar, and it may be 3-6dB less sensitive due to around double the nominal dispersion.
 
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OK a couple days after asking, the woofer seller sent me some data, I cant seem to copy and paste it here. I guess the 12"s are MCM 55-2952 never heard of them befor (heard mixed reviews from good for the buck to pooper scooper)there is a freq, plot on this data sheet. Would that help in figureing the crossover along with the horn drivers freq plot?
 
Ah so differant horn bell the plot is meaningless.
I got a feeling by the time all is said and done Im gonna end up ordering a hand full of mixed value caps and resistors and hanging the xover out the speaker jack hole and swapping stuff till it sounds OK.
Gotta reading an article on passive crossover and the math is beyond me.
 
Scarlett,

Been there, done that.
Use REW, far faster and easier than using your ears, and you can compare traces instead of trying to remember WTF went on.
 
Well I got the woofers today. I installed one and dragged a wedge down to the basement. Hooked it to the amp that will power it wide open. Plugged straight out my ancient EV console that lives down there. Mains at zero, channel at +5, channel flat and brought up the gain slowly with an SM58 hooked up.......lotsa ringing above 2K.......so having no real cables here at the house..........I dug up an old insert cable and inserted a 15 band graphic to the channel. By the time I got it to stop ringing and loud I had basically shelved down everything from 2K up. Which tells me the horns are too hot.
If I wanted to anaylize these and do it right, what ya think a ballpark would be between anaylizer mic, program, interface etc.... to do so? (then the fact that Im like a monkey with a cresent wrench with computers too LOL) I also have an old set of 2 way cabs I no longer use that could use a real crossover as the original is missing( I wired um for bi amp but that was a PITA for little cabs)
EDIT...Just had a crazy idea. Ya think if I took one wedge to a car audio shop and had them anaylize these and build a crossover for one and I copy the rest or are those kinda places pretty unreliable to get um flat?
 
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The only parts you have to buy are the mic & interface, the software is free (well donorware, so free to get started with at least).
The Behringer ECM 8000 mic & UMC22 interface should be about a hundred £/€/$, then you just need a mic cable, one TRS jack to jack cable for a loopback connection and one cable to get audio from the interface to your amp.
REW as linked earlier in the thread will do all the measurements you need, you will just need to be a bit patient and read the user guide carefully to get everything set up correctly at first.
 
Wow just looked that stuff up. Not expensive at all!! Just under $100 on amazon for the 2 items............now to find a freind who is computer literate to help me with the computer end. I'm much more illiterate than one might think.........like Encino Man bad LOL
 
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