Omni 15TB Crossover Points

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Stylojon
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Omni 15TB Crossover Points

#1 Post by Stylojon »

Hello All

Please be gentle, i'm not an expert - an enthusiastic amateur is my level

I've built an Omni 15TB and am happy with it.

Some time has passed and i've tinkered with it here and there.

I was wondering if it was possible to check and confirm the crossover points for the LF MF and HF elements?

I am currently running a tone from my phone through the omni and trying to gauge where each section of the speaker is doing it's share of the work.

I've removed the MF and HF elements from the equation and am starting with the LF

I was kind of expecting the LF section to begin to fade @ around 700hz as a guess, but my 2 x SM212 Beymas are not showing any signs of giving up...

I'm wondering if i've messed up with my crossover builds... LF section has been built to 4 ohms as using 2 x 12" 8 ohm drivers.

The Omni sounds "good" to my ears and all three elements are working, but not having something to compare it to is making me wonder whether "good" could be "amazing"

Could anybody advise a method of determining if my build is doing what it should?

Appreciate it's difficult to assess via my description, but any help would be appreciated

Thank You

Jon

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Bill Fitzmaurice
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Re: Omni 15TB Crossover Points

#2 Post by Bill Fitzmaurice »

You can't gauge what's happening with your ears, as hearing is more sensitive as you go higher in frequency. You have to RTA each element individually.

CarterKraft
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Re: Omni 15TB Crossover Points

#3 Post by CarterKraft »

This mic and the corresponding app on a cheap android tablet gave me a visual representation of what I was hearing.

Edit: not sure if it's available where ever you are though.

https://www.daytonaudio.com/product/111 ... microphone

https://play.google.com/store/apps/deta ... .AudioTool
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Grant Bunter
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Re: Omni 15TB Crossover Points

#4 Post by Grant Bunter »

You do need to measure, just to confirm that you're getting the reduction you would expect from the filters.

But don't stress to much at this stage.
You've disconnected the mids and highs, so the lows have no competition whatsoever.
The low pass LF filter IIRC is about 400Hz with 12dB per octave reduction.
Since 1 octave above 400Hz is 800Hz, you're going to hear 700Hz, but it should be, say, half as loud, which is totally subjective, so that's why you need to measure it...
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Seth
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Re: Omni 15TB Crossover Points

#5 Post by Seth »

Grant makes a really good point and you can perform rudimentary tests to verify the slope similar to the way you're already doing it. Download a free SPL meter app on your phone. Play a sinewave, take a measurement, and write the SPL down on a piece of paper. Don't adjust the volume between tests and measure all the following frequencies with the phone and speaker in the same exact place and orientation. Play the following frequencies and record your results. 100, 200, 400, 800, 1600, 3200, 6400, 12800.

The results you get are a combination of the response curve of the loaded driver, combined with the filter slope, combined with the response curve of the microphone in your phone, combined with any artifacts added by the space you are in when you measure. So, the measurements very likely will not result in an exact linear slope. However, it should give you quick easy measurements that indicate whether things are working or not.

But, the free RTA Cartercraft posted above would give you a more useful RTA read out, even without the calibrated microphone. RTA with pink noise.
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Stylojon
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Re: Omni 15TB Crossover Points

#6 Post by Stylojon »

Thank you chaps

I really appreciate your replies.

I will look into this.

The cab sounds good, but having nothing to compare it to makes me wonder whether it's performing as intended.

Cheers!

Stylojon
Posts: 12
Joined: Wed Jan 03, 2018 4:39 pm

Re: Omni 15TB Crossover Points

#7 Post by Stylojon »

Update

I've just had a fiddle with the cab and i've found the following

When fed a frequency with a "decent" amount of watts from a 300w/4 ohm shuttle 3.0, the Alpha 6CBMR's cone is resonating against the horn assembly. I put my finger on it gently and the resonance is reduced.

This is saying to me that the Alpha is seeing low frequencies that it shouldn't. As I was sweeping through with frequencies, I found putting 308hz into it was bad. As the frequency rises above approx 400hz, the resonance then disappears (i'm guessing the excursion is reducing as the frequency rises)

The alpha is listed as usable between 400hz and 5khz. The fact mine is misbehaving 100hz lower is giving me clues

So... I definitely have a problem somewhere.

I will have to take another look at the crossover and see whats occurring.

Annoying :(

Jon

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Bill Fitzmaurice
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Re: Omni 15TB Crossover Points

#8 Post by Bill Fitzmaurice »

You may need additional gasketing to keep the cone from hitting the baffle. But still do an RTA, there should be little power to it below 400Hz.

Stylojon
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Re: Omni 15TB Crossover Points

#9 Post by Stylojon »

funny you should say that Bill.....

I used a frequency generator to get the mid buzzing again. 320hz seemed to really get it angry.

I loosened the alpha mounting screws very slightly

Bingo.... no more buzz

I then made a gasket from the lid of a plastic paint pot this afternoon. The plastic measured 1.2mm thick

No more farting.

I have a feeling that being quite heavy handed has meant that my gasket had compressed a bit too much when I originally fitted the alpha.

My ears are still saying that there is too much grunt below 400hz. I will have to do some research concerning RTA - that's something I know nothing about.

Ive double checked the crossover comparing it to the diagram in the plans.

One more question if I may..... What element within the crossover would contribute to a lower than expected frequency reading? Is it to do with the 5mH coil? Or a combination of elements?

Still learning..... :)

Thanks again everybody

Jon

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Bill Fitzmaurice
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Re: Omni 15TB Crossover Points

#10 Post by Bill Fitzmaurice »

It's the combination. The 22uF cap prevents frequencies below the crossover frequency from reaching the driver, at a rate of 6dB/octave. The coil shorts to ground frequencies below the crossover frequency, at a rate of 6dB/octave. The 33uF cap adds another 6dB/octave, for a total of 18dB/octave. But an octave down from 400Hz is 200Hz, so content at 320Hz will still be audible.

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