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comp driver zobel help!
comp driver zobel help!
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Old 1st January 2018, 07:50 PM   #1
Drummer 35 is offline Drummer 35  Spain
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Default comp driver zobel help!

Hi!

Bought a pair of B&C DE500 comp. drivers. Want to cross them around 1400Hz but looking at the impedance graph I see a hump from 1kHz to 2kHz. It goes from 8 ohms around 900Hz to 20ohms peak around 1,5kHz and drops again.

What values do I need to apply on the zobel to get around 7ohms constant? Is it doable?

Thanks for your help!
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Old 1st January 2018, 11:26 PM   #2
eriksquires is offline eriksquires  United States
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Hi Drummer,

I'm sure it it doable, but not sure if "Zobel" is the right filter. That would help you for the impedance rise above 2kHz.

I am going to suggest you simulate the entire design first, and then see how big a deal it is. If you use XSim, use the "filter response (dB)" chart to see how that affects you at the end of the day. THEN you can go about working on an impedance compensation network.

Of course, alternatively, you could use something terribly simple like a plain resistor in parallel.

Best,


E
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Old 1st January 2018, 11:39 PM   #3
daqvin_carter is online now daqvin_carter  United States
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What are you using below 1400 hz? Mostly asking if you need to pad the driver.

Also, what sort of slope do you want?
If using at least a second order slope and padding by a few db, I would put about 25 ohms in series with the driver and also make the choke act as a transformer with a tap.
I like to avoid L pads especially with compression drivers.
Also, you might shoot for a lower q filter at 1400 hz an expect a higher q response in the end.
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Old 2nd January 2018, 12:10 AM   #4
eriksquires is offline eriksquires  United States
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Hi Drummer,

This sounded fun, so I did a little more work. First, I used Virtuix's SPL trace (under Tools) to copy an image of the driver's impedance. I'm not sure if it is exactly your driver, but it seems close. Imported that into XSim, and then used the "Circuit Blocks" menu to add a pair of notch filters. You can see the difference between the original, and filtered here:

comp driver zobel help!-z_improvements-jpg

Lastly, I added a 1kHz, 2nd order high pass filter so I could examine the filter's electrical response, which is now pretty close to ideal:

comp driver zobel help!-filter_with_z_notches-jpg




The final schematic is below:

comp driver zobel help!-schema_with_z_notches-jpg
Attached Images
File Type: jpg Schema_With_Z_Notches.jpg (24.9 KB, 333 views)
File Type: jpg Z_Improvements.jpg (139.3 KB, 338 views)
File Type: jpg Filter_With_Z_Notches.jpg (74.5 KB, 170 views)
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Old 2nd January 2018, 12:17 AM   #5
eriksquires is offline eriksquires  United States
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BTW, this is also a very good example of why simulation is a good idea. Depending on exactly where your HP filter is, and the Q, one or both of these impedance notch filters may become unnecessary, saving you some dough! Further, if we had been tempted to add an actual Zobel for the impedance rice at the top octave, the simulation shows that with this choice of HP filter there's really no need at all.

I almost forgot, here is a picture of the Electrical response without the notch filters:

comp driver zobel help!-filter_without_z_compensation-jpg

While it does look bad, your choice in HP filter could eliminate one or both of those notch filters. I am pretty sure the first one with the 4mH coil is pretty optional. Save yourself some dough!

Best,


E
Attached Images
File Type: jpg Filter_without_Z_compensation.jpg (92.9 KB, 167 views)
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Old 2nd January 2018, 12:28 AM   #6
eriksquires is offline eriksquires  United States
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Meh, I did even better. I'll just upload the whole thing so you can play with it.

The magnitude of impedance in the humps is now less than 1 Ohm, but again, your wallet uber alles! Make sure the notch filters actually matter to you at the end of the day.
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File Type: zip Compression.zip (165.1 KB, 10 views)
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Old 2nd January 2018, 11:37 AM   #7
EarlK is offline EarlK  Canada
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Usually the resistive padding ( necessary to match a compression driver to a direct radiating woofer ) smooths the impedance spikes into very benign bumps ( therefore; additional impedance compensations designed to flatten them out, become un-necessary ).

comp driver zobel help!-de500_seos12_10db-pad-jpg

Seen above is the effect of an Altec style, 10db "T-Pad" .

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File Type: jpg de500_SEOS12_10db-pad.jpg (69.4 KB, 163 views)

Last edited by EarlK; 2nd January 2018 at 11:42 AM.
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Old 2nd January 2018, 11:55 AM   #8
ICG is offline ICG  Germany
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Quote:
Originally Posted by EarlK View Post
Usually the resistive padding ( necessary to match a compression driver to a direct radiating woofer ) smooths the impedance spikes into very benign bumps ( therefore; additional impedance compensations designed to flatten them out, become un-necessary ).
That's exactly what I just wanted to say. The compression driver will very likely be too loud anyway, so a voltage divider is needed. On many horns a bypass-capacitor is also needed to boost the upper end, that's a lot cheaper than a zobel filter for the rising impedance (which is very tame for the DE500) and costs less power too.
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Old 2nd January 2018, 12:32 PM   #9
Drummer 35 is offline Drummer 35  Spain
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Erik thanks a lot!

Now I see how to approach to this.

I used box design software, but not for filters.

You're right it would work taming only the second impedance hump. (And I dont have 4mH coils for the first filter)

I need also some attenuation since I'll use a pair of beymas 8p300 running in parallel for mid/bass. Around 10db attenuation needed.

Daqvin, id prefer not using l pads but never used a transformer this way. Could you explain a bit more how to calculate the values?

And what about putting a parallel resistor to smooth that humps and use a first order filter at around 10-15k. I've read this can be used to pad down highs.

Anyway, when I get home, I'll play with the filter simulation.

Erik, thanks again for taking your time with this.

Regards.
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Old 2nd January 2018, 04:28 PM   #10
eriksquires is offline eriksquires  United States
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Drummer, I don't know enough atout T-Pads, but I do remember what EarlK is talking about, that is worth researching before my notch filters, certainly cheaper! You should examine the filter function for any idea before implementing.

Make sure you evaluate the wattage at each resistor too. XSim has a chart for that.

Best,


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