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Multi-Way Conventional loudspeakers with crossovers

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Old 8th March 2019, 04:08 PM   #741
jzagaja is offline jzagaja  Poland
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Quote:
Originally Posted by Zung View Post
Then no luck, both the Behringer and the Focusrite are sizable boxes. But then again, they have the XLR connectors and phantom power, convenient for measuring with a microphone.
It can be cheap card with line in-out. Most are 24/48. For speakers UCA202 may do the job?
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Old 15th March 2019, 07:35 PM   #742
tmuikku is offline tmuikku  Finland
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Default first timer, phase wraps

Hi all,
as a friday project I made quickie measurement jig on a plank of wood and tried LIMP for the first time.

All is ok, except the phase seems to wrap on the impedance peak. Never seen this on other LIMP screenshots. What might cause this, or is this normal?

Also, the impedance peak is mucho less than factory datasheet impedance curve, what might cause this? The driver is FaitalPRO 8pr155

Speaker was on the floor while measuring. Reference resistor is 23.3ohm. Using balanced inputs on my RME babyface using pin2 (probe) and pin3 (ground), grounds attached together (left & right). I calibrated input levels to green, put reference resistor value to measurement setup. I didn't put anything to cable compensation setup, cables from jig to sound card are maybe 30cm long. Speaker cables from amp and to speaker are few meters.

Sorry if this is already shown somewhere, didn't find any hints by googling.

Few attachments to illustrate

Thanks!
Attached Images
File Type: png phasewrap.png (82.5 KB, 85 views)
File Type: gif 8PR155_impedance_8_factory.gif (23.4 KB, 83 views)
File Type: jpg setup.jpg (499.3 KB, 76 views)
File Type: jpg jig_schema.jpg (248.9 KB, 75 views)

Last edited by tmuikku; 15th March 2019 at 07:37 PM.
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Old 15th March 2019, 08:07 PM   #743
DBMandrake is offline DBMandrake  Scotland
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Don't sit the speaker on a table or the floor, this will affect the measurement due to mass loading effects from the surface. Hang it from a wire through one of the screw mounting holes in the frame so that it's hanging in mid air at least half a metre from any other objects. (For example clamp a support wire to a table top or work bench and hang the speaker half way to the floor)

All published driver impedance curves for commercial drivers are taken with the driver suspended in mid air away from large surfaces.

Your phase curve does look odd, I wouldn't expect more than +/- 90 degrees phase shift to occur even with a very high Q resonance.

The impedance measurement is suspect as well, it should not be showing a lower impedance at 5Khz than 10Hz!

Before going any further I would perform some tests with reference resistors.

Replace the speaker with a reference resistor which you have measured accurately with a multi-meter. You should get a perfectly flat line of the correct impedance.

Check this with a few different values of resistor - say 6.8 ohms, 12 ohms and 33 ohms, and also check it with the speaker terminals shorted out to make sure you get 0 ohms.

My guess is that you have a problem with your measurement setup and you will not get a flat line. Probably due to reactive input impedance on the sound card interacting with a voltage divider that is too high in impedance.

Are you sure you haven't made an error in one of your resistive dividers ?
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Last edited by DBMandrake; 15th March 2019 at 08:12 PM.
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Old 15th March 2019, 08:40 PM   #744
iMat is offline iMat  Croatia
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Hi,
I suppose that your power amplifier has unbalanced output.
In that case you should connect pin 3 of both channel to ground.

In attachment I send you simple impedance jig that enables impedance measurement without power amplifier. Just use headphone output (RME outputs 3/4) as generator with reference resistor 47 ohms.
Use RME inputs 3 and 4 as unbalanced one.

Best,
Ivo
Attached Images
File Type: png impjig.PNG (17.7 KB, 70 views)
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Old 15th March 2019, 10:42 PM   #745
iMat is offline iMat  Croatia
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In my previous post I send a simple impedance jig schematic.
The possible question is how to do the calibration procedure of LIMP.
Do:
1) before calibration disconnect loudspeaker.
2) do calibration as described in LIMP manual (use averaging option)
3) after calibration, correct reference resistor with value that is equal to value parallel connection of reference resistor and sound card input resistance.
4) start measurements

This procedure gives good results, you can even correctly measure higher impedance (several kilo ohms).

More detailed explanation I will give in LIMP user manual that will be published next month.

Ivo
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Old 16th March 2019, 12:13 AM   #746
tmuikku is offline tmuikku  Finland
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Thanks all!

Operator error it was Cut the link between channel grounds (pin3 <> pin3) and it seems legit now. Next up TS parameters and semi-inductance for weekend fun
Attached Images
File Type: png 8pr155_measured_ok.png (80.4 KB, 33 views)
File Type: jpg arta_limp_jig_schema_fixed_rme_babyface.jpg (728.1 KB, 30 views)
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