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Author Topic: Acceptable deviation between boxes?  (Read 1179 times)

Matthias McCready

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Re: Acceptable deviation between boxes?
« Reply #10 on: July 26, 2018, 04:25:04 am »

the dessert wanderer may chime in.... 8) 

JR

Does he come bearing icecreams and other sweet things?  ;D ;D ;D
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boburtz

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Re: Acceptable deviation between boxes?
« Reply #11 on: July 26, 2018, 05:55:09 pm »

I would be interested to see the complex impedance curves of these boxes as well, so see if there is corresponding differences there too.
I'm using DATS for the impedance measurement, and it only allows 20 saved traces (10 biamp boxes) so I had to split it up into two groups. See anything alarming here?
« Last Edit: July 26, 2018, 06:13:39 pm by boburtz »
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Art Welter

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Re: Acceptable deviation between boxes?
« Reply #12 on: July 27, 2018, 01:12:28 pm »

See anything alarming here?
Bob,

+/- a coil winding (or two) can easily account for impedance minima/maxima variations. Though I'm not alarmed, the purple trace makes me curious, as the higher frequency peak around 2kHz (instead of around 1800 Hz) may indicate something is physically different in the cabinet, perhaps something in the HF horn.

Art
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boburtz

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Re: Acceptable deviation between boxes?
« Reply #13 on: July 27, 2018, 01:33:58 pm »

Bob,

+/- a coil winding (or two) can easily account for impedance minima/maxima variations. Though I'm not alarmed, the purple trace makes me curious, as the higher frequency peak around 2kHz (instead of around 1800 Hz) may indicate something is physically different in the cabinet, perhaps something in the HF horn.

Art
Thanks Art. Any chance this could be caused by old diaphragms or an issue with the passive crossover? I am going to dismantle the box and see if there is anything physically obvious.
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Art Welter

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Re: Acceptable deviation between boxes?
« Reply #14 on: July 30, 2018, 06:17:45 pm »

Thanks Art. Any chance this could be caused by old diaphragms or an issue with the passive crossover? I am going to dismantle the box and see if there is anything physically obvious.
I was not aware of the passive crossover in the path, either a diaphragm of a different impedance or slightly different passive crossover component values, or a combination of both could account for the differences.

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boburtz

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Re: Acceptable deviation between boxes?
« Reply #15 on: July 30, 2018, 07:28:29 pm »

I was not aware of the passive crossover in the path, either a diaphragm of a different impedance or slightly different passive crossover component values, or a combination of both could account for the differences.
The diaphragms both measure identically when removed from the path and they look fine when removed from the magnet assembly. There is nothing about the horn that is physically different than the other boxes, no blockage or cracks or otherwise. I suspect something in the passive crossover network. Incidentally, the magnitude and phase of this particular box measures more closely to the EAW spec than some of the others. Do you suppose it falls within the realm of normal or is it something I should pursue further? As always, thank you for the time you take to answer my questions.
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Matthew Knischewsky

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Re: Acceptable deviation between boxes?
« Reply #16 on: July 30, 2018, 11:57:50 pm »

I measured transfer functions of 16 KF730 boxes individually, each with the exact same measurement conditions. I am curious as to what is an acceptable variance box to box for stuff like this. Is there a standard?  They average out to something close to the published EAW spec, but individually there are some variations that seem to be pretty large.  Does this look normal? If not, do you have any idea what could be causing this or a suggested solution?

I did complete tear down and re-build of a 16 box 730 rig some 5-6 years ago. Your traces look (to me) to be very normal. The passive network in these speakers make these a bit of a bear to work on, not to mention each box has 2 drivers for each passband. Unless each driver is measured individually to get a baseline it becomes difficult to know for sure if it's any combination of the drivers, diaphragms, and the network thats contributing to a variance. This work is all time consuming, so how much time do you have? Are you the patient type?

The phase variance at the extreme top end is normal. That's minute variation in mic/cabinet placement. What's more important is what's happening at the MF/HF X-over point, 1k-3k or so. Phase shift here is going to have an impact on the magnitude response.

A note about the phase response- a lagging phase response might tell you something about a driver that you may not necessarily be able to hear - like a slight rub on a voice coil. On the 730s the original voice coil former was Nomex, a paper like material which tended swell up and rub over time. Later, the VC former was changed to Kapton, a high temperature plastic material which wouldn't expand over time/use.

Ultimately, you'll have to decide how much effort you're willing to expend for what returns. I was able to get most of "my" 730s within about 2dB of each other over most of their range, some a bit more like 3dB. But, my starting point I think was much worse- A thrifty person thought they were smarter than EAW's parts department. A long story for another day.
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Art Welter

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Re: Acceptable deviation between boxes?
« Reply #17 on: July 31, 2018, 03:11:19 pm »

Do you suppose it falls within the realm of normal or is it something I should pursue further?
Well within the realm of normal.
Forgot to mention magnet strength in the list of things that can change with age, though the difference you see between units could have easily been there from day one.

As Matt said "This work is all time consuming, so how much time do you have?"

As long as no coils are dragging/buzzing, I'd call it "good to go".

Art
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