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Old 8th March 2015, 11:04 AM   #1
vrfood is offline vrfood  Bulgaria
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Default help with Arta box/LIMP needed

Hello, guys. I've decided to build a test setup for measuring T/S parameters at home, because i've got quite a lot of speakers i don't have any information on (i am particularly interested in some old Telefunken/Isophon fullranges). I am gonna use a similar setup to these: Read research, parameter calculator. I can reproduce both, since i got a Feedback FG 600 generator and a good amplifier as well.
Now the hard (at least for me) part i need your help with. All the research i've done shows i need a REALLY accurate multimeter or even possible a low-frequency millivolt meter. Problem is, there are too many DMMs out there and i don't really know which one to buy. I was thinking one of these two:
  1. Keithley 177 Microvolt DMM - i think it will measure accurately down in the millivolt range, but the specs show frequency response from 45 hz and i need to measure lower (i think).
  2. UT61E True RMS DMM - same here, specs say Frequency from 10 hz, but response from 45 Hz.
So, i need some help here with the choice of DMM for the specific task. The budget is obviously of essence here as well, i know the 200$+ Agilen DMMs are more accurate
Thanks in advance

Last edited by vrfood; 23rd April 2015 at 02:18 PM.
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Old 8th March 2015, 02:20 PM   #2
DUG is offline DUG  Canada
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What about one of these?

HP 400EL Voltmeter AC Analog 10Hz 10MHz | eBay
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Old 8th March 2015, 02:38 PM   #3
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You should try the program LIMP from the ARTA software package.
You will need your computer, a sound card and Diy special cable which is very cheap and easy to build.
The LIMP manual explain clearly how to measure T/S parameters.
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Old 8th March 2015, 04:47 PM   #4
vrfood is offline vrfood  Bulgaria
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Quote:
Originally Posted by DUG View Post
That would work perfectly (i think), but as i am located in Europe i can't exactly get one of those off eBay.com. I can try to get one for a reasonable price from the UK eBay, tho. I was essentially looking for a more "modern" version of the same thing. I don't really need the extreme high range this offers, just the lower.

Thanks for the link, appreciate it.
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Old 8th March 2015, 09:15 PM   #5
vrfood is offline vrfood  Bulgaria
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Quote:
Originally Posted by ezavalla View Post
You should try the program LIMP from the ARTA software package.
You will need your computer, a sound card and Diy special cable which is very cheap and easy to build.
The LIMP manual explain clearly how to measure T/S parameters.
Downloaded the software and skipped through the manual really fast. It looks like it does exactly what i need, although i am not very sure how accurate it would be, just measured by the sound card. I suppose it would strongly depend on the quality of the card as well as other factors. I am gonna definitely try it, tho, since i've got a M-Audio Audiophile 2496 just lying around.

So, i take it you've used the software yourself. I might need a little help with it.. you said i'd need a custom DIY cable to connect the sound card to the speaker, right? Could you share some more information about the construction of the cable you've successfully used before?

Also, i noticed in the help section, that there are 2 options for measuring setup. One suggests to use the phone out and no amp, while the other says you need to use the left out and an amp. You said i needed an amp, so i should use the 2nd option then?

Cheers
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Old 8th March 2015, 09:19 PM   #6
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Do keep in mind that T/S are not scalars, but curves.

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Old 8th March 2015, 10:40 PM   #7
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Quote:
Originally Posted by vrfood View Post
...
So, i take it you've used the software yourself. I might need a little help with it.. you said i'd need a custom DIY cable to connect the sound card to the speaker, right? Could you share some more information about the construction of the cable you've successfully used before?

Also, i noticed in the help section, that there are 2 options for measuring setup. One suggests to use the phone out and no amp, while the other says you need to use the left out and an amp. You said i needed an amp, so i should use the 2nd option then?

Cheers
I'm curretly using the second option with an small "full-range" amplifier, but I guess you can skip on it if you don't try to measure acurately very low frecuencies.
As Planet10 says, T/S parameters aren't static values, but they are quite "stable ones" near the zone where they need to be used. And you must remeber that several parameters are "location dependent", specially the Vas, so.. don't be surprised if you find (not-so-big) differencs with parameters' values published by the manufacturer or by the web.

I suggest you to use the weight-method, where you add some weight to the loudspeaker 's cone to change its mass. There are several methods to do this, but I have got several small magnets and I've measured their weight in a high-precision balance. So, when I need to measure the T/S parameters, I first run an impedance measurement whith the "free" cone, and then I put the magnets in pairs on the cone, one on each side of the it with opposite polarity in order to activate the "attraction force" and keep them quiet during the measurement.

Bye!
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Old 8th March 2015, 10:47 PM   #8
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Oopppssss!
The cable needed can be found in fig. 3.2 if you use the second method. Just build the attenuators (voltage probe) inside a couple of RCA male connectors, and insert the 27 ohms resistor in a wire. Even when not shown in figure 3.2, don't forget to add an small SPST switch bypassing this resistor. You need to keep it open when measuring T/S parameters, and you need to close it in order to run a "calibration cycle" in LIMP.
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Old 9th March 2015, 12:22 PM   #9
vrfood is offline vrfood  Bulgaria
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Okay, i think i am gonna build it.. I am not very proficient in this, but i think i am gonna manage. I noticed that the SPST switches have "internal contact resistance", is that value important? Local store's website has a lot of them, most have a value of 50 mOhm.

Also, about the amplifier. I was planning to use an old Grundig Fine Arts A-905, since the frequency response starts from 10 Hz and i am inclined to trust it, because it is old, sturdy and German made. It is also quite powerful as well(2 x 120W @ 8ohm), tho, and that kind of worries me. Is that a problem when connecting to the sound card via the voltage probe?

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Old 9th March 2015, 12:33 PM   #10
AndrewT is offline AndrewT  Scotland
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You need a Vac measurement tool that can work effectively at very low frequencies.

If your Fs is around 20Hz, then you have to find the two frequencies that have 0.707 times the voltage at that 20Hz. These could be 15Hz and 26Hz. You need to be able to finely adjust the frequency until the Vac is exactly what you want to see.
Many DMM can't do that with their sample and average measurement method.
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