Self-contained LIMP jig.

[1100xxben]

Re: Self-contained LIMP jig.

I tried to use (unlicensed) LIMP recently, and was unable to access the menu entry for T/S params.
Of course, it may have been my error, but it looked like LIMP for T/S params had been removed
for the unpaid version.

No, it is available in the free version. You need to run a free-air sweep and save it as an overlay. Then, load the driver with mass or a closed box and run another sweep. You should see both curves. This is a good way to see if you have added enough mass and dropped the Fs enough (but not too much). Once you have both curves on the screen, you should be able to choose the T/S parameter menu entries. It will ask for Re, Sd, and the added mass or closed box volume.

[donradick]

Re: Self-contained LIMP jig.

No, it is available in the free version. You need to run a free-air sweep and save it as an overlay. Then, load the driver with mass or a closed box and run another sweep. You should see both curves. This is a good way to see if you have added enough mass and dropped the Fs enough (but not too much). Once you have both curves on the screen, you should be able to choose the T/S parameter menu entries. It will ask for Re, Sd, and the added mass or closed box volume.

Thanks 1100xxben - I'll give that another try. I've done it several times in the past.

[donradick]

Re: Self-contained LIMP jig.

REW supports measuring impedance and T/S parameter calculations. Here's some of their documentation on it:

http://www.roomeqwizard.com/wizardhe...ement.html#top
http://www.roomeqwizard.com/wizardhe...small.html#top

Thanks rhodesj -

REW looks SO great, I want to run it - but I'm on a campaign to remove Java from all my computers.
It's the infection vector for many exploits.

[johnnyrichards]

Re: Self-contained LIMP jig.

Thanks rhodesj -

REW looks SO great, I want to run it - but I'm on a campaign to remove Java from all my computers.
It's the infection vector for many exploits.

Good call.

Personally, I never was able to get used to the GUI that REW uses. Guess I am just an ARTA head

[donradick]

Re: Self-contained LIMP jig.

Good call.

Personally, I never was able to get used to the GUI that REW uses. Guess I am just an ARTA head

Yep, seems like we (or some of us anyways) get locked into specific applications.
As always, not enough time or motivation to learn a new package when the old one works perfectly.

[Whitneyville1]

Re: Self-contained LIMP jig.

I ran the T/S on a Peerless 830875 using my Berhinger UFO-202 and got with-in 4% maximum deviation from factory specs on a pair of them. I know these are very close to factory spec, as I purchased them used from someone here who ran them on a WT2. I'm using a 10K 2 watt 1% precision metal film resistor as that's close to the "line-in" impedance on the UFO. Just took cutting one phono-to-phono cord in half and soldering in the resistor. Quick, easy and cheap.

[1100xxben]

Re: Self-contained LIMP jig.

I ran the T/S on a Peerless 830875 using my Berhinger UFO-202 and got with-in 4% maximum deviation from factory specs on a pair of them. I know these are very close to factory spec, as I purchased them used from someone here who ran them on a WT2. I'm using a 10K 2 watt 1% precision metal film resistor as that's close to the "line-in" impedance on the UFO. Just took cutting one phono-to-phono cord in half and soldering in the resistor. Quick, easy and cheap.

A 10K series resistor is too much for this type of application, ESPECIALLY with a 16-bit sound card. ARTA measures the (loaded) output drive voltage and the voltage across the device being tested at the same time. If you were able to set the drive level so that you utilized 100% of the ADC input on your loop-back side, you'd only be utilizing 26 voltage steps (peak to peak) of the 65536 available when measuring a 4 ohm load, and a 1-bit error results in 4% error in the measurement. That also assumes that you have absolutely 0-bits of noise in the system, which we all know is never true, even of the highest quality sound cards. Even a 1K series resistor means you are still only utilizing 261 voltage steps. If you were to use a 1K series resistor on a sound card with a very low noise floor and a 24-bit ADC, then you'd at least be able to utilize almost 67,000 voltage steps in a 4 ohm measurement.

I prefer to use an external amplifier and a low series resistance that is close to the impedance of what I plan to measure. This maximizes the number of bits you are using in your measurements and drastically increases the SNR of the measurement. I use a Crown D-75 and a series 12 ohm resistor to take my impedance measurements with ARTA. I also use a 24-bit sound card with a VERY low noise floor, although the amplifier noise floor swamps out the sound card noise floor, so it really doesn't matter from that perspective.

[JasonP]

Re: Self-contained LIMP jig.

A 10K series resistor is too much for this type of application, ESPECIALLY with a 16-bit sound card. ARTA measures the (loaded) output drive voltage and the voltage across the device being tested at the same time. If you were able to set the drive level so that you utilized 100% of the ADC input on your loop-back side, you'd only be utilizing 26 voltage steps (peak to peak) of the 65536 available when measuring a 4 ohm load, and a 1-bit error results in 4% error in the measurement. That also assumes that you have absolutely 0-bits of noise in the system, which we all know is never true, even of the highest quality sound cards. Even a 1K series resistor means you are still only utilizing 261 voltage steps. If you were to use a 1K series resistor on a sound card with a very low noise floor and a 24-bit ADC, then you'd at least be able to utilize almost 67,000 voltage steps in a 4 ohm measurement.

I prefer to use an external amplifier and a low series resistance that is close to the impedance of what I plan to measure. This maximizes the number of bits you are using in your measurements and drastically increases the SNR of the measurement. I use a Crown D-75 and a series 12 ohm resistor to take my impedance measurements with ARTA. I also use a 24-bit sound card with a VERY low noise floor, although the amplifier noise floor swamps out the sound card noise floor, so it really doesn't matter from that perspective.

Thanks for that data. I always wondered why they chose such a small resistor and now I know why!

[JasonP]

Re: Self-contained LIMP jig.

I made a mistake and guessed at a box too small, so I had to wait until now to put it all together. Here is my final report:













Looks nice, works perfect. Awesome project.

[rhodesj]

Self-contained LIMP jig.

Can anyone recommend a small power amp for running impedance sweeps? The headphone out on the UCA202 I'm using rolls off pretty steeply around 20Hz, which isn't really helpful when I'm trying to measure a sub driver, Infinity 1260w, that has an Fs around 23 Hz, so I need solid impedance measurements from at the least 10Hz up.

Don't need a ton of wattage, just flat and clean, and hopefully not too expensive.

[bret191]

Re: Self-contained LIMP jig.

P-E's little Behringer's UFO202 USB/Audio Interface Phono Pre-Amp for 29.95 does a great job. It has a headphone output that will drive 32 ohm headphones to 350-500 mW (guesstimated). I patched it together like Johnnie and got more consistent readings than my ultra-precise Wallin-jig (ahem!). I was very seriously considering buying a DATS V2. Not so much now.

The no load voltage is 530 mV and with 150 ohms it was at 405 mV on my UCA202. It works kinda ok but I have been looking and came up with this site and pointed me to this same CM2606 as I was using A imm6 and on the UCA202 the input isn't stout enough to get a good mic sig. and the CM2606 is cheaper too.

[1100xxben]

Re: Self-contained LIMP jig.

Can anyone recommend a small power amp for running impedance sweeps? The headphone out on the UCA202 I'm using rolls off pretty steeply around 20Hz, which isn't really helpful when I'm trying to measure a sub driver, Infinity 1260w, that has an Fs around 23 Hz, so I need solid impedance measurements from at the least 10Hz up.

Don't need a ton of wattage, just flat and clean, and hopefully not too expensive.

If you're going to use a sound card that has single ended inputs, make sure you use an amplifier that has a non-bridged output (the negative terminal should be tied to ground). There are ways that you can safely hook up a bridged amplifier to a single-ended input, but it's much easier to just use an amplifier with single-ended output. You really only need 100mV or so of output level to get good results, but you need a very low output impedance such as an amplifier capable of driving speakers or headphones directly.

[Pete Basel]

Re: Self-contained LIMP jig.

I hooked up my limp jig to the card tonight and ran through two RMAA tests. Here are the results, nothing stellar (as expected) but fine for this kind of testing IMO.

[ATTACH=CONFIG]55752[/ATTACH]

Results on the left are at 50% output, and 100% output on the right. As you can see, nothing special here. The FR is pretty horrible, it starting to roll-off at 100 hz. It is -4 db at 40 and -8 db at 20. However, since LIMP is measuring the difference between channels, FR level isn't an issue at all. Channel matching between right and left is, and this card excels at that. LIMP calibrated with a channel difference of 0.02db, perfect. Tests I made conclude this to work just as well as my ASUS Xonar DX for low level measurement of T/S and impedance. I will not that even at 100% output, my DMM told me it was putting out 0.5 Vrms which isn't all that high. It did not seem to reflect in my measurements comparison in anyway however.

Nice work, so the LF rolloff means that you have even less signal down around 20 Hz for testing low
Fs woofers. If the response is fairly flat without the 100 ohm load then it is probably the output coupling
caps and it would help a lot to increase them by a factor of 10. Might not be so easy if they are surface
mount. I think this mod would help a lot.

[Pete Basel]

Re: Self-contained LIMP jig.

The no load voltage is 530 mV and with 150 ohms it was at 405 mV on my UCA202. It works kinda ok but I have been looking and came up with this site and pointed me to this same CM2606 as I was using A imm6 and on the UCA202 the input isn't stout enough to get a good mic sig. and the CM2606 is cheaper too.

I find it very odd that the site shows the frequency response on a linear scale, that is
ridiculous because it hides the low frequency response. And measurements shown by
Jason earlier in this thread found it to be quite bad when loaded with 100 ohms. It
seems like they want to hide the performance flaws perhaps in the hopes of having
people believe that there is very cheap hardware to use with their software.

Interesting that they list the line out output impedance as two ohms, quite good if
their measurement is accurate.

[bret191]

Re: Self-contained LIMP jig.

It
seems like they want to hide the performance flaws perhaps in the hopes of having
people believe that there is very cheap hardware to use with their software.

.

I believe your correct.


Nice job on the jig!