Self-contained LIMP jig.

[JRT]

this deserves a fresh bump to the top of the stack of threads

[JasonP]

Re: Self-contained LIMP jig.

Without the protection circuits, you just follow the diagram JR posted above but use the smaller resister instead of the 100 ohm.

[kevintomb]

Re: Self-contained LIMP jig.

Those circuits are for high power testing. Your sound card can't handle the voltage output of the amplifier directly, so it scales down the voltage to a level it can handle. The R1 and R2 function as a voltage divider to do just that. The two zener diodes placed across the sound card input will trip if the voltage is too high, create a short in the circuit, and protect it from possible overload.

If you are doing low voltage testing and have a way to ensure no accidental high voltages reach the sound card (using a DMM to test the output, etc) then you can just use the 16 or 27 ohm resistor setup with the amp output as presented.

Okay so they are showing the sound card LEFT and RIGHT inputs on the right side of the drawing, with the amp outputs from either side of the 27 ohm resistor as being on the left side of the drawing.

[JasonP]

Re: Self-contained LIMP jig.

What schematic would be used, when using an external Amp?

I plan on using about a 16 ohm resistor.

I see the mention in the manual about using zener diodes, and 2 resistors, but their schematic is confusing. ((Drawing 3.2 and 3.3)) Not sure how they are combining them.

Those circuits are for high power testing. Your sound card can't handle the voltage output of the amplifier directly, so it scales down the voltage to a level it can handle. The R1 and R2 function as a voltage divider to do just that. The two zener diodes placed across the sound card input will trip if the voltage is too high, create a short in the circuit, and protect it from possible overload.

If you are doing low voltage testing and have a way to ensure no accidental high voltages reach the sound card (using a DMM to test the output, etc) then you can just use the 16 or 27 ohm resistor setup with the amp output as presented.

[kevintomb]

Re: Self-contained LIMP jig.

As far as details, Kenny - just this schematic:

[ATTACH=CONFIG]52566[/ATTACH]

Can't take credit for it however, it is pretty standard. I drew this schematic since some of the examples I found on the interwebz did not make a lit of sense to me, and I actually used a pair of 200 ohm resistors in parallel.

What schematic would be used, when using an external Amp?

I plan on using about a 16 ohm resistor.

I see the mention in the manual about using zener diodes, and 2 resistors, but their schematic is confusing. ((Drawing 3.2 and 3.3)) Not sure how they are combining them.

[audioanarchist]

Re: Self-contained LIMP jig.

Thank you Jason. That's exactly what I needed to know. Seems like it's nothing to really worry about. I can't wait to get mine slapped together.

[JasonP]

Re: Self-contained LIMP jig.

The driver I have sitting loose now with the lowest Fs is a Dayton DA270 10" Aluminum. Fs is a mere 30 hz, and using my jig after a little break-in I got T/S matching the published spec nearly on the money.

The accuracy should drop the lower the Fs goes, but I think you'll be fine with drivers that have Fs 20 hz or above, in my experience.

[audioanarchist]

Re: Self-contained LIMP jig.

Either of you care to define the Fs limits of this device?

I have no clue that's why I was asking. This thing is awesome, I just wanted to make sure I know what it can and can't do.

[johnnyrichards]

Re: Self-contained LIMP jig.

Either of you care to define the Fs limits of this device?

[Pete Basel]

Re: Self-contained LIMP jig.

So would this not work as well for gathering t/s parameters of a subwoofer with a low fs?

No it would not, it will barely get it moving.

I think it is ridiculous not to use a power amp.

[audioanarchist]

Re: Self-contained LIMP jig.

So would this not work as well for gathering t/s parameters of a subwoofer with a low fs?

[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!

[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.

[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.

[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.