Self-contained LIMP jig.

[johnnyrichards]

So I decided to take my LIMP jig one step further, and source a USB sound card that would essentially allow me to duplicate the portability of the Woofer Tester/WT3/DATS format. Initially, I intended to use a small Creative USB soundcard, however it lacked line-in capabilities and I decided I was unwilling to build an L-pad in order to use it.

Some quick eBay'ing led me to this, and I thought - why not give it a shot?

The sound card proved easy to disassemble, was 100% plug and play on Win7 (did not need to install or find a driver), and LIMP recognized it immediately.

After setting it up in LIMP, my results are very similar (within 1.5%) of the S&L, and measuring components I am equally close to the results I achieve with my LCR meter.

Depending on what a guy has sitting around in his big box o' DIY crap, this can be built insanely cheaply and offers a nice "all in one package" alternative.







I left room in the box because, well because that was the box I had in the big box o' DIY crap, but I do plan on installing a polarity tester in the future. A simple switch, a AA battery holder and voila.

The nanner plugs I buy from Monoprice have a threaded-in nanner end - that is what I used for my leads.

I think all-in, even if I had to buy an enclosure and binding posts and 3.5mm cables etc, it could be be done for $20-25 dollars.

[kenny_k]

Re: Self-contained LIMP jig.

Hope you give some details.

[Chris Roemer]

Re: Self-contained LIMP jig.

Looks nice, Johnny.

[kenrhodes]

Re: Self-contained LIMP jig.

Very nice I will have to get working on one of those.

[JasonP]

Re: Self-contained LIMP jig.

Looks really nice. What soundcard did you tear down that is so cheap for this?

[JasonP]

Re: Self-contained LIMP jig.

Oh I see, you cleverly linked it into the text in your post. My bad! (hover over the 'this' in the original post).

[johnnyrichards]

Re: Self-contained LIMP jig.

As far as details, Kenny - just this schematic:



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.

[kenny_k]

Re: Self-contained LIMP jig.

I thought there was more to it. Thanks for setting me straight.

[johnnyrichards]

Re: Self-contained LIMP jig.

Played some more today...

The two left columns display how close the LIMP jig measures to my DER EE LCR meter. I tool LCR measurements all at 1K. The right column are T/S parameters I took of an Eclipse W6520R-4, and a snapshot of the audio device setup. That can confuse people and is a major reason why they have problems getting LIMP to work. It defaults to a microphone input selection rather than the Line-in.



I have four of the drivers sitting here, and can't remember if this is one I broke in or not. However, given the accuracy of the LCR measurements, I have no reason to really doubt that the T/S are fairly accurate for this driver, as-is.



LIMP used to require you supply your own DCR measurement, I just noticed today this rev (1.8.3) has an "estimate" DCR, but it read a little over 4 ohms which is too high for accurate measurements to be derived.

All in all, a pretty good afternoons playtime - now I am going to go make some sawdust!

[donradick]

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.

Yah, it's amazing how complicated some sites make that simple circuit! I drew my own also, but yours is even better than mine.
I'm stealing it - nyah, nyah
Very nice use of the ubiquitous sub sound card box. My limp jig takes a 3.5mm out from the headphone jack. I may have to do this -
problem is that I hardly ever use it - but when you need it, you really need it.

Very good tip about documenting driver settings that work. You always think you'll remember, until something goes wrong and you need the exact settings.

[JasonP]

Re: Self-contained LIMP jig.

This is a very cool project, and as I want a self-contained LIMP jig for laptop use I am starting it! I am still waiting on parts, but I got the USB sound card today so I figure I'd share more details about it:





I could not find it from JR's seller on ebay, but several sources have this model. I got mine here: http://www.ebay.com/itm/400852410469. It was 11.95$, pretty much the lowest cost USB audio card you can find. Inside is a C-media chip, a company renowned for making decent performing low-cost audio chips (you can find them on tons of motherboards). This one is the CM6206-LX:

PDF Datasheet: http://www.hardwaresecrets.com/datasheets/CM6206.pdf

Casual Youtube Review I found:

In essence, there is nothing wrong with this little board. Its not going to perform like a 100$ dedicated HiFi DAC, but its no slouch either. It'll work perfect for a LIMP jig like this. Looking at the CM6206 datasheet, its has a -87 db THD+N level across the spectrum and +0.1/-0.2 db frequency response up to a sharp roll-off at 20 khz.

Great find JR!

[Pete Basel]

Re: Self-contained LIMP jig.

I notice that your diagram shows line out driving the 100 R to the driver. Many OP amps
cannot reasonably drive 100 ohms, do you know what OP amp is used or is it on chip?
Most OP amps have trouble driving below 1K ohms.

Have you tested to see how many volts you can get undistorted? I use a power amp in
that path so that I can test at higher levels.

[johnnyrichards]

Re: Self-contained LIMP jig.

Just following widely accepted jig instructions, including from the author of the software. Seeing how close my results are to Woofer Tester II, though, I am really not concerned about the accuracy of the small signal testing.

[Pete Basel]

Re: Self-contained LIMP jig.

Just a suggestion. The NJM4556 OP Amp will drive about 2 volts into 15 ohms and about 5
into 150 ohms but still I'd probably use a power amp. Few if any other OP amps will do that.

[Silver1omo]

Re: Self-contained LIMP jig.

Pete, LIMP's manual suggests that circuit for headphone out and also has one to use with a power amp. Figures 3.1 to 3.3 in page 8:
http://www.artalabs.hr/download/LIMP-user-manual.pdf

Considering that the commercial offerings are USB powered, I guess they are not that different and seem to do more than OK.