Speaker Workshop jig

[Anthony]

Hey all,
Just wanted to throw out an idea I had. I want to start using speaker workshop to model and test drivers. It has a good component tester (resistors, capacitors, even inductors), provided you build the right jig.

You can also test distortion, frequency response, even T-S parameters. Again, though, you have to have a jig and calibrate it.

There have been a bunch of ideas thrown out there, the most popular being the Eric Wallin jig. There's also the claudio wire jig.

I wanted something a bit more complicated than the claudio, so I can do more with less fiddling. I also wanted something a lot less complicated than the Wallin jig. So here's what I came up with (attached):


First off, because I use the M-Audio mobilePre, I have a unique circumstance, no stereo line in. most of the jigs seem to assume you have that. Instead, I have 2 XLR balanced and 2 1/4" Tip-Sleeve inputs. So it looked like custom cables were in order.

However, soldering those tiny connectors of a 3.5mm and even 1/4" plug would be a pain. Soldering jacks for them, though, is easier.

Basically, the jig is a 3.5mm input from the sound card with an RCA jack to pass the left channel signal unmolested. Then there are three sets of binding posts.

One is a jumper between the left and right output channels (for making a loop cable or adding a resistor for testing).
One is a voltage divider resistor for any amplifier testing that would have a speaker level signal being fed back to the sound card.
Finally the last is the test port where any number of things can be plugged in (resistors, capacitors, inductors, and drivers). It is a parallel path between the red signal and ground.

With project box, an assortment of resistors, and the jacks, I think I can build this for less than $20 from Parts Express. Then I can use any 3.5mm or 1/4" cables, of which I have many laying around.

I post this for peer review. If anyone has used the wallin or claudio jigs, please chime in and let me know if this looks like it would work. Or tell me if I'm missing something very important.

Thanks,
Anthony

[Anthony]

Okay, I'm an idiot. Most of that jig would have worked except for the voltage divider. Basically, a voltage divider is a resistor in series with the signal path and in parallel between signal and ground. Marked S and P, respectively, the voltage seen at the signal output becomes P/(S+P). It's an easy and reliable way to cut amplifier voltage of say 48V peak to peak down to a more reasonable 4V, so as to not fry your soundcard.

My soundcard has its own preamp stage, so it could probably take it, but better to err on the safe side than fry a $100 soundcard.

So I was thinking the input impedance of the SC counted as the parallel resistor. Nope, that's what fries when you overvolt it. So my jig has to be redesigned to allow for another resistor to be inserted on the output stage.

I'll still stick with the banana plug design. I like to be able to visually verify what's shorted and what's got resistors. It's a good way to make sure everything's setup the way you want before applying the voltage.

As always, advice or criticism is appreciated.

[Anthony]

Real quick (have a meeting to run to).

Revised version of the jig.

1/4" mono jacks, except for the line in (1/8" stereo). The closed circles and test port are all banana binding posts.

Green is ground, black is hardwired, dashed is either open, shorted, or a resistor.

[Anthony]

Okay, this is a cross post from diyaudio.com (I posted over there because Claudio Negro and Jay Butterman frequent there and they are definitely speaker workshop experts).

Anyway, The first is a pic of the finished jig and parts. I have a nice label maker, so I will be making formal labels tonight. Right now it's grease pencil marks (or drawn in MS Paint).

Second is the open schematic. Bridge connects the channels and can be a short (0 Ohms), left open, or a resistor.

Series and parallel set up a voltage divider. This is only to limit the output voltage to protect the sound card. Most of the time "Series" will have a short in it and "Parallel" will be left open.

Testing begins tonight.

[Anthony]

Okay, well that box was a colossal failure. It did not work for me, mostly for sound card reasons.

Currently I have a prototype jig that I won't even bother taking a photo of, because I plan on making it nicer this weekend. But the entire process did give me some lessons learned to share.

1) Know the parameters of your soundcard before starting. You need to know what the input impedance is of your soundcard and how much current it can output before the voltage sags. I had both these problems. The relatively low load of the test resistors and speakers asked too much current from the sound card, which in my case was not even designed to power speakers, much less something like this.

PCI sound cards are usually designed to at least get some sound out of ******, unpowered speakers, so you may not have the same problems I did on that front.

As for inputs, for sensitivity and signal quality issues, you should strive for the jig output impedance to be 1/10 of the soundcard input impedance. I ended up doing this with a voltage divider circuit, although if you don't need to attenuate the voltage, you can still build a network that sets the impedance to what you want. More on this later.

2) If you can swing it, use a separate amplifier for all the tests. Sure there's a risk of burning up your soundcard, but if you test your jig, levels, and voltages before you plug it into the card's inputs, the risk is low. Power amplifiers are able to keep stable outputs at very low impedances, especially for low power levels. The amp I ended up using was a car audio amp powered by a Radio Shack 12V power supply (the big box, not the wall warts). I had them powering an old subwoofer a long time ago, but they've been collecting dust. Even still, the whole combination would be less than $125, and I'm sure you can find much lower power amps for cheaper with a built-in power supply.

This amp puts out 16V, unloaded at minimum gain with the sound card output set to maximum. So I designed the voltage divider to go 1/16 ratio and always keep the gain knob minimized. This way I have a lot of room to adjust the sound levels without worrying about a computer setting blowing up the sound card.

3) Always design your resistor networks around components you have. Resistors come in strange values and by wiring them in series or parallel, you can pretty much get any value you want, if you don't mind stringing together a dozen of them. That's a lot of solder joints to go wrong or introduce noise, so I kept things as simple as possible.

to recap, the voltage divider has three requirements: 1/16 voltage output, 1/10 impedance of the sound card input, done with resistors I already have and minimize connections.

The math is simple: Vout = Vin*R2/(R1+R2) R1 is the series resistor, R2 is the parallel one. Zout = R1*R2/(R1+R2) = Zsoundcard/10. The listed impedance is 3550 Ohms for the Mic/Line inputs and the Vout/Vin = 1/16. Solving, we get around 5680 Ohms for R1 and 380 Ohms for R2. Using resistors I had, I could easily make a 6k Ohm for R1 and 400 Ohms for R2, which was close enough.

Once these were selected, the rest was simple wiring, similar to what Claudio Negro did (www.claudionegro.com) for his cable jig.


I set levels, tested, verified, and was very happy. Everything seems to be working.

For my final version of the jig, I'm going to combine what Eric Wallin and Claudio did. If the switches I bought check out to have low resistance, I will hard-wire in the test and reference resistor to make life simpler. But since my soundcard powers my measurement mic directly, I can skip a lot of what Eric put into his jig. If all goes well, I hope to have it done this weekend.

[JCD]

Dude, I gotta give you props. Way more technical than I could ever be. Keep it up and good luck.

JCD