# Speaker Workshop jig



## Anthony (Oct 5, 2006)

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


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## Anthony (Oct 5, 2006)

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.


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## Anthony (Oct 5, 2006)

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.


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## Anthony (Oct 5, 2006)

Okay, this is a cross post from diyaudio.com (I posted over there because Claudio ***** 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.


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## Anthony (Oct 5, 2006)

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 ***** 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.


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## JCD (Apr 20, 2006)

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

JCD


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## mpotoka (Nov 3, 2006)

Anthony,

Are you successfully using Speaker Workshop and a Mobilepre? I am having problems getting it to work right (I've set it SW up before on other computers but I am having problems with the MobilePre)

Have you found the MobilePre is able to take Impedance Measurements by itself or is an external amp always required? I tried to go through the calibration of the reference resistor and it keeps giving me values around 4 ohms too high and tells me my series resistance is 3 ohms. When I try to measure a resistor or inductor it tells me that it is a capacitor. When I measure a capacitor I usually end up with about 1/2 the value it should be.

Also--when you take frequency measurements--do you need to run a loop back cable from an output do an input as a reference? Or don't you need the reference cable for Frequency Response measurements (can you just connect a mic to an XLR input and run an output cable to an amp?

Thanks so much!


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## Anthony (Oct 5, 2006)

First off, Welcome to the Shack!

As for impedance measurements, you have the same problem I had. Resistors showed up as capacitors and nothing seemed to measure right. What happens is that the line out voltage sags under even very mild current draw, so the readings get all kinds of screwed up. I use an external amp for all of it now. My current jig doesn't look like that at all anymore. I'll see if I can dig up a picture or schematic of what I ended up with.

Basically it maintains a 10x impedance while stepping the voltage down 1/16th of original. Since the amp puts out a max 16V, that guarantees that only 1V gets to the soundcard input regardless of what I feed the amp. Any amp you use might be different, but I'm paranoid and I always make sure the output voltage to the soundcard is less than 1V RMS (even though I know how I designed the jig). The jig does have banana plugs to screw down test resistors and inductors, so once it's set up measurements are easy.

As for sound measuring, I use an effects cord (Tip-Sleeve 1/4" plug on both ends) and loop one channel back to front (forget which, I think it's Right looped, opposite of Room EQ Wizard, which I also use a lot). Then I use the other channel for measurement through the XLR.

I did test here a week ago where I looped back XLR to line in with a voltage divider circuit to match the levels. I then did a calibration sweep. The difference between that and the 1/4" to 1/4" loop was almost zero. Something like a 1/2 dB at 20 Hz and the rest spot on throughout the frequency range. So I'm confident that's a good assumption.

Clear as mud?  Good luck -- I've had decent luck with the software, just not my designs


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## mpotoka (Nov 3, 2006)

Anthony said:


> Basically it maintains a 10x impedance while stepping the voltage down 1/16th of original. Since the amp puts out a max 16V, that guarantees that only 1V gets to the soundcard input regardless of what I feed the amp. Any amp you use might be different, but I'm paranoid and I always make sure the output voltage to the soundcard is less than 1V RMS (even though I know how I designed the jig). The jig does have banana plugs to screw down test resistors and inductors, so once it's set up measurements are easy.


Finally some confirmation! I've spent the last several hours trying to figure this out. What you wrote above is where I get a bit lost though. Might I just be better off getting a WT3 from Parts Express for measuring Impedance/caps/inductors? If you can at all enlighten me on how to do the voltage divider and what to do to make sure I don't fry the sound card that would be great. 

I will be using my Trends Audio Tamp--I haven't measured the max V but I know it is only rated to 15 watts. My electrical background is very poor so I don't really have a good grasp on impedance and voltage and how they all relate.




> As for sound measuring, I use an effects cord (Tip-Sleeve 1/4" plug on both ends) and loop one channel back to front (forget which, I think it's Right looped, opposite of Room EQ Wizard, which I also use a lot). Then I use the other channel for measurement through the XLR.


Well this is what I kind of thought I would need to do! I also want to use this as a portable REW rig. I haven't used REW for a couple years so I'll have to do a bit of re-learning.


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## Anthony (Oct 5, 2006)

You pick it all up again fast.

I'll dig up my schematic at work tomorrow. I think I ginned it up in powerpoint so it's just a matter of finding it.

The bottom line is simple, though -- feed the amp a 1kHz or 100 Hz (if you're testing a sub) sine wave signal at whatever output volume you are going to use. For testing, I max out wave volume and system volume to make sure I won't fry the card even under a worst case scenario. Then use a voltmeter that can read alternating current and voltage. Set it up in voltage mode across the + and - of the amp. If the voltage reading is less than 1 volt, you're fine. If it's higher than 2, you can cook the soundcard. Anything inbetween in gambling 

My amp also has a gain knob, so I tested that as well. The simple answer is: you just need to make sure that max out on your amp is stepped down to less than 1V at the sound card. 

It will make more sense when you see the jig schematic. Mine has two switches. One is test/loopback and the other is neutral or select 5 Ohm or 20 Ohm test resistor for calibration. 90 % of the time the jig is in Test/Neutral on the switches. It's actually simpler than the Wallin jig because the MobilePre has its preamp and gain stage built in. I made my jig for probably $15 or less if memory serves. Most of that was the jacks and box.


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## mpotoka (Nov 3, 2006)

Thanks for the input. I too made my own more simplified jig when I was using speaker workshop with my laptop's internal soundcard. I used an external preamp then but now I don't even need that. I am thinking I can just use it (or modify it) to work and I will just make sure before I take any impedance measurements I am only at the 1 volt. 

So when you are testing/calibrating the reference resistor you are using the amp then as well? I assume so...

And also when you say you put system and wave volume on max--does that mean you turn the knob on the front of the mobilepre all the way up?

Thanks again!
Mike


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## Anthony (Oct 5, 2006)

First, yes, I do use the amp for calibrating the jig. The connection goes:

Stereo Line Out (L) --> Amp input (L) -->Jig Amp Terminals 

Jig 1/4" Out (L) --> Mobile Pre In (L)
Jig 1/4" Out (R) --> Mobile Pre In (R) ----- This makes the loopback include the amp

As for maxing things out:
For safety testing only: Wav volume 100%, Direct Monitor 0%, Main Out 100%

For actual measurements, I went through the Speaker Workshop Manual steps to check for distortion and prevent clipping and all that. I use those values for test, as they are designed to minimize distortion by balancing all the gains available.

The gains in the MobilePre are a wildcard. I use them for fine adjustment, but they are usually at the 12 o'clock position. For safety testing, they don't matter, since it's not plugged in yet. I leave the line-in's unplugged and check them with a voltmeter to verify < 1V RMS coming out of the jig.

Still looking for that schematic. I'll post it when I find it.


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## Anthony (Oct 5, 2006)

Here it is:









I'll take a pic tonight to show you the final results.


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## mpotoka (Nov 3, 2006)

Ok--I got it basically running with a cable jig setup. However it still is listing my reference resistor as 17 ohms (not 11.5) and series resistance of 2 ohms. At least when I try to measure a component now it says the right thing--the inductors and resistors are "close" to the accurate value but my caps are coming in about 60% of value.

What does this mean if I try to do an impedance measurement? Is it going to be junk?


OOO wait--I see the CLIP lights on and I'm not doing anything in Speaker WOrkshop. How can I test for "noise" or a group loop or something like that? Should the - term from the amp output be connected to the negative on the 1/4" inputs? Thats how I have it maybe I shouldn't... Lets see I should look at your schematic closer..


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## Anthony (Oct 5, 2006)

First off, it's been a while, so if I fumble around some of the solutions, forgive me.

A cold solder joint or a loose wire joint could add the extra resistance you see. If you are seeing clip lights on the inputs with no signal, then there must be a lot of noise coming through the amp. Did you verify the voltage on a 1kHz sine wave was <1V?

Try a soundcard calibration (soundcard and amp) and post the results here. we might be able to see any noise, feedback, etc. Verify that the monitor volume is all the way down. That would introduce some feedback to the measurements which would throw them off.

That's all I can think of for now.

To test for ground loop hum, try plugging in headphones to the MobilePre and then hooking an old speaker up to your amp (assuming it can play down to 60Hz). You should be able to hear the static or hum through them. Another way to check is use REW on scope or RTA mode and see the noise distribution. It should be flat-ish in loopback (even through the amp and jig). If you see spikes at certain points, you know where the noise is.

I run my laptop on battery when doing measurements. In certain setups I got a hum that showed up nice and clear on the REW RTA.


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## mpotoka (Nov 3, 2006)

Maybe this doesn't mean much. I connected Line out to Amp In. Amp +/- to two 1/4" Mono Jacks to the 2 Inputs on the Mobile Pre. This is what my channel differences calibration file looks like. Not good I don't think.


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## Anthony (Oct 5, 2006)

Here's my amplifier calibration from a while ago. I was worried about it being too noisy. It is a tad, but good enough for my uses 









I don't have one of my channel diff calibration.

It really looks like you still have the monitor volume on, which causes the waviness. I think it's under Direct Monitor in windows sound and something else in the M-Audio mixer (can't remember right now).

Hmm, if I get a chance tonight, I'll snap a screen shot of my channel diff. IIRC, it was pretty flat with a slight tail at the end (20khz) Definitely not noisy.


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## mpotoka (Nov 3, 2006)

Direct Monitor is off--When I turn it on I hear noise in the heaphones. Have to do some work for now I'll come back to it again later.... At this point I care more about the FR stuff so I will try to test that later and see what happens


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## mpotoka (Nov 3, 2006)

Well for right now (I need to get some measurements done so i can paint next week) I went back to my Firewire Solo. I started from scratch (since I jacked up my Jig trying to get it to work for the MobilePre) I went through the steps and had it up and calibrated and running properly in about 15 minutes. All that work just to try and get rid of one power supply isn't worth it. I will just keep my MobilePre for REW and I can use it for SW Frequency too.


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## Anthony (Oct 5, 2006)

Found my last channel diff. file. Here's the graphic. It's different every session, but most of them are very close to this:


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