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Old 20th July 2007, 01:09 PM   #21
arc2v is offline arc2v  United States
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I'll see what I can draw up later today.

Worst case, I'll set it up for all the various tests and take a picture and add labels. That will have to wait until tonight, though.
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Old 20th July 2007, 01:52 PM   #22
arc2v is offline arc2v  United States
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Okay, here's the raw schematic of the jig. I whipped it together pretty fast, so I apologize for any mislabeling or errors.

bLack is Left, Red is Right, and Green is Ground

Input is a stereo 3.5mm, outputs are 1/4" unbalanced T-S.
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Old 20th July 2007, 01:54 PM   #23
arc2v is offline arc2v  United States
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Here are two examples of modes. I haven't tried them yet, but I used Eric's and Claudio's schematics in my planning so I hope this is accurate.

I do not show anything in the way of using the voltage divider or the bypass output (1/4" T-S jack), which I guess could also be used as an input if necessary.

Edit: clarification, jacks are defined relative to the sound card. So Out actually means Sound Card Out not jig output. It should be clear from the drawing, but I don't want to confuse anyone stumbling upon this.
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Old 21st July 2007, 02:23 AM   #24
arc2v is offline arc2v  United States
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Okay, I've spent the better part of the evening calibrating this thing, and I'm at a loss.

I can get the amplifier pulse and channel separation calibrations done. It gives me reasonable values for soundcard impedance and capacitance. And it calibrates the jig to almost exactly the reference resistor value.

So all that being said, it can't measure components accurately at all.

It identifies resistors and inductors as capacitors, and for capacitors, it doesn't come close to getting the value right. It seems to have repeatability, though.

I've tried different sampling rates and multiple MLS cycles for a better signal to noise measurement. Still nothing.

The jig checks out, though.

My friend seems to thing the M-Audio MobilePre might have a capacitive coupling on the input stage to take off the phantom power from the mic. This capacitor might be screwing up the measurements. I've searched for Speaker Workshop and MobilePre and I seem to be the first person trying this.

A lot of people have been trying SoundEasy and the MobilePre with mixed results and this might be why.

Ugh, if I can't get a resistor to measure right, there's no way I'll believe an impedance plot for a driver that I measure.

Advice appreciated. Until then I'll keep cranking at it.

Thanks.
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Old 21st July 2007, 01:31 PM   #25
arc2v is offline arc2v  United States
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Exclamation HELP

Okay, consider this a HELP flag.

I've done everything twice. I even built Claudio's jig as well and can't get comonent measurements to work.

Steps I took:
Built Jig(s)
Hooked up in pure loop mode (L-L, R-R) and tested using RMAA. Not the best numbers, but good and it was getting a signal, so I moved on.

Hooked it up in SW loop mode (L-R&L), no resistors.
Made a 1k sine wave. Did record, check levels, and repeat. Got good results at 25k w/ no clipping.

Did amplifier calibration (the soundcard compensation)
Did channel difference calibration. I had to adjust the interchannel delay to get flat phase up high (+23 us).

Soundcard calibration:
Hooked up 10k resistor to the jig. Ran the SC impedance test.
100kOhms resistance, 320pF capacitance

Jig Calibration:
Hooked up 10 Ohm resistor and ran the test with a 5.1 Ohm and 20 Ohm test resistors. It came back with:
9.8 Ohms for the reference resistor
0.15 Ohms for the series resistance

So far all the figures look in line with what I expected.

Then I hooked up a 4 Ohm resistor to check. It came back as a Capacitor of value 41 uF and resistance of 5.8 Ohms!!!

I tried inductors and capacitors and none are even close. Other resistors are way off as well. Even the actual ones I used to calibrate it with!

Results are the same with both jigs, although the Claudio jig has a lot less resistance, so I'm probably going to stick with it over the one I made. Oh well.


Sound card is a M-Audio MobilePre. By spec it's listed as a 16bit, 44.1kHz card, but RMAA says it will handle 96kHz, 32-bit. All tests were done at 16-bit, 44.1kHz.

Sample size was 256k, MLS warmup was 100 cycles.

I applied for an account over on the Audua forums, but it hasn't been activated yet.

Any ideas? I'm at a loss now. The current theory is that this sound card is not up to the task, but I'd like to exhaust all other options before giving up on it.

Thanks,
AC
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Old 22nd July 2007, 12:33 AM   #26
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I get accurate capacitor and resistor ratings.

I only use the sound card for my impedance measurements. I get accurate resistor and capacitor readings. I'm not certain my inductor readings are accurate, but at least SW correctly identifies what it is testing.

Make sure you sound card is set to equal input/output. It is better to have the volume lower - not higher. I find higher volumes lead to noisier impedance graphs.

My volume is set to only 9K peak to peak (sine wave 1KHz to verify). I set the wave output to 100% via SW and the volume is set to only about 1/4 (to get the 9K level).

I recommend you do a free air impedance graph and post that. you should be able to get it relatively noise free. The reason I think this will work (or could highlight other problems) is you are able to accurately measure you ref resistors.

FYI, I use wire-wound resistors. My values are approx 8.25 ohms for the reference, and I used 15.9 and approx 3.7 as my calibration resistors. I don't think quality of resistors will matter much.

So - lower the sound card volume and don't use an amplifier for your impedance tests and let us know how it goes.

I remember getting way out of spec values too. We'll get you there .

Jay and Claudio - if you are reading - any help appreciated.

PS: I also requested a SW forum account but don't think the moderators are active at the moment.

Cheers,
Davd.
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Old 22nd July 2007, 01:31 AM   #27
claudio is offline claudio  Italy
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Hello AC,
try this:

1) the mobil pre is a 48k S/R card, so stick with 48k, and repeat the channels difference calibration; don't use the amplifier calibration.

2) In the SC calibration, just place 47k ohm and 0 capacitance, assuming you are not using any voltage divider resistors.

3) Repeat the Jig calibration

4) Try again to measure the 4 ohm resistor, and report the results and the Vu-meter readings; also check the resistor under test is well connected to the jig terminals.
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Old 22nd July 2007, 04:30 PM   #28
arc2v is offline arc2v  United States
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Thanks for the replies, guys.

Okay, I did the folloing:
set 48k for the card.
Reset sine levels with 1/4 volume, max line in, and adjusted the pots on the front of the unit so the clipping lights do not light.

VU levels were 18k for both channels.

Did channel calibration and separation. With 22us of separation, I get a perfectly flat phase and value line.

VU levels were 22k for both channels, MLS repeat count was at 10.

Sample Size was set to 256k.

Then I tested an MLS file, with perform correlation checked and Sample Size (of the signal itself) at 131k.

Record levels were 23.4k for each channel.


Redid the Jig Calibration:
VU levels were only 869(L) and 533(R) to -1195(L) and -541(R). 1st Resistor (5.1 O)
VU levels were only 1496(L) and 1075(R) to -2184(L) and -1510(R). 2nd Resistor (20.5 O)

That seems really low for testing. Nothing was changed between the level setting and the jig calibration.

The results of the calibration were good, though:
10.06 Ohms for the reference and 0.26 Ohms for the series.

Immediately, I went to test the calibration resistors again:
DMM Measure ----- SW Measure ----- +VU (L/R) ----- -VU (L/R)
20.5 ----- 20.853 ----- 1498/1075 ----- -2181/-1509
5.1 Recognized as a Capacitor, 80.4 uF, 5.83 Ohms
Levels were 867/531 max to -1194/-539

I repeated the calculation to make sure it was repeatable, and it was.

On a whim, I upped the system volume from about 1/4 to 1/2. Same results, although the VU levels are 1207/767 max to -2195/-1094 min (L/R).

This has pretty much been my problem so far.

Free Air impedance to come.
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Old 22nd July 2007, 04:55 PM   #29
arc2v is offline arc2v  United States
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Free Air Impedance of a Dayton RS 8" Woofer, 8 Ohm

Phase axis is -60 to 60 degrees.

I copied the VU levels onto the chart.

Overall it looks close, but not prescise compared to the spec:
http://www.partsexpress.com/pe/showd...number=295-366

The first attachment is a pdf with FR and Z charts.

Thanks for any help,
AC
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Old 22nd July 2007, 05:55 PM   #30
arc2v is offline arc2v  United States
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Same free air with master volume maxed out. Nothing else changed.
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