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Old 18th July 2007, 04:42 PM   #11
JMB is offline JMB  United States
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If you can figure out how to post a picture either here or at the SW forum, it would be helpful. I have found phase measurements to be accurate. SW measures actual phase and not derived phase, so measurements from the same position for both drivers will provide a more accurate ability to develop loudspeaker designs.
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Old 18th July 2007, 04:49 PM   #12
arc2v is offline arc2v  United States
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I'm sorry, I wasn't very clear.

I was confused by the phase measurement in the THD output. I didn't know if it was really phase, something else, or what. I turned it off to clean up the chart, but I wasn't sure how useful that information was for the THD measurement.

I certainly am going to use it to test the actual drivers.

I'm got some web storage I'm not using (I think I an hotlink images directly). I'll upload a few pics tonight and post them here so you guys can set me straight.

thanks,
AC
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Old 18th July 2007, 11:32 PM   #13
arc2v is offline arc2v  United States
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Okay, I think I figured out the problem and its mostly on my end.

I was using two guitar effects cords (short T-S 1/4", about 1 foot long) to loop the inputs 1 and 2 to their corresponding outputs on the MobilePre.

This worked great for the Right Mark Audio Analyzer.

However, SW is expecting L/R IN, but the same signal from L OUT. Since I have Left Out -> In and Right Out -> In

Now I seem to be getting input on both channels, so this may not be it. But I'm getting good results from RMAA, and I have NOT built my jig yet, so this is the best theory I have so far.

I hope to make the jig tomorrow night and will test again. If things still look odd, I'll post the charts here for some help.
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Old 19th July 2007, 12:05 AM   #14
owdi is offline owdi  United States
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Quote:
Originally posted by arc2v
Okay, I think I figured out the problem and its mostly on my end.

........

Whenever I have a problem with measurements, that's usually what I find too. :-)

Dan
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Old 19th July 2007, 02:08 AM   #15
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arc2v,

Unless you can be sure the toggle switches you order for your jig have low (and consistent) contact resistance - go the hardwired way.

I built the Wallin II jig - and when I had impedance all over the place due to the lousy switches I was using - I retrofitted the jig Claudio style.

I therefore use as follows
J1 links to BP2. J1 is connected to sound card left output
J2 links to BP1. J2 is connected to sound card left input
J3 links to BP3. J3 is connected to soundcard right input
J4 links to BP3. J4 is connected to microphone preamp output
BP4 is the earth connector

The above are permanent. I then use BP1, BP2 and BP3 to wire in the modes as follows:
Loop mode - wire from BP1 to BP2 and from BP1 to BP3
Impedance calibration mode - wire from BP1 to BP2, then calibration resistor between BP1 and BP3
Passive component / driver impedance test - wire from BP1 to BP2, 8 ohm (or whatever) calibration resistor between BP1 and BP3 then DUT between BP3 and BP4
Frequency response measurements - same as loop mode, but amplifier input connects to BP1(+) and BP4(-) and plug your pre-amp output J4

You have to be careful not to overdrive your amplifier or you risk burning out your soundcard.

I'll post photos and a schematic on my website in due course.

If you put spade connectors on your wires and calibration resistors etc... you can stack up any combination under the binding posts, change them reasonably quickly and have a low and consistent contact resistance.

I see the above as getting the simplicity of the Claudio setup (and avoid cost of switch components, variability in resistance and line level / sound card protection resistors), but some not having to use a screw driver on a connector strip to change modes (benefit of Wallin jig with binding posts).

David.
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Old 19th July 2007, 03:55 AM   #16
arc2v is offline arc2v  United States
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Thanks Dave. I have a plan in place for my "version" of the jig.

Since I use a MobilePre, I have a 1/8" stereo line out from the soundcard and 1/4" T-S jacks for line out and line in. This is in addition to the balanced XLR for my microphone.

So I was already going to modify the jig for use with the different jacks.

Upon reading all the instructions, one thing that troubled me was no clear singular purpose for each switch. The combination of all of them was what made it do mode X. Since frying the soundcard was a major concern, I always wanted to see what resistor was in the signal path and voltage divider, in addition to making it easy to test.

So basically the jig is going to be a series of banana plugs and various jacks (no switches). It will have line out (from soundcard), lines in, a port to plug into an amplifier and the banana plugs to put:

Device under test
Jumper or resistance between channels
Series and parallel resistors to make a voltage divider on the right channel input.

It will take longer to calibrate, but once I make some dedicated jumpers and solder some banana plugs to my common test resistors (5, 10, and 20 Ohm), it should go faster.

I have a graphic on my office computer. I'll post it tomorrow morning.

Thanks for the help.
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Old 19th July 2007, 01:39 PM   #17
arc2v is offline arc2v  United States
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Okay, here goes: my version of the jig.

Edit: I attached the image (below) instead of hotlinking it. That should avoid any file-not-found issues if I redo my hosting.

Some notes: green is ground, black is signal, and dotted means it can be anything: open, shorted, or a resistor.

Unless otherwise indicated, jacks are banana terminals with a screwdown option. Most of the inputs and outputs are unbalanced 1/4" jacks (I have a T-S to RCA cable for hooking the bypass up to an amplifier). The input is a stereo 1/8" (3.5mm) to take advantage of the line out port on the soundcard. Left is all that will be wired. Right will remain open (ignored), as per Eric and Claudio's jigs.

I plan on soldering some dedicated 5, 10, and 20 Ohm jumpers as well as shorts for common testing modes.

Eventually, I may install Erics zener diodes to clamp down the output voltage just in case. I have an e-mail to the M-Audio tech support to see what is the max voltage the inputs can take.
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Old 20th July 2007, 10:04 AM   #18
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yours looks good. Simpler is always better in my book.

Actually I was wrong about burning out the soundcard.

for freq. measurements - the amp is never connected to the soundcard so no risk.

If I did amp calibration or calculate the transfer function of the xo (which I haven't yet), but I'm sure it can.

Cheers,
DAvid.
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Old 20th July 2007, 01:36 PM   #19
arc2v is offline arc2v  United States
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Okay, the jig is done and checked out okay. Design was revised a bit for space considerations.

When I moved into my house the previous owner left an assortment of HAM radio parts: jacks, knobs, pots, resistors, micro-screws, etc. So I ended up using most of the mini-plugs to be my test/bridge ports.

Every jack is grounded, as is the black banana test port. Grounds were run as close to the lid as possible. Signal leads were run off the lid and kept as far apart as possible. Internal wiring was 18 gauge solid.

Unfortunately my DMM test leads have a break in them and I can't seem to get accurate resistance readings, although open/short seem to work. I'll have to pick up some new test probes (I think the wires fatigued at the strain relief in the test probes and are barely hanging on -- a 5 Ohm resistor measured 17 and a 10 Ohm measured 23).

Oh, and I have an extra ground on the above powerpoint diagram. Only the jacks, the one test port, and the black banana plug should be tied to ground.

The leads shown there should be self explanatory. Two are shorts for bridging or bypassing the voltage divider section. The two resisors that are wired up are 10 Ohm, for most tests, and 10k Ohm for calibrating the sound card. I will also wire up voltage divider resistors once I figure out what the sound card can take.


On that front, I heard back from M-Audio regarding input impedance for the MobilePre:
3550 ohms instrument and Mic
4800 ohms for the stereo mic
Maximum input is -5.2 dBu

Finished around 11 last night, so no actual tests. I want to make sure my DMM is working right first anyway. Hopefully Rat Shack has replacement probes in stock.
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Old 20th July 2007, 02:04 PM   #20
JMB is offline JMB  United States
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Any chance that you would be able to post an actual schematic? I am a little confused with the current presentation but it looks like a wonderful design.
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