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Old 12th January 2004, 04:42 PM   #1
sobazz is offline sobazz  Denmark
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Default Measurement mic question

Hello guys

I'm trying to dig into measurement microphones in order to be able to adjust the crossover in a current speaker project, but at this point I find myself not getting any furter and therefore I ask for your help. I would certainly like to go the DIY way in order to save some money (which I'm rather sure is possible).

So, I know electret capsule are the way to go, and modifying the Panasonic capsules as shown by Mr. Linkwitz should be possible. I have a source for the capsules (WM-61A), but they cost approx. 8$/piece - probably less if I buy 5. Can anyone help me with a cheaper european source for Panasonic electrets?

I've been looking at Rod Elliot's measurement mic preamp project and housing, and almost decided to go that way. But the preamp outputs a balanced signal and I'll be using my computer for the analysis, so I need an unbalanced signal.

How should I convert from balanced to unbalanced while maintaining a proper signal level (I guess somewhere in between 1-2v in order not to overdrive the Audigy input, or am I very wrong here)?

Please help me out... :-)
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Old 12th January 2004, 06:40 PM   #2
sreten is offline sreten  United Kingdom
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IMO if you are just trying to sort out a smooth crossover, rather
than take reference measurements a standard electret omni
mike should enable you to do the job, there are plenty of these
available for the standard PC soundcard mike input.
I think one of these is also good enough to sort out baffle step correction.

What you cannot assume is the broad picture is correct.

JMO sreten.
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Old 12th January 2004, 06:50 PM   #3
sobazz is offline sobazz  Denmark
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Well, yes. Using a simpler setup would be a solution. I suppose it would be able to sort out baffle diffraction step , which I suspect is one of the problems the speaker has. I also need to adjust the tweeter level.

However, I would like to do the more reference microphone like project in order to be prepared for future more complex projects.
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Old 12th January 2004, 07:49 PM   #4
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Hi,

The same Panasonic capsule is sold in Europe by Monacor with part number MC2000. Paid about 4 EU for one last year at my electronics parts shop around the corner.

Cheers
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Old 12th January 2004, 07:58 PM   #5
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Hi Sobazz,

I just got some wm60AY mics the other day (decidedly more expensive than $8 when you add in digikeys $6 international handling and $17 postage).....

I'm going to make this preamp http://www.gti.net/wallin/audio/preamp2/preamp2.html

have got everything bar the electros, IC and voltage reference which I'm going to have to order from farnell.

You could simply hook the mic capsule directly to your audigy's mic input, I did this on the weekend, The mic is definitely better than the computer mic I was using. (note it only worked on one of the two channels (Right I think)....

Below is a screen grab from in speaker workshop. I tested each speaker in the box individually with no crossover (nearfield), applied 1/16 Oct smoothing, scaled the results so that they were all around the same sensitivity then added each data set to a graph. As you can see the drivers aren't really suitable to be used together. when I measure the whole system, there are big dips at 500Hz (first xover freq) and again at 2Khz (upper xover freq is 3Khz)...... edit: all with the mic connected to the audigy's mic input.

The thing I find most strange is that they still sound quite good regardless..... I may change that opinon once I build my next set .

Tony.
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Old 12th January 2004, 07:59 PM   #6
sobazz is offline sobazz  Denmark
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So Monacor relabels Panasonic capsules? That certainly is nice to know. The Linkwitz mod may easily destroy a couple of capsule so the price is important.

Thank you very much for letting me know.
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Old 12th January 2004, 08:21 PM   #7
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Hi Sobazz,

Yes Monacor does. For removing the earth link use a new very sharp scalpel. Be careful, the aluminium housing is very thin and is easily damaged. I did not solder the ground connection but used conductive glue for it. Also be careful with soldering the other two connections. The solder blobs at the back are the pins of the internal fet which are very short.

The mod is only needed if you want to measure above 90 dB SPL. Then the capsule starts to compress. For normal x-over checks you rarely go beyond 90 dB, so you can hook it up the usual way with a resistor at the drain. And do not use more than 4 - 5 V power when hooked up with a drain resistor to limit noise (caused by the gate leakage current).

Cheers
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Old 12th January 2004, 08:22 PM   #8
sobazz is offline sobazz  Denmark
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wintermute, the preamp circuit certainly looks like an interresting alternative - and cheaper than making both phantom supply and balanced receiver for converting to unbalanced for use with Rod's mic.

If I decide to go for Rod's preamp and housing, however, what would I need in order to power the preamp and convert the balanced signal?

A phantom feed supply: http://sound.westhost.com/project96.htm

And the receiver shown here: http://sound.westhost.com/project87.htm

Or am I wrong?
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Old 12th January 2004, 08:27 PM   #9
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I use a Panasonic Mic Capsule with a modified Radio Shack SPL meter which works as a preamp, it's quite accurate, the different scales will provide different biasing. Just remove the factory mic and replace with a detachable mic wand (I used n Arrow shaft).
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Old 13th January 2004, 07:35 PM   #10
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Hi Sobazz,

I haven't looked at the ESP mic projects in depth, are you looking at project 66 or 93. I had a quick look at 93, and he says you can run it off either a phantom supply, or a 9V battery.

I like the simplicity (and cleaness) of a 9V battery myself, and personally would be opting for that if supported.

I don't know about the balanced stuff haven't read about it.

Tony.
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