Lilmike's Mic - AVS Forum
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post #1 of 119 Old 03-28-2011, 04:08 PM - Thread Starter
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With all the extremely capable subs being built and tested, I figured I'd share a measuring mic that I came across so that we can get accurate measurements. This is extremely capable (SPL from 50 dB to 130 dB with no ranging or preamp, frequency from the single-digits to >20 KHz +/- 3 dB with no corrections) and extremely inexpensive. The only downside? You gotta make it, but this is the DIY Forum.

Pretty much anyone familiar with a soldering iron should be able to put one of these together and have change left out of a $20, maybe even a $10, depending on what you've got sitting around.

Anyhow - I can not claim the design - that is all from Linkwitz. This is 1/2 of the microph2.gif from the linked page, with a 10uF capacitor substituted for the 2.2 uF to lower the low corner to ~4 Hz.

The other use of these mics? Bootleg recordings of live shows. This is known as a "battery box".

Without further ado - here is the schematic:


EDIT: This is the corrected schematic.... 4-7-2011

Pin 1 of J2 is the (+) signal to the sound card (tip of the RCA or miniplug), pin 2 is the ground (ring of the RCA or miniplug).

Parts List:

Panasonic WM-61A Mic Capsule, modded per Linkwitz
R1 10 KOhm resistor
R2 100 KOhm resistor
C1 10 uF/16V capacitor
9V battery clip
9V battery
plug compatible with your PC Sound Card
perf board and small project box

Deluxe versions can have switches, LEDs and attenuators too

Point to point wiring worked fine on mine, notnyt made one too, and he took better pictures than I did.

Anyhow - here are the parts:


Here it what it looks like all put together with one of my plumbing-aisle wands:


Detail of the back side of the circuit board I used:
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post #2 of 119 Old 03-28-2011, 04:12 PM
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You can get the mic capsule at digikey: Panasonic WM-61A Mic Capsule

Here are some pics and measurements of the one I built.

Linkwitz's schematic for modifying the mic capsule and the circuit design. You can substitute a 10uF cap for lower rolloff (2hz). This was originally a stereo design, but I removed the unnecessary channel for a measurement mic.


Mic capsule pre-mod


Mic capsule with 2-3 lead cut


Mic capsule soldered up with 1-3 joined


Board assembled


Not much soldering required


Assembled in a project box. I know the mic capsule isn't in the best position, but it works for me.


linear measurements from 75db up to 125db. I clip the input to my PC at around 130db.


131db input. The distortion is from my walls/mic input/amps



EDIT:

adding some more pics from second build.

modded mic


wires soldered


all parts for build


mic holder




assembled


closed




This is the EMM-6 measurement, with soundcard+mic buddy cal, and mic calibration.





Now, relative to this thread, here is a calibrated and properly configured EMM-6 compared to two WM-61A microphones.

EMM-6 vs WM-61A #1 (mic in pen tip, 10uF capacitor). These are very close up until just over 1khz.


EMM-6 vs WM-61A #2 (no mic extension, 4.7 uF capacitor).


Radio Shack C Weighted VS WM-61A VS Audyssey VS EMM-6


And here is a comparison between both WM-61A microphones.


Unsmoothed close mic comparison from 5-90hz of various mics
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post #3 of 119 Old 03-28-2011, 04:13 PM - Thread Starter
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Moved the parts list up to my first post.

This will drive a soundcard directly, no preamp required. Just plug it in, turn it on, and go. Still need a SPL Meter to reference your measurements to a real sound level, but that's it. No corrections really needed in the subwoofer testing range, these capsules are typically flat from below 20 Hz to above 10 KHz. They're within 3 dB of flat from 20 Hz to 20 KHz as delivered from the factory. The Linkwitz mod is a bit fiddly, but if I can do it, nearly anyone should be able to. I have no dexterity....

The modded capsule is the key here. The modification greatly reduces distortion and increases SPL capacity.

I can't get my mic to clip. I can overdrive my soundcard, and I can overdrive my speakers, but the waveform from the mic is fine.

120 dB was no problem. I ran out of amp and voice coil there...
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post #4 of 119 Old 03-28-2011, 04:24 PM - Thread Starter
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OK, to allay the fears, I am not an engineer.

I am a geologist, not an electronics technician, I have zero formal training in electronics.

....ok, ok, I'll come clean and admit I'm an amateur radio operator....

I made most of this with stuff I have laying around, but as I am a HAM, I might have a better-stocked junk-drawer than most.

The wire I used is a car RCA cable. The wand? Seriously - it is a PEX water supply line. The capsule fits perfectly in the end.
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post #5 of 119 Old 03-28-2011, 04:41 PM - Thread Starter
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The capsules are available at Digikey.

They have ~79,000 in stock. $1.92 each as singles, but that drops to $1.55 each if you buy 25. Buy 3 capsules at a minimum, that way you can compare the results and make sure things are right. They are tiny, I have melted them trying to get things soldered and modifying them.

Everything else can be had at Radio Shack, but since you're already shopping at Digikey, you can get it all there too.

Once the mod is done, it is a good idea to seal the back of the capsule. I've used clear nail polish, superglue, and hot-melt glue. Superglue is probably the easiest and fastest, especially if you use accelerator.
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post #6 of 119 Old 03-28-2011, 06:52 PM
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Very nice... I like lilmike's mic
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post #7 of 119 Old 03-28-2011, 07:08 PM
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Quote:
Originally Posted by Oklahoma Wolf View Post

Very nice... I like lilmike's mic

me too!~
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post #8 of 119 Old 03-28-2011, 07:17 PM
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Quote:
Originally Posted by lilmike View Post


Once the mod is done, it is a good idea to seal the back of the capsule. I've used clear nail polish, superglue, and hot-melt glue. Superglue is probably the easiest and fastest, especially if you use accelerator.

I used some 5m epoxy.
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post #9 of 119 Old 03-28-2011, 07:19 PM
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post #10 of 119 Old 03-28-2011, 07:33 PM - Thread Starter
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Quote:
Originally Posted by notnyt View Post

Did you get to compare this to the EMM6 yet?

Not yet.

Gotta cook some dinner...
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post #11 of 119 Old 03-28-2011, 07:42 PM
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post #12 of 119 Old 03-28-2011, 09:02 PM
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Super mic !!!!

With a little more tweaking you will have a Dayton "OmniMic" clone.

If you can pull the 9v from the USB port and send the audio signal to the sound card via the USB port you'll have it.....

CH
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post #13 of 119 Old 03-28-2011, 09:08 PM
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USB is 5v. You'd need to emulate a USB HID microphone for that.
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post #14 of 119 Old 03-28-2011, 09:15 PM
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The Mobile-Pre USB interface by M-Audio and other USB interfaces provide 48V Phantom Power to the mic via the USB. Are they getting it direct from USB or stepping up 5V ?

CH
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post #15 of 119 Old 03-28-2011, 09:26 PM
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post #16 of 119 Old 03-28-2011, 11:36 PM
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Expensive but I'm sure you can build one yourself.
http://www.dimensionengineering.com/AnyVoltMicro.htm

You can search *bay for dc dc converters.
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post #17 of 119 Old 03-29-2011, 07:19 AM
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Then you would need an ADC and a usb controller chip, which increase this project in complexity and cost by a large factor =]
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post #18 of 119 Old 03-29-2011, 07:25 AM
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Cool stuff!! but it seems like a lot of tedious work to save $30??

It is not "open-minded" to reject knowledge - Bob Lee
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post #19 of 119 Old 03-29-2011, 07:38 AM
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Pretty cool Mike!

Inexpensive too. Bonus!

Great contribution to the community.

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post #20 of 119 Old 03-29-2011, 07:40 AM
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Quote:
Originally Posted by penngray View Post

Cool stuff!! but it seems like a lot of tedious work to save $30??

Penn, time to turn in your DIY card
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post #21 of 119 Old 03-29-2011, 07:42 AM
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Penn--doesn't it save on the Mic and the preamp/external sound card? So it's not $30, it's ~$150-20 = $130 saved .

Lilmike--thanks for bringing this to the attention of the collective. Now there's a mic I can actually afford! I'm really looking forward to your comparison tests. You sir may have just got me on the road to owning measuring equipment .

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post #22 of 119 Old 03-29-2011, 08:01 AM
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Quote:
Originally Posted by lennon_68 View Post

Penn, time to turn in your DIY card

lmao, I would never build passive XOs for the same reason why I will never build this mic

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post #23 of 119 Old 03-29-2011, 08:02 AM
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Quote:
Originally Posted by Starkiller4299 View Post

Penn--doesn't it save on the Mic and the preamp/external sound card? So it's not $30, it's ~$150-20 = $130 saved .

The external sound card is because 99% of all internal cards on notebooks suck. Sure some people lug a PC around to do the testing so maybe they have decent sound cards. There is still a voltage requirement here, is there not.

Quote:


Lilmike--thanks for bringing this to the attention of the collective. Now there's a mic I can actually afford! I'm really looking forward to your comparison tests. You sir may have just got me on the road to owning measuring equipment .

Of course I 100% agree with this as a viable option for many DIYers!!

It is not "open-minded" to reject knowledge - Bob Lee
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post #24 of 119 Old 03-29-2011, 09:21 AM
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One of the benefits of this mic is the incredible range without clipping.

I'm still waiting to see some measurements compared to a pro mic though. Too bad I don't know anyone on long island with one.
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post #25 of 119 Old 03-29-2011, 09:59 AM - Thread Starter
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Quote:
Originally Posted by penngray View Post

Cool stuff!! but it seems like a lot of tedious work to save $30??

Ummmmmm........

No.

Soldering two resistors and one capacitor to a piece of perf-board is not tedious. One of these takes me less than an hour to put together, and the parts for a basic setup cost me less than $10. Sure, I can spend more money and make it nicer-looking, but it measures exactly the same.

Show me something that I can use to measure 50 dB to 130 dB in a single range setting, with a single calibration point, with a frequency response that's +/-3 dB from the single digits to well over 20 KHz for $40, including the necessary cables, power supplies, and preamp.

Seriously - if I could have done it cheaper, I would have. I may be cheap, but I am definitely results-driven. Saving that buck was a waste if the results are compromised.

If you're not worried about response below 20 Hz, the onboard sound on my Dell Latitude 620 is plenty good (it has a switchable line-in, not just a mic-in). True, this setup requires a sound card for the super-low stuff, so add $30 for a UCA-202. Still comes in under $50, which is cheaper than the EMM-6 mic alone. Most of us would prefer to use an external USB sound card anyhow - so that cost is a wash as far as I am concerned, as we'd use it regardless.

Seriously, nothing I could find comes close in this price range. I've looked for years. Closest I could find is the EMM-6 and the Behringer Xenyx 502. That pair added up to over $100 when I bought it, not counting cables. Over 10X the cost, but no where near 10X the capability. In fact, the EMM-6 mic handles less SPL (127 dB) and has higher distortion levels (1%) than the Linkwitz-modded WM-61A.

At least I can use the mixer for other stuff.
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post #26 of 119 Old 03-29-2011, 10:03 AM - Thread Starter
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Re the USB power issue.

I could probably power it with USB, the 5V supply should be sufficient. Might need to reduce the values of R1 and R2 though.

Biggest issue I see though is that it will pick up all the lovely noises that come from switch-mode power supplies and my 50 dB noise floor would become a 90 dB noise floor.

Batteries are cheap, and extremely effective.
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post #27 of 119 Old 03-29-2011, 10:06 AM
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So this does not need any voltage to run??

Im not interested in any posted measurement of any system at 130dB myself but others are. Its definitely something cool and cheap so well done!!

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post #28 of 119 Old 03-29-2011, 10:08 AM - Thread Starter
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Quote:
Originally Posted by penngray View Post

So this does not need any voltage to run??

Im not interested in any posted measurement of any system at 130dB myself but others are. Its definitely something cool and cheap.

Just the 9V battery in the picture.

It measures just as accurately at 90 dB.

It just does not clip until >130 dB.
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post #29 of 119 Old 03-29-2011, 10:12 AM
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Quote:
Originally Posted by lilmike View Post

Just the 9V battery in the picture.

It measures just as accurately at 90 dB.

It just does not clip until >130 dB.

I definitely understood that it works well below 130dB

I wonder if it will have issues up high, reflection stuff (because the box,etc).

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post #30 of 119 Old 03-29-2011, 10:17 AM - Thread Starter
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Quote:
Originally Posted by penngray View Post

I definitely understood that it works well below 130dB

I wonder if it will have issues up high, reflection stuff (because the box,etc).

That might be an issue. I use a wand on my mics. It is the same diameter as my EMM-6, and is made out of a piece of PEX water supply line. The capsule slips into the end perfectly. A piece of shrink-tube for a strain relief works great.
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