Now, I didn't believe it either... but there are a couple of sites on the Internet selling such mics. After some searching I came across the website of audio engineer Siegfried Linkwitz.
Linkwitz discovered an interesting trait of the Panasonic condenser mics: If you cut the trace between the mic element and the internal FET and then reverse the polarity of the mic element it greatly reduces the distortion, especially in high sound pressure level (SPL) environments.
Linkwitz' panasonic mic information is here:
http://www.linkwitzlab.com/sys_test.htm#Mic
Linkwitz' Actual modification details & schematic:
http://www.linkwitzlab.com/images/graphics/microph2.gif
So, armed with the relevant information, I set out to build my own. I decided to buy the WM-61As from Digikey and ordered 10 of them.
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I bought a $5 pair of headphones from Target and took them apart to use to house the mics. I removed the original headphone speaker modules and kept the wires to use for the mics themselves.
An exacto knife was used to easily bore the holes in the ouside of the headphones to hold the mic cartridges. I actually wanted the mics pointed outward, instead of forward, to more realistically capture the stereo seperation experienced at a concert.
After modifying the capsules as per Linkwitz' instructions, I soldered them to the wires formerly used by the headphone speakers. I then put them in place and applied silicone to them to hold them in place and protect the wiring. I let the silicone set for a few hours before handling the headphones.
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For the battery box I followed Linkwitz' schematic above as well, using the setup for no attenuation and 220uf capacitors. I happened to have some 220uf caps handy. They're rated for 200V, so they are a bit larger than need be, but it'll prove whether this whole setup works or not.
I assembled the parts onto a RadioShack project board and fit the assembly, battery, and switch into a altoids tin. To mount the Input/Output jacks I made pilot holes on the back spine of the tin with a small nail and then drilled larger holes with a standard drill.
After some consideration I decided to leave the power switch inside the tin to keep me from accidentily operating it while the unit is in my pocket.
I originally wired a "power LED" & resistor across the battery terminals to indicate whether the unit was on, but after reconsidering the schematic I removed it because I determined that it would also act as an attenuator.
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A closeup of the finished mic is below:
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Testing:
Some preliminary testing in my home helped me determine that the only real
test of these mics would be in a high SPL environment. They performed
okay in low SPL environments, but there was no real indication of the
fidelity- everything just sounded like quiet background noise.
So, with the headphones around my neck and an Altoids can and MiniDisc recorder in my pocket I headed off to the nearby Star Bar to check out a local band. A $5 cover got me in to see Yum Yum Tree, a local Atlanta band that had apparently been around for a number of years.
| Yum Yum Tree - 2005.09.17 The Star Bar Atlanta, GA |
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|---|---|
| Filename | Size |
| YY01.mp3 | 5 MB |
| YY02.mp3 | 6 MB |
| YY03.mp3 | 7 MB |
| YY04.mp3 | 1356 KB |
| YY05.mp3 | 6 MB |
| YY06.mp3 | 8 MB |
| YY07.mp3 | 5 MB |
| YY08.mp3 | 6 MB |
| YY09.mp3 | 6 MB |
| YY10.mp3 | 7 MB |
| YY11.mp3 | 8 MB |
To ensure that the test was conducted under realistic conditions I downed three or four Meyer's & Cokes while waiting for the show to start. ;)
Yum Yum Tree took the stage and started playing with no introduction, so I quickly powered on the battery box and MD recorder and then walked over towards the center of the venue. I jumped around a bit during the first few songs to see what effect it would have on the recording. After the show, I went to the bathroom to "disarm" my equipment (and deal with the four drinks I had prior.)
Once I got home and reviewed the MD recording I was impressed! The only problem encountered was that the level on the right mic was about 25% lower than the left. The overall fidelity and recording quality is excellent! The stereo seperation is good enough that you can even clearly "feel" me walking from the bar to the center of the venue during the first 20 seconds of the first track.
I transferred the audio to the PC using Audacity and boosted the right channel to make up for the level difference. This boosted the MD recorder noise during the quiet parts, but overall it sounds better this way. Other than the level boost, I did no post processing. You can hear the results in the MP3 files posted in the table to the right.
Update 2005.09.25:
Bench testing with an o-scope leads me to believe that I damaged the
internal FET of the right Mic module when trying to complete the
modification, hence the dramatic level difference. I have since replaced
the module. I plan on testing it this weekend and will post the results.
Update 2005.10.02:
Last night I took the monkeyphones out to The Earl and recorded two shows.
The replaced mic cartridge made a big difference! The shows are here:
The
Press Live @ The Earl, 2005.10.01
Tenement
Halls Live @ The Earl, 2005.10.01