I used the plans from http://www.arches.uga.edu/~tidmarsh/binmic.html. I'm no electronics expert but can solder, and it was very easy. If you want to build a battery box you'll probably want some fine gauge wire and a little piece of circuit board in addition to the components listed.
--------------||---------------------------hot--- (left)
| 1 or 2.2 microfarad
from left mic | input capacitor
|
---hot--------+----10k ohm-- "+"
\
---ground----------------------)---+--------------------shield-----
/ |
---hot--------+----10k ohm-- "+" "-"
| resistor 9 volt
from right mic| battery
|
--------------||---------------------------hot--- (right)
1 microfarad
input capacitor
Steve Smith ([email protected])
After reading about the DIY binaural mics detailed on the main minidisc home page, I thought I'd have a go at making them, using parts from TANDY (as Radio Shack is known in the UK) and MAPLINS (a retail electronics parts supplier). I was surprised at how good the results were from a home-built mic. set. I have an old set of binaural mic/headphones from JVC circa 1977 (they cost GPB40) with which they compared favourably.
I tried the mics both with and without the "battery-box" and the results were much the same (the battery box supplies each mic with 9V).
Curious to know why, I discovered that the bias voltage, from the MS200 is about 4.5V. I measured this with a digital multimeter, with the MS200 running from the internal Li-ion battery. I concluded that there must be a DC-DC step-up circuit inside, to give this much better voltage than the typical 1.5 (watch) battery. It is about midway between the recommended minimum and maximum voltages for the sub-miniature electret capsules I'm using. The capsules may/not be Panasonic WM60s, they have no markings on them at all.
I wonder whether this voltage step-up circuit is used anywhere else to improve the fidelity of the analogue side the the MS200, in a similar way to my old portable JVC CD1635 which internally steps up 9V to 20V.
I built some stealth microphones last week and I thought I'd let others hear of my experiences. Firstly, it took three attempts to make the mics work. I couldn't understand how to convert the schematic diagram into a working prototype. Eventually, I contacted Steve Smith (who writes above). Steve kindly sent me back a veroboard diagram to work from and I had the mics working very quickly. His comments I've added below.
Due to my poor soldering skills and the size of the microphone capsules, I bought a pair of ready made lapel mics and cut off the plugs. I looked for a compromise between size and performance. The brand I bought (no name from Taiwan, probably not available in your country) were very small with performance specifications of 50 - 18,000 and sensitivity of -52dB.
I built the circuit and put it into the smallest box I could find. I hard wired the leads from the mics straight onto the circuit board, and I used the wire from a good pair of stereo headphones with gold-plated L shaped 3.5mm plug for the output. I knotted all the wires inside the box, to reduce strain, and put a big blob of epoxy glue around the wires to secure them to the box.
I taped the microphones to my glasses and fed the wires under my shirt, the mic preamp was in my back pocket and I held my Sony R30 MD recorder in my hand. The first band I taped, was in a very hot sweaty pub. I stood about 30 feet from the stage, in between the PA speakers. I fed the mics into the mic input and left it on automatic recording with the mic sensitivity set to low. For the second set, I moved closer, to avoid some drunks standing near me.
Afterwards, I listened to the MD. I was amazed at the bass, far better than I'd been lead to believe, but there was some slight distortion in the high end. That distortion wasn't evident through the PA, so it was a recording problem. The second set was heavily distorted, due to me moving closer. However, the presence and "up-front" sound made me think that this was one of the best live-recordings I've ever heard, ignoring the slight distortion!
So, I thought I'd try again, because I wasn't sure if it was a) the mics, b) the powered mic input c) the automatic level control or d) my closeness to the stage that was causing the distortion.
The next show was in a restaurant club, with the dining tables right in front of the stage. Therefore the sound wasn't very loud and unfortunately not mixed very well. I decided to try the mic input again and kept it on automatic. The recording was excellent (through headphones; I'm querying the quality of the R30's line out in the MD Community Page, because it seems to flatten the sound quite noticeably) though not as up-front as the previous recording.
One thing you must not do is move your head! I slightly moved my head to the right to kiss my girlfriend and, on playback, the whole band shifted to the right too! I hadn't expected this, because I thought that with such a narrow separation between the mics, slight head movement wouldn't matter.
This weekend I'll try again with the line in input, rather than mic, and I'll use the manual recording setup, which is very hard to use in the dark - very small buttons! I also plan to replace the mic capsules with the Panasonic capsules. (Those $2.75 Panasonic capsules in the USA are $35 here in Australia - about USD$42.50 with the current exchange rate!)
I'm delighted so far, but not 100% convinced that I would trust the mics to record an important concert. I'm going to have to try and experiment more, so I know how to use this equipment in any location.
By the way, nobody noticed the mics, including the bouncers who regularly walked past me. The trouble I did have was that to keep the wires hidden, they had to be tightly tucked away, which restricted my head movements. At times, I probably moved like Frankenstein!
The Sony R30 is good for recording live music; it has its advantages and disadvantages. Firstly it is so small, slightly bigger than a compact cassette box, but that makes all the buttons very small and hard to use in the dark. Secondly, with its maximum recording length of 74 minutes, it is a nuisance changing MDs. However, remembering how the computer 3.5" disk moved from storing 760K (from memory) to 1.4m, I'm sure extended recording times with the MD setup won't be too far away.
This is Steve Smith's diagram and comments:
I drafted a layout on a post-it would you believe! I used a strip of vero-board about 1.25 inches long and five strips wide. I didn't connect anything to the "top" strip; I was going to earth it as a kind of shield, but didn't bother in the end. I'll try an ASCII version of the circuit here:
( wire to negative battery terminal)
|
|
GND o-----o-------------------o GND (Strip 1)
IN OUT
2.2 uF
Left o-----o-----||------------o Left (Strip 2)
IN | OUT
\
/ 10K
\
|
+9V o-----o-------------------o (Strip 3)
(positive |
battery \
terminal / 10K
\
| 2.2 uF
Right o-----o-----||------------o Right (Strip 4)
IN OUT
The horizontal lines show the veroboard copper strips. The vertical lines show connections across the strips, or components. The "o"s show some of the soldered connections on the board. I can't easily show the holes in the veroboard strips.
The capacitors are shown as "||" and you will need to space the leads apart three holes and cut the strip in between (with either the correct tool, or a suitable small drill tip, carefully held in the hand). I used two 2.2 microfarad Tantalum Bead capacitor, one for each channel. They are only accurate to 20% but this is all a bit of a lash-up anyway.
The two 10k ohm resistors were the metal-film type (1% accuracy). They are shown in the schematic as vertical zigzag lines. The caps and the resistors were all mounted vertically to save space across the board.
The IN connections go to a stereo jack socket. The OUT connections go to a lead connected to a stereo jack plug.
You don't need to make the battery box initially. You can build and test the mikes by connecting them to the stereo jack plug and plugging them straight into the R30 mic-in. It provides the power that the battery box would supply (except only a few volts for the R30 I believe). MAKE SURE: you only connect the left out, right out and ground connections to your output cable and that you have no shorts across any of the strips or across components. I put the components and leads on the blank side of the veroboard. The diagram above is symmetrical across the strips, so it doesn't matter which side of the veroboard you lay in out. I do STRONGLY recommend that you sketch out your veroboard layout first. Make it to at least twice the final scale.
I stuck the veroboard onto the side of the battery holder with one of those "sticky fixers" that you can use to stick pictures to walls/cupboards etc., and put the lot into a Nuttalls mints box (about 3.5"x.75"x2").