So, you want to build a pair of binaural microphones?

Small binaural microphone pairs are popular for unobtrusively recording musical and other events. A number of people sell ready-made sets, namely Core Sounds, Sonic Studios, and Marcsounds. All of these microphones are based on the Panasonic omnidirectional electret mic capsule, which sells for around $2 a capsule, and the other associated parts and electronics bring the cost of a set of home-brew binaurals to about $20, far less than the price of a set from the above manufacturers. Be aware, however, that all of these manufacturers match their capsules and make a number of modifications to the capsules. Is the cost of a microphone set worth the extra money compared to the home-made version? That's a matter for your ears, not mine.

The Parts:

2 Panasonic wm-60AT or WM-61 capsules
2 10k ohm 1% metal film resistors
2 1 microfarad metallized polypropylene capacitors
1 9 v battery holder
1 1/8" stereo mini jack
2 1/8" stereo mini plugs
An appropriate enclosure (I used an Altoids box. You could buy a metal project box, or use your imagination.)
Appropriate cabling (I cannibalized 2 patch cords: a 3 ft to go from the battery box to the deck, and a 6 ft to go from the mics to the battery box.)
9/32" brass tubing
mini alligator clips
epoxy

The Electrical Components:

The values for the resistors and capacitors can be varied. The size of the resistor controls the bias current. A larger resistor raises the point a which the capsule overloads, but it also increases odd-order harmonics, which can make the capsule sound unpleasant. 10k was suggested to me as a good compromise, and it seems to work well. Others have used 2.2k resistors as well.

The size of the capacitor affects the low frequency rolloff. The input impedance of the deck and the capacitor in series produce a low cut filter with the 3 dB down point set by

f=1/(2*pi*R*C) with f, r and c in Hz, ohms and farads.

For my D3, the input impedance is 10k ohms, so with a 1 microfarad capacitor, my 3 dB rolloff is

f=1/(2*3.14159*10,000*.000001)=15.9 hz

Using a different capacitor, you can adjust the bass rolloff to match your tape deck to give more or less rolloff (or you can roll off the frequency later with an equalizer on playback).

Capsule Matching

The Panasonic capsules are variable both in their frequency response and sensitivity (output level). You might be lucky as I was and happen to buy 2 capsules that sound pretty much the same. To improve your odds, you might want to order capsules from Mars Electronics (phone number below) or Sound Professionals for smaller quantities (link below) with a "-102" suffix, which are more closely matched (within 2dB) than the standard capsules available elsewhere. You should also match the capsules, though detailed instructions for properly matching them sonically using pink noise is far beyond my meager knowledge. For a rough job of matching, check the resistance across the terminals of the capsules and use 2 that have equal resistance. It won't give you a sonically perfect match, but it will be better than nothing and at least get the output levels close, if not the frequency response.

The Circuit

This circuit, and the appropriate cheesy ASCII diagram, are adapted from the Rastocny PZM modification section of the DAT-heads microphone FAQ. For simplicity, I have shown only one channel. Obviously, you'll need one for each capsule.

UPDATE:Mike Feldman contributes this clearer, non-cheesy ASCII-diagram and circuit description:

                           1 - 4 microfarad
                         polypropylene capacitor

                                  ||
     o--hot--------+--------------||------------------------hot---o
                   |              ||
     ^            <
     |             >
     |            <  2-10k ohm
  electret         > metal film
 mic capsule      <  resistor
 (shell side)      >
     |             |           
     |             + - - - - - duplicate above here for 2nd channel 
     |             |
     |             _  +
     |            ___
     |             _   9 volt 
     |            ___ battery
     |             _
     |            ___
     |             |  -
     v             |
                   |
     o--ground-----+------------------------------------shield----o

I've broadened the value ranges for the components. Panasonic says 2.2kohm for the bias resistor, but Tidmarsh says he likes a higher value for less distortion (and less cross talk between channels). Resistors should be matched for stereo pairs along with the capsules. Actually, you could vary one side's resistor to make up for capsule sensitivity differences. Yesterday I tried to hand rig a capsule matching setup and failed miserably. One the battery box is assembled, you could measure output from both capsules together, and add a high value trimmer resistor in parallel to the weaker side. The DC blocking (AC coupling) output capacitor value depends on the input impedence of the tape recorder or preamp input stage and the desired bass roll-off frequency (F3). Note that the Sony TDC-D8's mic input needs 10x more capacitance than it's line input for the same roll-off corner frequency. I'm thinking of building a switch or two into my next battery box so that I can adjust for boomy rooms and wind noise, but I have yet to find switches that don't stick out and won't accidently get changed when I pull the box out of my pocket; I had that happen with the D8's auto-gain switch! There are other improvements I could try, like adding buffer capacitors to the battery (I'm surprised how "good" a 9 volt alkyline battery sounds). A 100 uf electrolytic with a .1 uf poly bypass should do it. You could do a pair with separate resistors between the battery and the final bias resistor to decrease channel cross-talk. -- Mike Feldman

For simplicity and minimal potential for connector failure, you can hard wire the entire setup, but then the mics will draw power whenever the battery is connected. I used an 1/8" stereo minijack to connect the microphones. When they're unplugged, the circuit is broken and there is no battery drain. Additionally, the mics wired this way can be used with the plug-in power on the mic input of the Sony TCD-D7/8 or WM-D6, although they will have less dynamic range and distort at lower SPLs because of the lower bias voltage. Note that Radio Shack makes a cheap stereo microphone (cat. no. 33-1065). The circuit board inside follows this circuit, so if you want to be fancy about it, you can upgrade the yucky electrolytic cap and cheap resistor from their board and use it as the basis for your battery box, which is what I did since I had an old one lying around. Note that the new parts are not exactly the same size as the old ones. The resistors fit fine, but you'll have to bend the leads on the capacitors, which are much larger than the orginal Radio Shack components, to get them to fit the circuit board. Others have simply wired everything together.

Soldering the Capsules:

Since the mic capsules are so small, soldering is difficult. Tin the tips of your wires and hold them against the solder pads on the capsules. Then just touch the tip of your soldering iron melts them together. Be careful, or the solder will flow all over the back of the capsule and short out the connections. After you've soldered both contacts, use a bit of epoxy to secure the cable and seal the back of the capsule. Click here to see Mike Feldman's photos from a recent capsule soldering session: Mic Cap Photos

The Housing.

I used 9/32" brass tubing (from a hobby shop that sold model airplaines, etc.) and alligator clips to build a mic housing. I soldered an alligator clip to each end of the tube, and then used a hacksaw to cut each one approximately the same size. I then used a chainsaw file to smooth the edges and a flat file to even the tube lengths. I clamped them together by the alligator clips and then filed both ends smooth and even so that both mics would end up the exact same size. I don't know how crucial this is, but it seemed aesthetically important.

Assembling the Microphones

I used epoxy to mount the capsules in the housing, flush with the front. I also used more epoxy to fill in the back of the capsules. Doing so should improve the sound of the mics by giving the mic body more mass and thus more stability relative to the diaphragm. The epoxy also further accomplishes the goal of sealing the back of the capsules from the air.

The Plugs

On small recorders such as the Sony TCD-D3 and TCD-D7/8, the input jacks are a serious weak point. The 1/8" stereo mini jacks are soldered to the main circuit board and can break loose from strain. To minimize this, I made right-angle plugs. I couldn't find any pre-made right angle plugs, so I took regular heavy-duty 1/" stereo plugs and cut the contacts short. I then soldered the wires in at a right angle and used several coats of epoxy to seal and smooth the plugs, and I used short pieces of flexible heat-shrink tubing as strain relief for the cables. Another option is to use the regular heavy duty plugs (with the advantage that they can be disassembled and repaired if necessary) and get a right angle adapter from Radio Shack.

Using the microphones

Now that you've built your mics, test them out with your home stereo to get the hang of using them, and then record some live music with them.

Improvements:

Enlarging the hole so that the entire surface of the diaphragm is exposed is supposed to improve the sound of the Panasonic capsules by smoothing out some of the harshness in the high frequencies. I destroyed 10 capsules trying to figure out how to do this. Andrew Jones wrote with these instructions for opening small mic capsules non-destructively, and Mike Feldman reported these results. You might also want to check out Mike Feldman's homepage and his dummy head project.

Acknowledgements:

This project is based on a number of posts to the DAT-heads mailing list which described a similar project, as well as information from the DAT-heads microphone FAQ. I also received excellent advice from Vincent (who posted his information to DAT-heads) and David Josephson, proprietor of Josephson Microphones (if you need some excellent real microphones, check out his product line).

Comments

Whaddaya think? Drop me a line if you like. Let me know if these instructions were helpful (or not).

So, you want to build a pair of binaural microphones?

The Parts:

The Electrical Components:

Capsule Matching

The Circuit

Soldering the Capsules:

The Housing.

Assembling the Microphones

The Plugs

Using the microphones

Improvements:

Acknowledgements:

Links

Binaural Information and Recordings

Binaural Microphones

Binaural Microphone Construction

Microphones

New & Used Microphones and Equipment

DAT Resources

Parts Sources

Digicon Modification

Comments