How to Build Harmonica Microphones.

I have plans for two types of microphones, a ceramic cartridge microphone and a electret condenser microphone.   The cartridge microphone works well for that blues sound that has some distortion, the electret finger mike works well for a more acoustic sound.  I plug both into my amp through different inputs and use the one that fits the song better.   Both microphones sound great and are far less expensive than buying commercial microphones.  They also are much more satisfying because they are self made.  They both take at least a couple of hours to make but do require some time in the middle to allow glue to dry.

 

Dynamic Cartridge microphone:

 

The cartridge microphone is smaller and lighter than a Sure “Green Bullet”, so you may find it easier to grip and cup.  The cartridges I could find that were cheap enough to play around with (cheap crystal Kobitone) yielded a more treble sound, but still get that nice overblown sound when you cup tight and blow hard.  The details were taken from

http://www.planetharmonica.com/ph2/VE/TMI-micUK.htm, and I have added some of my own comments, they are in bold.

Michel Triste is a harp player from the Paris area, and a electronics fanatic. He developed this little beauty that is the i-Mic for us, with second hand stuff that anyone can find in their local supermarket plus a few components that can be found in any electronics shop. At the worst the parts should cost around $8. (Probably a bit more for a good cartridge, maybe $25 for something good on ebay)

The results are so impressive that today Greg Szlapczynski uses an i-Mic on tour ! And all the testers on our panel found the i-Mic to be one of the most interesting mics they tested ! So if you're worried about spending the $60-$100 that's the minimum entry price for a harmonica microphone, why not build one yourself first ? I also think the mike I made sounds good, and their sound test, while subjective, does support the quality of these microphones.

Plan of the i-Mic

1. Mono 6.35 Jack
2. Mineral Water Plastic Bottle (ex : Badoit)  It is easier to make with a slightly wider mouth plastic bottle
3. 10 kohm b (Log) Potentiometer (small)
4. Potentiometer button
5. Dynamique microphone capsule  I used a Kobitone Dynamic Cartridge from Mouser, 25LM032,  they are $4.50 each.  Crystals are more fragile, but at this price you can buy a few and break them at will.  Just make enough to have an extra at the gig!  The metal part on the black with the black spot is ground (goes to outside ring of the mono Jack trough the volume potentiometer.
6. Mic Head (Cap and part of a plastic bottle of milk with holes drilled in)  Gatorade bottles are more heavy duty, also I cut a round hole in the cap then glued in a brass screen.
7. Not shown : red/black isolated copper wire

(On the drawings Rouge or R means Red and Noir or N means Black)

i-mic2.gif (1913 octets)

A few bits of advice :

The place to cut the plastic mineral water bottle in order to obtain the length L is to be determined by the diameter of the milk bottle used for the mic head, which itself should be sufficiently wide to host the capsule. Also be sure that the part that holds the jack is wide enough to fit it in so the leads don’t short out when you cram it in.  I used Gorilla Glue to put the 2 halves together, 5 minute epoxy works too.  The Gorilla glue expands, but you can trim off the excess.  I also suggest using some sand paper to rough up the plastic edges before gluing, drilling any holes before you glue, and before and after you glue test that all parts fit in. You want to make it as small as possible, but don’t want the leads of the potentiometer to interfere with the jack or the back of the cartridge.

·        The capsule can be attached with 1 or 2 strips of 5 mm adhesive foam as used for isolation of doors and windows. Alternatively you might want to put the capsule further away from the head in which case you might need a plastic tube of a diameter corresponding roughly to that of the milk bottle. Another top of mineral water bottle with the cap on might do the trick.  The weatherstrip works great, and it is somewhat waterproof, so it protects the electronics behind the front of the microphone from condensation and spit!

·        Expect to spend at least several hours of work. The parts that are the easiest in theory (like drilling holes in the milk bottle cap) are the longest and most delicate.  That is why I ended up drilling a big hole and gluing a screen to the “inside” of the bottle cap.

·        You will need a drill, welding utensils and a lot of patience and inventiveness. If you're not the type to spend hours doing home improvement, you might hire a bandmember, family member or friend who is to help you out the first time.

·        If you make the neck of the mike narrow enough, it can be held between the ring and pinkie finger of the cupping hand.  It is way easier to get a seal on this type of mike than a bullet microphone.

·        By using a Badoit bottle and green capped bottle of milk you'll have the original i-mac look. But just like the new i-macs, you can vary the color codes by changing brands. Please send us photographs of your finished models.  The bass player claims mine looks like a bong.  You judge.

ELECTRET FINGER MICROPHONE

The finger mike allows for cupping the harmonica and getting the wah or flutter sound by moving your hand.  These microphones are also better for playing fast than the cartridge microphones, the sounds don’t blur together.  My hypothesis is that the magnet in the cartridge is harder to move than the very small condenser parts so the cartridge microphones are less responsive to very fast changes in sound.  No data to back it up, but to my ear you can hear runs better on these.  Also the electret microphone allows for higher volumes, so if you are having trouble cutting through in gig or practice situation, you can turn up the cleaner electret to higher volumes  before getting feedback. The microphone circuit has a high impedence, so works well with guitar amps. If you need a long line, use the line to the microphone from the battery box, not from the box to the amp or the PA board.

This bit is lifted heavily from

http://webpages.charter.net/tidmarsh/binmic/  So, you want to build a pair of binaural microphones?

 

These microphones are constructed for people who like to record live music and put a premium on sound quality, so they come out sounding pretty good.

The Parts for 1 microphone:

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 at 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. That is what I used for my harp mike. I used a variable resistor that doubles for some tone control and volume control.

 

Here is some discussion on bass rolloff,  I just used the 1uF caps

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).
 

I substituted a 10K pot and you can adjust the tone of the mike a bit, but it is a fine line between feedback and an improvement in tone. You may want to try a linear rather than audio taper here.

The Circuit

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)      >
     |             |           
     |             + 
     |             |
     |             _  +
     |            ___
     |             _   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 (which could be useful if you tend to overload the mike,  tube distortion sounds better so get volume from your amp if you can).   There are other improvements you could try, like adding buffer capacitors to the battery (I'm surprised how "good" a 9 volt alkaline 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  coments modified by Walter Dodds

For simplicity and minimal potential for connector failure, you can hard wire the entire setup, but then the mic will draw power whenever the battery is connected. I used a 1/8" stereo minijack to connect the microphones. When they're unplugged, the circuit is broken and there is no battery drain.

You may want to add a small 10k ohm log or linear pot to microphone for volume control.

Soldering the Capsules:

WM-61 capsules

Since the mic capsules are so small, soldering is difficult.  Be sure to use a low wattage solder iron with a fine tip.  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. The best way to do this is to put masking tape around the outside of the capsule so the capsule can sit sound-sensitive side down.  Then you can put epoxy into the cylinder of tape and fill the back of the capsule to the wires.

The Housing.

The best housing is very small and connects the capsule securely to the microphone cable.  Electrical tape works fine for this.  Alternatively the right diameter heat shrink will work but be careful not to damage the capsule with excessive heat.

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.  Once the microphone is connected to the cable, and the circuit is made, plug it into an amp to see if it is working before making the ring.  The ring is the biggest work.  Find a friend who likes to do wood work if you don’t know how or don’t like to work with wood.

 

Making the Ring to Hold the Microphone

Take a block of wood (use a nice hardwood) about ½” thick and 2x2”.  First drill a hole that your 4th finger will fit comfortably into.  Drill a hole through the 4th side that goes just out from tangent to the ring that is large enough for the microphone cable to slide through.   Then use a saw to remove the excess around 3 sides of the ring to get it as thin as you can without breaking it.  Use wood-working tools to carve down the ring and make it rounded and comfortable.  Sand well and stain.  Use a cable tie to attach the microphone.

If you use a volume control, make a space in the ring to hold the volume control.  The next one I make will have this.

 

Using the microphones

Plug into altoids box, plug box to amp. Put it on your 4th finger of your wah hand with the microphone pointing in.

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).