Microphones for accordion

This page describes how to make microphones for accordion, which can be done by any handyman with a minimum knoledge in electronic circuits.
These microphones are based on the back electret technology and can be powered either by a phantom power supply from the mixer or from a 9 V battery.
Reminder: the phantom power supply from the sound mixing console is balanced and floating, which limits the parasite noises even when using long cables. The system as proposed here is balanced, but can be powered from a 9 V battery and then become unbalanced.
The electronic diagram should be done as shown here, but the technical building which is explained below in this document only represents the solution I designed and everyone's imagination can improve it.
The total cost for two microphones should not exceed 75 €, which is far less than the commercial equipment with similar charasterictics.
Of course, this type of microphone and associated electronic can be adapted to other instruments by modifying the physical support.

Charasteristics

• Subminiature back electret type microphone
• Type : omnidirectionnal
• Frequency bandwidth : 20 Hz to 20 KHz +/- 2 dB
• Sensitivity : 6 mV/Pa (-43 dB)
• Power supply : External phantom (balanced) or internal 9V battery (unbalanced)

Components for two microphones

• Microphone : Sub-miniature capsule Panasonic MCE-2000 x 2.
• Resistors 0.6 W 1% : 47 ohm x 4; 1,0 Kohm x 4; 1,5 Kohm x 4; 3,3 Kohm x 2; 6,2 Kohm x 2, 10 Kohm x 2.
• Condensators 50 V : 33 microF x 6; 10 microF x 2.
• Zener diodes 1 W : 10 V x 2.
• Operational amplifiers DIP8 : OPA 2134 (double) x 2; Support CI x 2.
• Miscellaneous : double inverter slide switch x 1; Box 120mm x 80 mm x 35 mm; 9 V battery connector; 9 V battery support; prototyping circuit board 65 mm x 45 mm.
• male XLR sockets x 2; 2.5 mm female stereo jack socket x 2; male stereo jack 2,5 mm x 2; Cable 2 wires + shield 1 m x 2.
• Accessories : belt clip for the junction box x 1; succion cups for fixing on smooth surface x 2; 4 mm2 copper wire 50 cm x 2; spiral wrap 3.7 mm x 50 cm x 2; miscellaneous screws and elements for fixing the capsule on the copper wire.

Diagram (double for two microphones)

Manufacturing with photos

The simple mock-up for checking the concept and functions, has been very usefull, before going further.
Prototyping circuit boards with punched holes are very convenient for such small circuits. Notice the two idendical setup for each microphone. The 9 V battery support is on the left, the two microphone inputs (2.5 mm jack) are on the rught, the two XLR outputs are at the bottom and the 9 V / phantom supply switch is between the XLR.
The electronic box is now finished. the two microphone inputs are on the right (2.5 mm jacks) and the two XLR outputs which are connected to the mixing console by means of standard XLR x XLR cables. The belt clip is there for attaching the box to the musician's belt for avoiding the cable weight, and allowing easy displacements without pulling directly on the microphones.
All the elements used for the microphone supports are there: a suction cup with a hook (bathroom or kitchen use), a 2.5 mm2 copper wire, a crimp ring terminal, a copper rivet having the same diameter than the suction cup hook, the spiral wrap (not shown) ant that's it.
The copper wire is soldered on the rivet head (a small hole having the same diameter than the wire (2.5 mm2 is drilled on the rivet for a better centralising and robustness). The terminal is crimped at the other end of the wire. A piece of shrinkable tube is placed on the rivet for tightly adjusting the diameter to the suction cap hole. You can notice the little parts and screws for fixing the rubber microphone holder on the terminal.
Assembly of the 1 m cable (2 wires and shield) on the 2.5 mm2 jack and microphone capsule. The shield is not connected on the microphone side for avoiding parasite noises. The capsule is soldered before being inserted into its rubber holder.
Here is the detail for the mounting of the microphone capsule holder, as I made it, but it can be adapted according to one's possibility. This rubber holder is very efficient for preventing transmission of shocks and vibrations from the microphone support to the capsule.
Everythinh is now assembled and the cable is maintained along the copper wire by means of the spiral wrap.
The complete assembly (microphone, support and cable) is now ready to be placed on the accordion.
The right microphone is in place: thez suction cup is place at the top of the melodic box, and the support is shaped for placing the microphone at the centre of the sound emission zone. It is also possible to stick the sucction cup onto the bottom face of the melodic box, but the weight of the system acts against the suction, as it acts in the same direction when placed at the top side.
The left microphone is placed in a similar way than the right one, but the shape of the wire is adjusted for avoiding contact with the musician's hand in he strap.

This type of assmbly using succion cups, could give random results depending on the surface smoothness of the body, therefore I have imagined and made a new system where the microphones are attached on the musician's wrists. This has the advantage of keeping constant the distance from the microphone to the sound source: at the right hand, it follows the the actuated valves and at the left hand, it follows the bellow displacement.
The microphones are mounted on an aluminium blade (10 mm wide and 2 mm thick) in order to limit the weight while ensuring a good rigidity of the assembly. A 8 mm hole is drilled at one end for inserting the microphone sensing element. the elastomer holder is fixed using two small screws. Each blade is sewed at its second end on an elastic bracelet (8 cm wide), which will be placed around the musician wrist. The seam is made across two holes drilled in the blade, which is also maintained by means of a special covering sheath. This bracelet should be at the same time comfortable and firm, for ensuring a good position of the blade and microphone during the musician is playing.
Lengths and shapes should be adapted to the instrument, and I only show here the principle I have made for a Castagnari Handry.
Here is the right hand microphone in position on the musician's wrist. it follows the sound source by staying close to the actuated valves. The slight swing movements during the action does not generate parasite noises.
Here is the left hand microphone in position at a constant distance of the sound source in opposition to a microphone on a stand.

Conclusion

The first tests have been successfull and the capsule quality, which are yielding a linear response versus frequency, contributes a lot. The circuit does not modify this repsonse and add any parasite noise, for a very reasonable cost. All this makes possible the use of a detachable microphone system on a diatonic accordion, which respects the instrument integrity and the amateur's wallet.
Of course a fine tuning of the microphone position in its rubber holder and relatively to the sound source will be necessary, but it will always be better than a microphone on a stand.
The suction cups system could also be a problem, but if the accordion surface is clean and smooth, a little bit of salive or electrocardiogram gel will improve sticking. There are some more powerfull suction cups with a lever, but I could not find some sold alone. I found the suction cup system more clever than the scratch, which is bonded on our best musical button box friend.
The microphones on the musician's wrists gave very good results during a bal where they have been compared to microphones on stand and commercial instrument microphones (AKG).
In addition, this system allows the musician to change from an instrument to another without disconnecting the microphones, which remain in place on his wrists.
I just wanted to share my experience and give amateurs a way of adding some microphones to their instruments with a minimum cost, and I hope it will help.