Making a Condenser Microphone

This is an instructable on how I made a condenser microphone. The microphone is largely based on a measuring microphone designed by Linkwitz (find the like to his website here). In his design the particular electret condenser capsule used (Panasonic MW-60AY) is very difficult to acquire. I'll be trying to make do with a generic Chinese capsule instead and then test the resultant microphone to see how well the substitute works.

To cut a long story short, I found that Linkwitz uses the particular Panasonic capsule for good reasons. The generic capsule made for an all right microphone for speech but a very poor measuring microphone. None the less it was a very inexpensive and fairly easy project that turned out a microphone that works quite well for recording speech.

Materials

1. Electret capsule
2. Op-amp - I used a NE5532
3. Two 9v batteries and connectors
4. 3.5mm headphone jack
5. Passive electronic components

Tools

1. 3d printer
2. Soldering iron

The first step was to take apart one of the capsules in order to have an idea on how this particular capsule works. Opening it up revealed a capacitor consisting of a movable plate and a fixed plate. The movable plate or diaphragm was attached to the aluminium casing. The fixed plate was attached to the gate of a JFET used as a common-source amplifier along with an externally attached source resistor. The FET amplifier provides an amplitude gain of three with the recommended source resistor. This condenser capsule is exactly the same in design as the Panasonic capsule Linkwitz used and is the reason for basing this microphone on his plans.

The movable plate of the capacitor is connected to the source of the JFET through a pcb trace. This trace is cut and replaced by a source resistor in order to turn the source amplifier in to a source follower. This is done as a source follower offers better linearity than a source amplifier and hence reduces distortion.

In this step a preamplifier will be made in order to compensate for the gain lost in the previous step. Switching from a source amplifier to a source follower meant loosing a voltage gain of three. This will be compensated for with a Op-amp based non-inverting amplifier set to a gain of three.

The circuit was made on a 10x24 point Perfboard. The Op-amp used was a Texas instruments NE5532 audio Op-amp but almost any would do. The Perfboard layout for the preamp along with a bill of materials is present in a pdf attached at the bottom of this step.

The enclosure was 3d printed from PLA. The stl files of the models used have been attached at the bottom. The assemble was pretty straightforward and everything was held together with some E6000 glue. One of the models attached is for placing the microphone on a camera tripod with a 1/4" threaded mounting bolt.

The calibration curve for the microphone was obtained but comparing it to a Behringer EMC-8000 measuring microphone. A frequency response measurement was made of a loudspeaker by placing a microphone one meter in front of a loudspeaker. Seven individual measurements were made while moving the microphone 10cm after each measurement and averaged. This was done with both microphones and then responses were divided in order to get the calibration curve of the microphone.

The calibration curve obtained shows that the microphone had a bandwidth of 5.8KHz. This is enough for recording speech but not for musical instruments or for use as a measurement microphone.