This page is a work in progress. This page will be updated as time permits.
We love tinkering with hardware as well as software. We have maintained our microphones page for years, and hope you find it useful.
Click here on Microphones->LegacyPage, or in the menu above, to see the old microphones page.
This new page will focus on new information on DIY microphones.
New Designs
We will present here microphone designs that are both approachable for DIY individuals and high quality for better recordings. These new designs will most likely be based on large diaphragm capsules that are inexpensively available now (see below).
Diaphragm Bias
Unlike the electret condenser capsules used in our legacy projects, large diaphragm capsules require a bias voltage applied across the diaphragm. As sound strikes the diaphragm, the varying capacitance creates a voltage from the bias that is the output of the capsule. This is typically a 30V to 60V DC voltage. And the level of the output is higher in proportion to this voltage. However, there is an upper limit, depending on the capsule, so that the voltage cannot be increased to arbitrary values. High voltages can cause the diaphragm to collapse to the back plate. So, one factor in your mic design is what voltage to apply.
Note:
Electret capsules do not require a separate bias voltage because they are manufactured with a permanent voltage charge on the backplate.
Impedance Buffer
The next design challenge is the very high impedance of the condenser capsule design. The high impedance is accomdated with an impedance converter (see diagrams). The buffer usually consists of a JFET transistor due to their very high input impedance. The analog signal is then passed to an output stage.
Note:
Vacuum tubes can also be used as high impedance converters. But our designs here will focus on using JFET transistors.
JFET Source Follower
The JFET impedance converter design that is often chosen is the "common drain", also called "source follower" (see below). In the circuit, the value of R1 is generally as high as possible, 100meg to 1gig ohm. R1 effectiviely represents the input impedance of the ciruit, but still manages to let the JFET gate to "see" ground. Such a high value is necessary so that the capsule is not loaded down. The value of R2 sets the bias for the JFET and must be chosen for each individual JFET (even of the same type!).
Note:
There is a large variation in the internal values of JFET transistors which requires them to be viewed as unique devices requiring supporting component values to be selected for each JFET!