The circuit described here is a microphone amplifier, adapted for use with the modified WM-60A, WM-61A Panasonic omnidirectional electret capsules.
These microphones are "a low cost - high quality" solution. They have a frequency response that is almost flat from 20 to 20000Hz.
The left schematic is the microphone's internal circuit. The terminals are as noted in the photo.
The diaphragm capacity appears in the schematic as C (~10pF). The R emulates the leakages of the diaphragm spacer.
The value of R is in the GOhm range , but is enough to bias the jfet's gate.
The jfet used in WM-61A is probably the
2SK3426
As seen in the schematic, the jfet is connected as a common source amplifier with terminals 2 , 3 sorted. The terminal 1 (jfet's drain) supplied from +2V voltage source through a 2.2Kohm resistor. The values (2V and 2.2Kohm) are proposed from the manufacturer of capsule.
This topology produces distortion at high sound levels. There are two reasons for this. The common source topology is much more nonlinear than the proposed, later, source follower topology, and the +2V bias supply limits the headroom.
The modification that described here is to rearrange the terminal connections, so that the jfet of the capsule can rewired as source follower. This can be done by cutting the track that connects terminal 2 to ground and short the terminals 1 , 3.
The new topology supplied by 9Vdc through a 9.1 Kohm resistor. The supply voltage increase, from 2V to 9V also increases the headroom. The coupling capacitor has a value from 1 to 10 uF.
The resulting schematic after modification.Capsule terminals 1 and 3 needs to be connected.