Dynamic microphone to electret microphone input

Design and copyright Tomi Engdahl 1999

• Summary of circuit features: Circuit which works as an preamplifier which allows you to replace an electret microphone with a dynamic microphone.
• Circuit protection: No special protection circuits used
• Circuit complexity: Very simple one transistor circuit
• Circuit performance: Worked well, although distortion and noise performance is not very good (but usable for many non-hifi applications)
• Availability of components: Uses common and easily available components
• Design testing: I have built this circuit and it worked well with Sound Blaster 16 sound card
• Power supply: 3-9V current limited power from electret microphone input (same as normally supplied to two wire electret capsule)
• Estimated component cost: Electronics components cost less than $10 including connectors
• Safety considerations: No special electrical safety considerations.

Circuit description

This is a simple microphone preamplifier circuit which you can use between your dynamic microphone and any equipment designed to work with an electret microhone (2 wire connection to electret capsule). This amplifier amplifies the low level signal to the levels used by electret microphone input and uses the power from the device.

The circuit is a simple one transistor amplifier to convert the sub-millivolt level voltage from electret microphone to current changes as generated by electret capsule.

Component list

R1   470 kohm
C1   220 nF
Q1   BC547

Example of using the circuit

Connecting the circuit to 3.5 mm mono electret microphone input

The circuit below is a typical wiring used with electret micrphones connected to 3.5 mm mono plig unput (as used in vidoe cameras and some portable audio recoders). Toy cna use the wiring below just by replacing the electret capsule on the left with my dynamic microphone amplifier circuit described above.

     +------+                                                / \
     |      |-------------------------------- audio --------|   |
     |      |mic+                                            \ /
     | mike |                                               +===+
     |      |                                               |   | 3.5mm mono plug
     |      |                                               |   |             
     |      |mic-                                           |   |
     |      |---------------- ground -----------------------|   |
     +------+                                               |   |
                                                          +=======+
                                                          |       |

Connecting the circuit to soundcard

The circuit below is a wiring to connect a two wire electret capsule to Sound Blaster 16 soundcard (works with many other soundcards which use electret capsule also). The same wiring works well with my dynamic microphone circuit when you replace the electret capsule with my circuit.

     +------+                                                / \
     |      |---------------+---------------- audio --------|   |
     |      |mic+           |                                \ /
     | mike |               |                               +===+
     |      |               +-------------- bias +5V -------|   | 3.5mm plug
     |      |                                               |===| to soundcard
     |      |mic-                                           |   |
     |      |---------------- ground -----------------------|   |
     +------+                                               |   |
                                                          +=======+
                                                          |       |

Basic electret circuit using external power

The circuit below is a typical electret microphone powering circuit. This works well with my adapter circuit when you put my circuit on the place fo the electret capsule in the circuit below.

       vcc
        O
        |
        /
        \
        /
        \  EQUAL TO REQUIRED IMPEANCE
        /  EX.= 1000 Ohm
        \
        |
        |     CAPACITOR MOST ANY VALUE 10uF 16v
|---|   |  + | /
|   |---0----| |-------   AUDIO OUT
|   |        | \
|   |----O--------------
|---|    |
         |
       -----
        ---