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Dual-Mode Microphone Pre-Amplifier 2011-12-17

This is a microphone pre-amp which I designed for use with a SoundBlaster(TM) AWE 64 Value sound card; but it should be suitable for any compatible card or PC audio input which provides a 5 Volt supply (via a 2k2, or thereabouts, built-in current-limiting resistor) at the ring contact of a stereo jack plug inserted into the "Mic In" socket of the card/PC. (N.b. it has only been tested with the AWE 64 card.)

The circuit has the following features :-
(1) It will automatically detect whether a mike plugged into it is of the electret (condenser) or dynamic type and will switch between respective "low-gain" and "high-gain" modes accordingly. The auto-detection relies on use of a stereo plug fitted to an electret mike and use of a mono plug fitted to a dynamic mike.
(2) It is powered only by the 5 Volt available at the "Mic In" socket.
(3) It is small enough to be housed in a standard plastic 35mm film canister (remember those?).
(4) A trimmer allows the relative gain for the two microphone types to be adjusted.
(5) The gain is sufficient to give about 60mV peak-to-peak at the "Mic In" socket, required for full recording level in WaveStudio.
(6) In conjunction with the card's built-in input amplifier, the circuit provides a flat frequency response over the audio range, with -3dB points at < 10 Hz and 19 kHz.
(7) Input impedance > 10k. Output impedance < 1k
Component Values
R1 = R7 = 4k7; R2 = R3 = 47k; R4 = 39k; R5 = R6 = 100k; R8 = 220; R9 = 10k; R10 = 2k2 (all 1/8 Watt).
VR1 = 10k horizontal pre-set.
C1 = 47uF; C2 = C3 = C4 = 10uF (all 10V or greater, miniature radial electrolytics).
Tr1 = Tr2 = Tr3 = Tr4 = Tr5 = BC108, BC184, BC547, ZTX337 or similar low-power (preferably low noise) NPN transistor with e-b-c pinout to suit the printed circuit layout below.
Construction notes
Assuming the printed circuit layout above is used, begin by inserting the wire link for the C1 ground connection, then VR1, TR1-Tr5, C1-C4 (noting polarity), and R1-R10. Use thin twin screened cable for connections to the PC and mikes. Use insulated hook-up wire (or "garden wire") threaded through the holes provided in the board, around the cable and twisted tight to make strain-reliefs for the cables. At the mike cable end attach a stereo socket. At the PC cable end attach a stereo plug. Ring, tip and ground contacts of the plug and socket are connected as shown in the circuit diagram. Take care to avoid any short circuits within the plug/socket.

Circuit operation

When an electret mike is plugged into the circuit, the current drawn via R1 and the stereo plug ring contact will result in point 'A' being at about 1.6V. This will turn Tr1 on and hence turn Tr2 off. The voltage gain of Tr4 will be determined partly by R4 and by R8 in series with VR1 and will be about 2.5. For a dynamic mike however, the ground contact of the mono plug shorts point 'A' to ground, so Tr1 is turned off and Tr2 is turned on by base current via R2. Tr2 thus shunts VR1, so Tr4 voltage gain will be determined partly by R4 and R8 and will be about 40. Tr3 is a stage with a voltage gain of around 1-5 (depending on the input impedance presented by Tr4, which in turn depends on whether Tr2 is on or off). Tr5 is a unity-gain emitter follower to give a low output impedance.


Total harmonic distortion is around 0.035 %.
The graph below shows the frequency response measured for the circuit in "high-gain" mode, fed from a sine wave source of 150 Ohms impedance and with the circuit driving 60mV peak-to-peak into the "Mic In" input of the AWE 64 card.
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