Description

Most cards of sound in computer, are deprived stereo input for microphone, on the contrary have stereo input for high level [ Line ]. The circuit uses the input Line of card of sound, in order to we place a two mono stereo microphone, in the input of card of sound. Simultaneously he is more functionnal, after we have the possibility of regulating the gain of stage of microphone and regulating the final level of sound externally and independent for each channel of microphone. This solution was selected in order to we can if we want mixing two mono microphones. Also exist the classic supply for Electret microphones. Simultaneously exist stereo input Line, with two choices plugs of input and regulation level of input with the RV4, making him mix of sounds from the microphones and the input Line, easy affair. The input A, use two plugs RCA [ JF3-4 ] and input B, use a classic stereo plug jack 3.5 mm of nail. The IC2A-B, make the addition of signals and they drive the stereo output, that becomes from the JF6 jack. In JF7, we can give the supply, bigger than + 9Vdc, from a simple external power pack. This voltage is stabilised in + 5V, from the IC3. This low voltage was selected in order, to does not exist danger overdrive of card of sound. If the level, is not in the level that we want, we can change the prices of resistors R14-19 and R21 until R24, with other smaller or bigger, so that we change also the percentage of gain of proportional stage.

Circuit diagramParts list
• R1-8-31-32=3.3Kohms
• R2-7-27-30=100ohms
• R3-4-9-10=1Mohms
• R5-11-21-22-23-24=10Kohms
• R6-12-15-17=1Kohms
• R13-14-19-20-28-29=47Kohms
• R18-16-25-26=22Kohms
• RV1=2X100Kohms Lin.
• RV2-3=47Kohms log.
• RV4=2X47Kohms log.
• C1-4-7-9-12-17=10uF 16V
• C2-8=470nF 63V MKT
• C3-24-27-28=100nF 63V MKT
• C5-10=10pF ceramic
• C6-11=1uF 16V
• C13-16-20-21=47pF
• C14-15-18-19=1uF 63V MKT
• C22-23-25=100uF 16V
• C26=4.7uF 16V
• C29=470uF 25V
• IC1=TLC274
• IC2=TLC272
• IC3=7805
• D1=1N4007
• D2=LED 3mm
• JF1-2-5-6=female jack 3.5mm stereo
• JF3-4=RCA female plug
• JF7=female jack 3.5mm mono

All the Resistors is 1/4W 1% metal film