LM3900 Audio Mixer

This audio mixer circuit doesn’t use a low impedance input to mix no ideal sources, but use many amplifiers to provide ideal sources before mixed through simple resistors. Ideal source means a sources with low impedances, make the interaction between signals in the output doesn’t affect the input. Please note that this mixer circuit has high impedance output, so you’ll need an next processing equipment with appropriate input specs. This audio mixer circuit is designed around an LM3900 quad op amp and combines 2 line and 2 mike inputs and sums them at the output terminal. To vary the gain (around +23 dB), we can change the R7 through R10.

Line Driver

Description:

A stereo line driver for feeding long cables or buffering an audio source.




Download this schematic diagram.




Download semiconductor parts datasheet. (PDF Format)

1. BC109C

Notes

This preamplifier has a low output impedance, and is designed to drive long cables, allowing you to listen to a remote music source without having to buy expensive screened cables. The very low output impedance of around 16 ohms at 1KHz, makes it possible to use ordinary bell wire,loudspeaker or alarm cable for connection. The preamplifier must be placed near the remote music source, for example a CD player. The cable is then run to a remote location where you want to listen. The output of this preamp has a gain of slightly less than one, so an external amplifier must be used to drive loudspeakers.

Buffer Amplifier

Description:

A hifi preamplifier designed to convert high output impedance amplifiers to 600 ohm outputs.










Download this schematic diagram.










Download this image.

List datasheet semiconductor parts. (PDF Format)

1. BC547C
2. BC557C
3. 2SA1358
4. 2SC3421

Low Impedance Microphone Amplifier

You can download the schematic, pcb layout and component list from here





Download this schematic diagram.

This circuit is a microphone amplifier for use with low impedance (~200 ohm) microphones.
It will work with stabilized voltages between 6-30VDC. If you don’t build the impedance adapter part with T1, you get a mic-amp for higher impedance microphones. In this case, you should directly connect the signal to C7.

Component list:

R1=15k		C1= 3k9		U1= TL081
R2= 150k		C2= 100u		D1= 1N4148
R3= 2k2		C3= 22u		CN1= SIL6
R4= 820		C4= 4u7
R6= 10k		C5= 470u
R7= 10k		C6= 10u
P1= 1M		C7= 100n
		C8= 47u UNIPOLAR