5 Channel Mixer
This is the actual circuit that I'm using in my system. it works very well. the schematic is handwritten by me since I have no idea how to do it in the computer.
Mixer Schematic. just a note, the schematic is really big.
The circuit is pretty basic. so I guess I will only have a little explanation to do.
The input circuitry is made up of the 12 ohm, four 10kohm resistors, 10�E�F capacitors and the LF353 dual op amps. these form a semi-balanced input which rejects ground loop noise. the 10�E�F caps are there to eliminate DC offset to prevent scratchy operation of the pots when they get dirt and also eliminate the "pops" and "ticks" when the switches are pressed. the 20kohm slide pot and 4.7kohm resistors are wired so as to make a crude log pot. the next set of 10kohm and 47kohm resistors and OP amps are the summing amplifiers to combine the signals from the different sources. the main section has pre-out and main in jacks to allow connection to processor loops before sending the signal out.
The circuits in the lower right hand corner of the schematic are for the VU meter and an LED indicator as a visual indication of signal at the outputs.
The powersupply circuit that I used has the LM317T and LM337T variable voltage regulators. they are said to be more quiet than the 78XX and 79XX series of regulators, although the 78XX and 79XX series can also be used.
The gain was intentionally set high to allow the connection of low output level signal sources and still have enough output to drive the least sensitive amplifiers.
The circuit is also flexible enough that most available OP amps can be used for it. An option could be to use NE5532 OP amps for the output since they have very good driving capability - a requirement when using very long cable runs. The use of high quality Burr-Brown devices are also recommended but the suggested devices are sufficient for general use.
input volts: 850mV-8V RMS
output volts: 8V RMS
input impedance: 10kohms
output impedance: 1kohm
gain (max): 9.4 (voltage gain) or 19.4 db
Audio Designing With Opamps - What they are, what they do, and how they do it -Rod Elliott, ESP
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