Five stages are employed within the layout; DJ mixer stage; Mono headphone amplifier stage; Balanced-microphone preamplifier stage; Stereo VU circuit stage, and General purpose preamplifier stage.
A basic ceramic cartridge preamp is displayed that looks so straightforward that it could be constructed on the input sockets itself!
When using the stages outlined above practically any audio options could be mixed or blended by the user to get a stereo transmission specifically suited for driving power amplifiers straight away.
The mixed signals may also obviously be applied to feed headphones etc. The inputs from CD players, microphones, Ipods, cellphones etc should be appropriately matched up to the inputs of the mixer board. To get this done the proper preamplifiers needs to be determined and built.
Even so, the audio mixing range could be practically endless. Before you start building consider which preamplifiers you might need, consider which kind of sockets you would like to work with, and the number of channels you would like (although demonstrated as 4 channel, the mixer could be extended with the addition of further control pots and mixer resistors).
BALANCED MICROPHONE PREAMPLIFIER
The best thing about this balanced microphone circuit is that it gets rid of a pricey line transformer.
Even though intended for 600 ohm input and 40dB gain, various other impedances and gains could be dealt with by using R1 = R4 input impedance divided by two R5 = R11 voltage gain multiplied by the value of R3. The very first equation functions for impedances of approximately about 5k.
Over this figure R2 + R3 need to be incorporated in the computation. Even though we all posses just one mouth, the output from this circuit makes it possible to griddle the output through stereo by utilizing a couple of 10k resistors or a 20k linear pot using the wiper attached to the output enables you to pan the output via left to right.
In case a balanced MIC is applied R2 values will be as follows microphone R2 = 4k7 (limiting R2 47k) if employed with balanced preamp as input to restrict R2 = 15k
MIXER AND POWER SUPPLY
Due to the excellent feature of excessive ripple rejection ablity by the incorporated integrated circuits across different segments, the power supply specifications actually tend to be quite straightforward. A simple bridge rectifier, large smoothing capacitors having a RF bypass capacitor and you own a good power reference.
Courtesy: ETI Circuits
The inputs through cellphones, SD cards, USB microphones etc has to be ampliﬁed or possibly equalized with a pre-amplifier prior to any kind of controls positioned to process them.
The output of each of such preamps is variable using a volume control or fader, previous to being added to lC1. The total gain of the mixer stage could be modified through RV1.
If various preamps possess largely varying output voltages the value of Rl-R4 could be improved in order to match them.
The output of lCl is connected subsequently to the tone control stage. lC2 typically features a unity gain when the pots are moved at the center of the dial.
However, this gain is actually variable, with regard to frequency, when the tone controls are not around the center the output of the tone control stage specifically toggles the main power ampliﬁers.
This output is additionally rectified by Dl to run the meter circuitry. The mixer provides stereo outputs which is accomplished by replicating the circuitry for the second channel.
The exemption may be the tone controls that are dual gang potentiometers.
Remember that the volume controls are separate units.
The power supply is actually a full wave rectified supply using a centre tap offering about 1 VDC
How it Works
The resistors linking Left and Right channel outputs are positioned to get a composite mono signal, without severely deteriorating the main mixer stereo separation.
The signal is actually picked by SW2-SW5 and raised on to a buffer having adjustable gain (IC3). The output can now be given to a LM380 power ampliﬁer that runs the monitor headsets. Just like the mixer the input resistors may be made higher, to minimize high signals towards the other channels.
The mixer is actually a standard summing amplifier using adjustable feedback (ie gain), accompanied by a Baxandall tone-control system.
In case input ranges aren't of the identical value, the 27k input resistors could be modified to decrease the highest signal by increasing resistor value. Avoid lowering under 27k because this may decrease overall level of sensitivity of the mixer.
The remaining details of this mixer project can be learned in the NEXT concluding PART 2 which includes the universal preamplifier. headphone amplifier and the VU meter circuits stages.
Courtesy: ETI Circuits