Integrated Circuit Based Mixers
The Simple I.C. Line Mixer
Using the LM387 Op Amp I.C.
Integrated Circuits (ICs) were developed to provide "more Circuits"
per square centimeter. The development of the Integrated Circuit was the turning point in "Electronics Revolution" of the
20th Century. Not only did it lead to true "Micro Circuits" it was the keystone event that led to the development of the
"Micro Processor" and made the development of the "Personal Computer" Possible.
The development of "Integrated Packaged Electronics" led up to the OP-Amp "Operational Amplifiers"- whole circuits on a single "Chip Device"
called a "DIP Circuit". With the use of OP-Amps a 4-Channel mixer could be designed and constructed that occupies little more than
a few square inches of Printed Circuit Board space. This meant that the mixer could have "Special Effects", "Frequency
Equalizer's", "Automatic Gain Control", or even other wild and exotic "custom circuitry". The sky was the limit!
Some Semiconductor Manufacturers actually "make" Mixer IC's. Some are called "Analog Mixers", "Analog Switches", and/or
"Digital Switches". The only problem you will encounter is to sift through the spec sheets and see which are mixers and
which are actually switches.
A true "Analog Mixer" is in reality a "Signal Summing Device". This means that it sums up the signals presented at the
multiple input points and produces a signal at the output that is a sum of the total signals presented. I guess you
could call this a signal adder. The difference between simply adding up the signals and summing them up may seem a mute
point - but consider this. When you add the signals, you are not just combining them, you are actually adding them,
like adding the audio frequencies to give additional frequenciews, and adding the levels to give even higher signal
levels (such as adding a - 3 dbm signal with a +1 dbm, and a -10 dbm to give a total level of +-6 dbm). Instead we
are combining the signals which would result in a +1 dbm signal output (because the output will be the output of the
highest input signal (after loss and amplification - called the "gain" of the mixer circuit). In effect, the signals
are not interacting to produce ratios of frequency and levels. We are not averaging, we are combining.
When a "linear" Mixer Circuit is properly designed and constructed, you have a quality output. A cheap and poorly designed audio combiner
circuit can be non-linear and can produce poor quality results, as well as a noisy and distorted output signal.
Remember - the ultimate goal of any "Mixer" is to produce a faithful
signal output that is "clean" and distortion free. The proper design requirements include: Linearity, Frequency Response,
Low Noise and Low Distortion Values.
This mixer is a fixed-gain summing amplifier based on an LM837 Integrated Circuit. It sums up
all the six microphone preamp signals into a single output. This is called the MIX OUT, and
normally appears on the rear panel as a MIX OUT socket. The single output is split into two
outputs, to allow easy connection to external stereo equipment. The idea behind this is to
allow connection of an external effects unit or graphic equalizer between the mixed output
and the power amplifier.