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MX10 CIRCUIT DESCRIPTION
We will use the schematic diagram to step through the circuit and find out what
makes it "tick". There are plenty of different devices performing different tasks
in the MX10. The most common active component is the opamps. These are
used in a number of ways in the MX10, one of which is mixing.
To perform mixing, an opamp is set up in what is called a summing amplifier.
This amplifier does exactly what it says. It takes all of its inputs and adds them
together on the output. Following is an example of a summing amplifier in a
mixer, similar to the ones in the MX10.
There are three inputs to this amplifier. The 10K ohm resistors determine how
much gain each of the inputs has, and the 33K resistor sets the overall gain. To
find the gain of each branch, there is a simple equation:
R
=
A
bracnch
R
R9, R22, R14, and R58 determine how much line level signal is actually sent to
each branch of the summing amplifier by using slider control potentiometers.
So for the circuit up to this point to have a gain of one, the control will be set at
1/3 of its full scale setting.
The microphone amplifier is slightly different from what you may have
expected. Not only does it have gain, but it also has two diodes in the feedback
of one of the amplifiers. First we will talk about the gain of the circuit. A typical
microphone when talked into will have an output of about 50mV, while line level
audio has a level about 1V. To get the microphone signal up to line level, we
will need an amplifier with a gain of around 20. In our case we have chosen an
non-inverting amplifier for the job. A non-inverting amplifier has the nice feature
of a very high input impedance. This prevents loading on some high impedance
microphones which causes poor sensitivity. To find the gain of a non-inverting
amplifier, the equation is as follows:
=
+
A
1
mic
RF = 33K and Rin is any one of the three branch resistors.
f
In this case the gain is 33K/10K = 3.3. To find the output
level with a given input level such as 1V P/P, multiply the
in
input voltage by the gain so 1*3.3 = 3.3V P/P out.
R
In our case we chose R23 at 220K ohms, so for a gain
23
of 20, R21 needed to be about 1/20th of that value or
R
about 10K. (Notice the values around U5:A)
21
MX10 Page 5
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