MX-5 Page 5
MX-5 CIRCUIT DESCRIPTION
We will use the schematic diagram to step through the circuit and find out what
makes it “tick”. As you can see, there is only one IC in the entire circuit, but
there are four individual operational amplifiers in the one IC. U1:A and U1:B are
the main components of the mixer. They are set up in what is called a summing
amplifier.
If we were to look at the left channel only,
we can see what one of the summing
amplifiers looks like. The three inputs to this
amplifier are seen as R1, R9, and R10.
These resistors determine how much gain
each of the inputs has, in this case they are
all equal. To find the gain of each branch,
there is a simple equation:
RF = R4 and Rin is any one of the three branch resistors. 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 input
voltage by the gain so 1*3.3 = 3.3V P/P out.
R3 and R13 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 100mV, 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 10. 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:
In our case we chose R23 at 220K ohms, so for a gain
of 10, R21 needed to be about 1/10th of that value or
24.4K. Since that is not a standard value we used a
22K ohm resistor which is close enough. (A gain of 11)
The next stage of the microphone amplifier has two diodes in the feedback
loop. What are these for? you may ask. Well they are called clipping diodes.
Diodes have a property of needing about .7 volts across them before they turn
on. On signals under .7V P/P, the gain of the second stage is determined by Ri
of 10K (R18) and Rf of 10K (R16). This gives us a gain of 1. But if our signal
in
f
branch R
R
A=
21
23
1R
R
Amic +=
