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MX10 Page 6
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 if the forward
bias direction before they turn on. On signals under .7V P/P, the gain of the
second stage is determined by Ri of 10K (R28) and Rf of 10K (R27). This gives
us a gain of 1. But if our signal becomes greater than .7V P/P, then the diodes
D1 and D2 begin to turn on. This brings R30 into the gain equation as well.
Now you have an Rf = R27 in parallel with R30. This brings the overall gain
down to less than .1, now the amplifier is working as an attenuator. The best
part of the diodes is that they don’t just “snap” on, they have some variance
before they are on fully, so this creates what is called “soft clipping”. This soft
clipping is a close relative of distortion, but much more tolerable. This soft
clipping circuit prevents a person from overloading amplifiers or speakers by
preventing high volume levels from exiting the mixer.
U6:A and U6:B are both RIAA equalization circuits. This compensates for the
peculiar frequency response you get on records due to the way vinyl and the
phonograph needle interact. This circuit boosts the bass and reduces the treble
to give a level response throughout the hearing range. The nice part of this
circuit is that it is easy to modify into a line input instead of a record input. This
is described later in the manual.
U4:D and U4:C are the summing amplifiers. These take the audio from the line
level inputs, the phono inputs and the two microphones and adds them all
together. R26 then adjusts the summed outputs to the peak hold meters, the
earphones, and the output jack.
The earphone amplifiers consisting of U3 and U2 are set up as summing
amplifiers as well. Each branch of the summing amplifier is connected the same
as the ones in U4. When you switch between CUE and play, the signal is just
redirected from the earphone summing circuits to the line level summing
RIAA Equalization
-40
-30
-20
-10
0
10
10
15.
8
4
9
2
5.
11
9
39.81
1
63.09
6
1
00
15
8.
4
9
251.1
9
398.1
1
630.
9
6
100
0
1584.
9
2511.
9
3
981.
1
630
9.
6
1
000
0
1
584
9
Frequency (Hz)
Gain (dB)