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.
RIAA Equalization
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Frequency (Hz)
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