Ramsey Electronics FM100B Instruction Manual page 8

consider the Right channel due to the Left channel being practically identical.
Right off you will see we have two sets of RCA input jacks for the two channels
of audio we are able to mix together. This audio passes right through the front
level controls so that we can control the audio level for proper mixing and
distortion-free sound. U5:A is the right-channel audio summing amplifier. It
takes Line 1 and adds it to Line 2 giving you an amplified output on U5:A pin 1.
This summed or "mixed" output then goes through R52 before encountering
Q7. Q7 acts like an open switch when it's off or a closed switch shorting out the
audio to ground (so it doesn't go beyond this point) when it's on. Q7 is
controlled by a combination of U1 (the microcontroller) and U13 (the AGC
amplifier for the microphone). U1 enables or disables the Auto AGC feature of
the FM100B. The Auto ACG feature works by U12 detecting a level of audio
from the microphone, and if loud enough, U13, set up as a level detector,
mutes the audio coming in by turning on Q7 and Q8. We will get back to the
microphone circuit in a minute.
If the AGC circuit is off, audio continues to U5:D (another summing amplifier).
This stage sums the audio from the microphone circuit and the audio from our
current line input signal together. The level of microphone audio mixed in is
controlled by R16. Notice that there is another transistor (Q2) on the
microphone line. This works just like Q7 and allows U1 to mute the microphone
signal when necessary.
Once mixed, the sum of the audio from the two RCA jacks and the microphone
audio enters the low-pass filter. Filtration is provided by a complex mix of two
low-pass notches and a regular low-pass filter to achieve a steep cutoff
frequency. A notch at 19 kHz prevents any higher frequency audio components
from interfering with our stereo multiplexing signals. If you look at the schematic
you will see a section boxed off to indicate where these filters are located. You
can see there are quite a few parts involved! The components were chosen to
keep a nice bright sound as well as maintaining good stereo separation. I won't
FM100B • 8