FM35 5
FM35 CIRCUIT DESCRIPTION
We will begin by talking about the new and improved power supply section of
the FM35. First off take a look at L1, C8, and C7. These parts form a low-
pass filter designed to remove any RF from escaping through the power jack
on the FM35, but more importantly to keep RF from entering back in! RF can
easily get into power regulators and other components and cause them to
“flake out” as we call it. The leads to hum and excessive noise in some
cases, and can be very hard to remedy without these filters.
D1 is designed to protect you in the unlikely event that power is connected in
reverse polarity. The diode is placed in-line with the power. Since a diode can
only conduct in one direction, it prevents a reversed power supply from
damaging your new FM35! We figured the addition of a this part was worth
the peace of mind. C5 is another part used to reduce RF interference.
C13 is used to “smooth” any ripple there may be on the input to VR1, a
12VDC low dropout regulator. You may ask “what does low dropout mean?”
No it doesn’t mean the regulator was made by a bunch of kids from a
privileged school, it means that it can regulate lower voltages than it’s non-
low dropout versions can. In this case to regulate to 12V, VR1 only needs
about 0.8V across it, this means it can regulate 12.8V and up!
C9 is used in conjunction with VR1 for good regulation of the +12V. The
+12V is then fed to VR2, a standard 5V regulator. This simply steps the +12V
down to +5V for our logic and audio circuits. At this point our voltages are
very well regulated for lowest noise! C10 works with VR2 for good regulation.
Now on to the audio input and control circuitry!
Audio is introduced into the FM35 at J8 (left) and J10 (right). For now we will
only talk about the left channel, as the right channel is a duplicate of the left.
Immediately after J8, you will notice L2 and C28. These parts are to prevent
EMI produced from transmitting from getting into your FM35 and ruining the
audio quality, similar to what we’ve prevented in the power supply above.
After this filter the audio goes into U4:A, a simple rail to rail opamp. R46,
U2:A, and R43 set the gain of the opamp.
You may wonder, where is U2:A? I don’t see any pots! Well, U2:A is a
digitally controlled pot. Inside the IC are a bunch of transistors switching in
and out resistor arrays to make a digitally controlled variable resistance. In
turn, this variable resistance controls the gain of U4:A, thus U2:A is the
volume and balance control! We just happened to draw the IC like a pot in
the schematic software to make more sense.
After this gain stage, the audio moves on to U5:B, another opamp
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