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AF1 • 5
THEORY OF OPERATION
Let’s have a look at what makes our audio filter so special. Have a look at the
schematic diagram and follow along.
First, having a look at the power supply section of the AF1 notice that diodes
D3, 6, 7,and 8 form a full wave bridge rectifier. We used this configuration so
that an AC or DC power source may be used for the unit (and the polarity
connection for a DC supply can be either way). This “raw” voltage is smoothed
by several large capacitors and routed to the voltage regulator IC to provide a
crystal clear source of DC voltage for the filter IC’s. You’ll also notice a few
bypass capacitors on the supply in case any of those nasty RF signals try to
get in and mess up the power supply.
At the heart of our kit is a pair of the MF8 switched capacitor bandpass filter
IC’s. Without these little marvels of technology this kit would not even exist!
Included on each chip is a pair of bandpass filters which can be cascaded to
provide sharper filter characteristics, a “Q LOGIC” binary input to set the filter
bandwidth, an adjustable internal oscillator to provide the center frequency for
our filter, and an extra op-amp to boot! Even the usually stingy data book
devotes 16 pages to the design possibilities for this hard working IC. The
overall bandwidth is controlled using switches S2 and S3 to provide a binary
input at the B, C, and D inputs of the MF8 IC. Resistors R12 and R13, along
with capacitor C8 provide the RC timing component necessary for the CMOS
oscillator.
The rest of the circuit is pretty straight forward - with whatever features could
be built in. The power switch doubles as an audio bypass, or feed through
when in the “OFF” position. Input protection diodes D1 and 2 limit the audio
input voltage to protect the valuable filter IC’s. Headphone jack J2 is set up to
accommodate either stereo or mono headsets, and headphone use will switch
the external speaker output off when the jack is inserted.
The headset / speaker output is from the LM380 audio amp IC. This chip
provides about 3 watts of noise free audio with a minimum of external
components.