Elecraft AF1 Audio Filter User Manual

Assembly and operating manual

Elecraft • www.elecraft.com • 831-662-8345

L E C R A F T

A F 1 A c t i v e A u d i o F i l t e r

Assembly and Operating Manual

E740102: Revision B, February 8, 2012

Introduction

The Elecraft AF1 is a versatile audio filter that can be used with any receiver or transceiver. It can improve intelligibility of
CW, phone, or data signals, and is especially well suited to radios that have inadequate I.F. or audio filtering. The AF1 uses
straightforward analog circuitry and no surface-mount components, so it’s an ideal project for first-time kit builders.

A rotary switch on the unit allows you to select a low-pass characteristic with adjustable upper frequency roll-off, or a narrow
bandpass characteristic. The bandpass filter offers two levels of selectivity, and its center frequency may be tuned from about
350 Hz to about 950 Hz. The low-pass filter is active during bandpass operation, allowing you to further control the upper
frequency response. The output amplifier drives low impedance phones or a small loudspeaker. Power can be supplied via
either an on-board 9-V battery or an external supply. An LED indicates power on/off status.

Specifications

Size:

5-1/2” L x 2-7/8” W x 7/8” H (14 cm x 7.3 cm x 2.2 cm). Height not including feet or knobs.

Internal Power: 9V battery.
External Power: 7-14 V (8 V min. recommended).
Current Drain: 10 mA minimum volume to 100 mA full volume, typical.
Functions:

Low pass plus two levels of bandpass with tunable center frequency in bandpass mode.

See

Typical Response Plots on page 9.

Tools Required

You will need the following tools to build this kit:

 Fine-tip temperature-controlled ESD-safe soldering station with 700 to 800F tip (370-430C). Recommend a spade

tip no greater than 0.05” (1.3 mm) wide.

 IC-grade solder (Kester #44 or equivalent). Small diameter solder (e.g. .031”) is easiest to work with on small

printed circuit boards. DO NOT use acid-core solder, water-soluble flux solder, additional flux or solvents.

 Small, #2 Phillips screwdriver.
 Needle-nose pliers.
 Diagonal cutters.
 Small wrench or driver for 4-40 nut.
 Digital Multimeter (DMM) with voltage, resistance and diode-checking functions is useful for confirming the value

of components. A DMM with capacitance measurement capability is desirable, but not required.

 Desoldering tools and supplies are invaluable. Narrow solder wick or a good vacuum desoldering tool such as the

Soldapullt® model DS017LS are recommended.

Refer to www.elecraft.com for tool sources and solder recommendations.

Parts List

We strongly recommend that you do a complete inventory before beginning assembly. The inventory helps you correctly
identify all the parts to avoid mistakes during assembly.

Identifying Parts

In some cases the component marking is self-evident. A 1N4007 diode, for example, will have “1N4007” stamped on its
body. When the markings are not so obvious, the marks you should find to identify individual components are shown in
parenthesis in the parts list and text. These may be numbers such as “(103)” on capacitors or colors on resistors such as