Elecraft AF1 User Manual
ELECRAFT AF1 Active Audio Filter
Assembly and Operating Manual
E740102: Revision B, February 8, 2012
Copyright © 2012 Elecraft; All Rights Reserved
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 800F tip (370-430C). 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
Elecraft • www.elecraft.com • 831-662-8345
“(brn-blk-red)”. Look for the color band sequence starting near one end of the component. Some resistors have dark blue
bodies that make identifying the colors difficult. Use your DMM to verify the value if you aren’t sure. Remember that your
DMM usually will indicate a value close to, but not exactly what is shown, due to normal tolerances in the components and
the DMM.
There may be more numbers, letters or color bands on a component than those shown in parenthesis. The additional markings
may change from unit to unit and are not important for identifying the part supplied with the kit.
Reference. Designator Description Qty
Misc. AF1 Audio Filter Printed Circuit Board 1 E100310
B1 Battery Holder 1 E980074
C1, C10 1µF 50V Electrolytic Capacitor 2 E530022
C2, C3, C11 470µF 25V Electrolytic Capacitor 3 E530084
C4, C7, C8 , C9, C13 .01µF (103) Capacitor 5 E530009
C5 .0022µF (222) Capacitor 1 E530054
C6 .1µF (104) Capacitor 1 E530011
C12 .047µF (473) Capacitor 1 E530025
R1 1.5k (brn-grn-blk-brn-brn) 1/4W Resistor 1 E500034
R2, R4, R7, R14 1.0k (brn-blk-red) 1/4W Resistor 4 E500013
R3 15k (brn-grn-org) 1/4W Resistor 1 E500060
R6 10k (brn-blk-org) 1/4W Resistor 1 E500015
R8, R11 180k (brn-gry-yel) 1/4W Resistor 2 E500068
R9, R12 680Ω ((blu-gry-brn) 1/4W Resistor 2 E500040
R17, R13 360k (org-blu-yel) 1/4W Resistor 2 E500264
R16 10Ω (brn-blk-blk) 1/4W Resistor 1 E500054
R18 22k (red-red-org) 1/4W Resistor 1 E500090
R10 5k dual potentiometer, linear taper, 6 pin 1 E520019
R15 10K (10KD) potentiometer 1 E520018
R5 50K (50KB) potentiometer 1 E520017
D1, D2 1N4007 Diode 2 E560001
D3 LED, Red, Ultra bright 1 E570025
J1, J3 Jack, Stereo, 3.5 mm 2 E620027
J2 Jack, 2.1 mm, 12VDC power 1 E620026
SW1 Switch, Rotary, 3 Pole, 4 Position, Non-Shorting, PC 1 E640033
U1 LM348 Integrated Circuit 1 E850231
U2 LM386 Integrated Circuit 1 E600022
Hardware Screw, Pan Head, Black Machine, 4-40 4 E700008
Hardware Lock Washer, Internal Tooth, #4 4 E700010
Hardware Nut, #4-40 4 E700011
Misc Knob w/set screw for 1/4 inch shaft 4 E700118
Misc Rubber Foot 4 E700024
Misc #24 Wire, Solid, Green, Insulated 6” E760008
Part
Number
Assembly Options
The AF1 is designed to operate as a stand-alone unit without an enclosure. In the event you want to build it into existing
equipment or its own enclosure:
1. Four holes are provided in the board that you may use with your own #4 hardware and spacers as needed to mount
the unit instead of using the rubber feet.
2. The battery may be mounted on the top (component side) or bottom of the board or you may choose not to install the
battery holder if you are going to power your AF1 only from an external source.
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Assembly Procedure
The finished audio filter is shown in Figure 1. The knobs were left off to better show the components on the board. We
recommend you follow the assembly procedure below, and refer to this figure as needed.
Figure 1. Assembled Audio Filter.
i
Use adequate ventilation when soldering; avoid inhaling smoke or fumes. Always wash your hands after
handling solder, as lead residue is highly toxic.
When applying solder, use the minimum amount required to surround the component lead and make good contact with its
printed-circuit pad. You don't need a "fillet" (build-up) of solder. This will avoid unwanted solder bridges and any need to
clean the PC boards.
The solder must flow onto both the component lead and its PC board pad. To ensure that both will be heated at the same time,
the tip of the iron should contact both the component lead and the PC board pad before solder is applied.
Solder joints should be clean and shiny. If a joint appears dull or has fine cracks, it is probably cold. Cold solder joints should
be cleaned and re-soldered. First, use solder wick (desoldering braid) to remove the old solder. Then apply fresh solder. If
you have many cold solder joints, it probably indicates that your soldering iron temperature is too low, or that the tip or
solder itself is defective.
flush on the bottom. If you position the board so the legends are right side up, you can work from left to right across the
board following the order given below. Save one of the clipped leads. You will use it to make a ground test point later.
Install the fixed resistors listed below. Position each resistor within the outline on the PC board, solder and trim the leads
__ R18, 22k (red-red-org)
__ R7, 1K (brn-blk-red)
__ R1, 1.5k (brn-grn-blk-brn)
__ R17, 360k (org-blu-yel)
__ R3, 15k (brn-grn-org)
__ R8, 180k (brn-gry-yel)
__ R12, 680Ω ((blu-gry-brn)
__ R16, 10Ω (brn-blk-blk)
__ R2, 1K (brn-blk-red)
__ R4, 1K (brn-blk-red)
__ R6, 10k (brn-blk-org)
__ R14, 1.0k (brn-blk-red)
__ R13, 360k (org-blu-yel)
__ R9, 680Ω ((blu-gry-brn)
__ R11, 180k (brn-gry-yel)
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