I recently completed an operating accessory that I’ve found quite useful and impressive. It brings my little KX1 trail radio’s single conversion receiver’s performance right up there next to my K2.
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. I built mine in about 45 minutes and that includes making a cup of tea halfway through.
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.
There’s a picture of it at the end of this post but first, I’d like you to listen to it in operation. The following recording is 45 seconds long. You’ll hear the unfiltered 40 meter signal of VE3YAA calling “CQ” and then I’ll switch the filter in. Toward the end of the recording, I’ll switch it out and then back in again. I’m sure you’ll agree that it really makes the CW pop out.
Anyone interested in more information can go to the Elecraft website.