AF2 - Audio (CW) Filter
The AF2 is a two-pole "active" filter, using selective feedback to attain a good CW response to audio frequencies centred on about 800 Hz.
Use of close tolerance components in the frequency determining section results in a very sharp peak bandwidth without any 'dips' and fairly steep 'skirts' - but not so steep as to result in 'ringing'.
Unlike the "passive" type of filter, which introduces an insertion loss into the AF amplifier chain, an active filter can provide an actual gain at its response peak.
Typical bandwidth of the AF2 is 210 Hz @ 6db, 1600 Hz @ 30dB down the response curve.
Plugged directly into the low impedance output socket of your receiver, the AF2 provides more than adequate volume for headphones, but bear in mind that the unit is not really intended to drive other than a small 'speaker.
It can, of course, be built directly into a receiver. If used in this manner it should be placed in the AF amplifier circuit before the output stage, probably following the AF gain control. In the case of a typical Direct Conversion Receiver, the AF2 would represent the entire audio filtering requirement and could immediately follow the mixer stage.
A flashing LED indicator is incorporated which is a useful battery saving device. Its current consumption is a mere 0.3mA - compared with a possible 8 to 10 mA if the LED was continuously lit. The eye-catching flashes, about 80 per minute, will remind you to switch OFF after use. PP3 batteries are not cheap and always seem to run down instantly - the moment you forget to switch them off!
The AF2 kit is complete with case, 1/4" jack plug for connection to your receiver and 1/4" socket for 'phones. The PP3 battery is not supplied.
Kit : £16.50 Ready built : £26.00
Postage (U.K.) £1.50