User manual

Your Autek QF-1A audio filter is the product of
several years of development by the originator of
commercial active audio filters for shortwave
communications since (1972).
Each filter is thoroughly tested twice before shipment.
Please read the instructions carefully for best results,
and save them for future reference. If you require
adaptors or other parts not supplied, please obtain
these locally as we do not stock them.
A. INITIAL HOOKUP
Plug the QF-1A into your receiver’s phone jack or
speaker output. Any impedance is O.K. Obtain an
adaptor if size is not compatible. Connect any
impedance phones or speaker to the rear-panel phone
jack output. To avoid hum or distortion with “Hi-Fi”
headphones, use a series resistor of about 46 ohms.
(See section: “In case of trouble”.)
The filter input may also be connected to the receiver
speaker wires. Just be sure the input cable shield
(connected to QF-1A chassis) is connected to the
grounded speaker wire; otherwise, there may be hum.
Note: This is rare, but some Barlow-Wadley radios
and CB sets require a load resistor across their speaker
or phone output for a DC return. The filter’s high
input impedance does not provide this. Try a resistor
in the range of 10 to 100 ohms installed in the radio
across the output being used. Otherwise the set may
appear to be dead, as the output stage bias is wrong.
B. FAMILIARIZATION
The filter selects or rejects certain frequency
components of signals you tune in. This allows it to
reject undesired signals, such as noise and
interference, while passing the desired signal, so long
as the noise and signal are not on the same frequency.
This frequency selection/rejection is optimized with 5
controls:
1. Auxiliary Notch Frequency
A notch rejects a narrow band of frequencies, such as
a whistle, or a code station. The Aux Notch is present
at all times. To effectively disable it, set it to either 80
or 11,000 Hz. The Aux Notch is very wide, for easiest
tuning, and very deep. Its 135:1 frequency range is by
far the widest ever available for communications, and
should cover any signal.
2. Function Select Switch (PK, NOTCH, LP, HP)
This switch and the Selectivity/Frequency controls
adjust the response of the main filter (all but the Aux
Notch). PEAK (band-pass) passes a narrow band of
frequencies, and rejects others -- just the opposite of
NOTCH. LOWPASS passes low frequencies, while
rejecting high frequencies (e.g. hiss). HIGHPASS is
the opposite of LOWPASS: it passes high frequencies,
while rejecting low frequencies. An exception occurs
at high selectivity, where LOWPASS and HIGHPASS
take on some of the characteristics of PEAK.
3. Selectivity Control
In PEAK, this control determines the filter bandwidth.
Peak bandwidth as narrow as 14 Hz (- 3 dB) or 20 Hz
(-6 dB) is available at 300 Hz. The bandwidth
gradually increases to 20 Hz at an 800 Hz center
frequency, and reaches a few hundred Hz at the
highest frequency setting (2500 Hz). This bandwidth
increase is deliberate and makes for easiest tuning. At
min. Sel., the filter is almost flat.
In NOTCH and LOWPASS, the panel suggests the
best settings. More selectivity rotation (clockwise)
makes the notch narrower, and shallower, hence
harder to tune: in LOWPASS and HIGHPASS,
excessive selectivity rotation can lead to ringing at
some frequencies.
The ideal “theoretical” best selectivity for least
peaking in LOWPASS and HIGHPASS occurs when
the selectivity control is rotated about two-thirds the
frequency control, i.e. if the frequency control is
rotated 50%, rotate the selectivity control about 30%.
However, this “rule of thumb” should be broken under
many conditions (see Tables 1 and 2).
4. Frequency Control
This tunes the main filter frequency from about 250 to
2500 Hz, the entire usable communications range for
voice, CW, and digital (RTTY, SSTV, etc.) signals.
5. In/Out (ON/OFF) Switch
This turns off the AC power and bypasses the filter.
There is an unavoidable click when thrown due to the
power supply charging or discharging. The filter
draws less than 1/2 watt if left on accidentally.
Note: Volume is adjusted with your receiver’s gain
control. Gain at high selectivity in PEAK (and LP) is
high, and very little noise comes through; so keep
receiver gain low under these conditions to avoid
saturation of the QF-1A power amp (distortion) when
the desired CW signal comes through!