INSTRUCTIONS
QF-1A AUDIO FILTER
Your new audio filter is the product of several years
of development by the originator of commercial
active audio filters for shortwave communications
(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 In case of trouble .)
The filter input may also be connected to the rcvr
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 (bandpass) 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).
