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IMPROVING RECEIVER AUDIO QUALITY
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IMPROVING RECEIVER AUDIO QUALITY
WITH AN AUDIO GRAPHIC EQUALIZER
-------------------------------
Source: Monitoring Times
Reprinted by: John Johnson KWV8BP
An easy and effective way to improve the audio quaility of your scanner or
shortwave receiver is to use an audio graphic equalizer. While more expensive
shortwave receivers provide sharp filtering for Continuous Wave (CW) and
Radio Teletype (RTTY), they may provide little or no audio filtering or only
fixed Single Side Band filtering at all. CW and RTTY filters are much too
sharp for audio filtering and SSB filters, as mentioned, are fixed and cannot
be varied for other types of signals. By using a graphic equalizer, we can
tailor the audio to our particlular need. A graphic equalizer is a device
that consists of a number of fixed audio filters, each having its own volume
control. Grahpic equalizers are used mainly in home and auto stereo systems
to compensate for excessive loss or boosting of frequencies because of
inadequate speaker systems, amplifiers, poor room acoustics, and so forth.
Each filter frequency is called a "band." Graphic equalizers may have as few
as three bands or as many as 20. Different manufacturers often select
different frequencies to represent the bands. For example, one graphic
equalizer may have 500 Hz, 1 Khz, 5 Khz, etc., while another manufacturer may
have 450 Hz, 1.5 Khz, and 5 Khz, etc. With careful adjustment of the graphic
equalizer controls, we can boost useful information while attenuating
everything else. Or, if we desire, we can notch out one or more select
frequencies, leaving the others alone or even boosting them at the same time.
In effect, what we have is a variable bandpass filter and a variable notch
filter all in the same box. Obviously, the more bands that the graphic
equalizer has, the sharper each band will be increasing the notch and band
pass resolution of the grahic equalizer. As mentioned earlier, the better
receivers will provide filtering for CW and RTTY, but what about Facsimile
(FAX), Slowscan TV (SSTV) and other forms of telemetry, each of which have
different bandpass characteristics? The graphic equalizer will allow you to
change the audio bandpass of your receiver to meet a particular need. When
used with a scanner, a graphic equalizer can work wonders on weak and noisy
signals. A signal that is barely discernible through all the popping, frying
and other forms of static will come through loud and clear with the proper
equalizer settings.
Two Types of EQ
There are basically two types of graphic equalizers on the market. One is an
"inline" type commonly found in home stereo systems. It needs an external
power amplifier to drive the speakers. This type of graphic equalizer is most
often found to have ten to twenty bands. The other type, which I like to
refer to as an "outboard" type, is most commonly found in car stereo systems,
and usually has only three to seven bands. The car stereo graphic equalizer
almost always has a power amplifier to drive the speakers directly. Its input
is taken from the tape player or radio speaker outputs. Home stereo graphic
equalizers usually come with separate controls for the left and right
channels, while the car stereo graphic equalizers usually have one set of
controls for both channels. Combining the two channels into one set of
controls as well as the reduced number of bands makes it easy to adjust the
car stereo graphic equalizer while in traffic. Wiring up the equalizer is
fairly easy and straight forward. But before you tear open your
receiver/scanner and rip the graphic equalizer out of your car, read this
article thoroughly! Be aware that opening your equipment may violate any
existing warranties! Neither the publisher nor the author will assume any
liability for any damages to your equipment relating to this article!
Connecting the "Outboard Model"
The outboard or car stereo graphic equalizer is the easiest to use and
requires no modification of either the scanner/receiver or the graphic
equalizer. If you are (understandably) squeamish about performing surgery on
your receiver/scanner, then the outboard or car stereo graphic equalizer is
for you. It will require a 12.6 VDC 2 AMP (at least) power supply. Booster
type equalizers may require a higher amperage. Nearly all receiver/scanners
are equipped with an earphone jack. Use the audio from the earphone jack to
drive the outboard graphic equalizer. The typical car stereo graphic equa
lizer comes with a minimum of six wires not including the power supply leads.
Some models may have more wires and may come with auxiliary power leads
intended to power another device through the equalizer. These wires are often
identified somewhere on the graphic equalizer. Be very sure you know what
each wire is before you start hooking up the graphic equalizer to your
receiver/scanner or you could wind up "smoking" the graphic equalizer and/or
the receiver/scanner. Follow the same procedure for wiring up the outboard
graphic equalizer to the receiver/scanner as you would for wiring up to a car
stereo. Only one channel of the graphic equalizer will be used. Be sure that
if you use the LEFT channel input, that you also use the LEFT channel output.
Wire a connector that matches the earphone plug of your receiver/scanner to
the AUDIO input wires of the graphic equalizer. Solder an 8 to 16 ohm load
resistor to the output of the unused channel. This load resistor must be
EQUAL to or GREATER in wattage than the channel it is wired to. Failure to
install the load resistor may destroy the power amplifier for that channel
(It is quite possible, if you wish, to wire one channel up to a scanner and
the other channel up to a shortwave receiver. This might make it difficult,
however, to use both receivers at the same time.) Before you apply power, be
very sure you have everything wired up properly and that the volume control
of the receiver/scanner and the graphic equalizer are at a minimum. If the
graphic equalizer has a fader control, set the fader control to mid range
position. Apply power and slowly adjust the volume controls of the
receiver/scanner and graphic equalizer. If the graphic equalizer has a BYPASS
switch, make sure it is set in the "equalize" position. If you get no sound,
immediately turn everything off and recheck ALL your wiring. Once you have
everything working, adjust the graphic equalizer for the best sound possible.
Experiment with different settings and adjustments. Even similar devices,
such as two-way radios, will have different bandpass characteristics. Compare
the different between "normal" audio and "equalized" audio by alternating
with the BYPASS switch. You can really appreciate the difference on weaker
signals.
Wiring the "In-line" Equalizer
The in-line home stereo type equalizer will require some modification to your
receiver/scanner. Despite this fact, the inline graphic equalizer is the one
I prefer. This is because, at will, I can hook up practically any kind of
inline equalizer simply by plugging in the one I want. This modification also
provides a handy output for a tape recorder or external power amplifier.
First, drill two 1/4-inch holes approximately 1 inch apart at some convenient
place on your receiver/scanner, and install a female phono plug in each hole.
You will then need to locate the volume control potentiometer (pot). It is
here that you will "break in"and "place" the graphic equalizer (see figure 1)
.Turn on the receiver, and using a signal tracer, find the "high" side of the
pot. This is the contact that has a signal that does not vary with the pot
setting. If you don't have a signal tracer, you can make one from just about
any cassette tape recorder. Make up a test cable from about two feet of
microphone cable. On one end install a connector to fit the AUXILIARY (AUX)
or MICROPHONE (AUX) jack of your cassette recorder. On the other end of the
cable, solder a .01 uf 25V or greater capacitor (CAP) and place some heat
sink or sleaving on the free lead of the CAP leaving about 1/8 inch of lead
bare. Use this lead as a simple probe to find the "high" contact (see figure
2). Plug a small speaker or headphones into the EAR jack of the cassette tape
recorder and put the cassette tape recorder into RECORD mode by inserting a
blank cassette and pressing RECORD and PLAY.
Be very careful not to short any pins or contacts while probing around! Once
you have found the "high" contact, deslder the wire from this contact and run
it to one of the phono jacks. Label this jack OUTPUT (to equalizer). Using
only the shortest length of wire necessary (AWG 28 gauge), run a wire from
the contact left bare to the other phono jack. Label this jack INPUT (from
equalizer). If the pot is mounted on a circuit board (as was the case with my
scanner), find the "high" contact on the circuit board in the same manner
described previously. Once you have found the "high" side contact on the
circuit board, locate the trace that runs to this contact. Carefully cut
completely through this trace using a sharp craftsman knife and remove
approximately 1/16 inch of trace. Scrape back the lands at the break point
about 1/4 inch on each side of the break until the copper is shiny. Carefully
tin the exposed copper surface on each side of the break, taking care not to
bridge the break with solder (see figure 3). Locate the trace that connects
to the pot. Using the shortest length of wire necessary, solder one end of
the wire to one of the phono jacks and label this jack INPUT (from equalizer)
. Using the same procedure, solder a wire to the tinned area of the remaining
trace. Solder the other end of this wire to the remaining phono jack and
label this jack OUTPUT (to the equalizer). Next, make a jumper by taking a
2-inch piece of AWG 20 gauge wire and solder a male phono jack at each end
(see figure 4). This jumper will be needed whenever you use your
receiver/scanner without the graphic equalizer. When you're all done,
double-check your work, making sure your solder connections are good and
that there are no solder bridges anywhere. Once you are sure that your work
is okay, apply power to the graphic equalizer and the scanner/receiver. If
you get no sound turn everything off and recheck your work. If you get sound
but the equalizer controls have no effect, make sure that the BYPASS switch
is in the proper setting. Most graphic equalizers come equipped with a bypass
switch to let you return to a "normal" setup. BYPASS may be part of the
ON/OFF switch function. If you still arent getting any sound, make sure you
have the patch cable going to the proper inputs and outputs of the graphic
equalizer and the receiver/scanner. When you are positive everything is
working okay, put your receiver/scanner back together.
An advantage to the inline graphic equalizer is the ability to cascade the
left and right channels. By feeding the left channel into the right channel
(or the other way around) this will increase the overall selectivity of the
graphic equalizer (see figure 5). The Realistic equalizer that I am currently
using is a model 12-1867 seven band car stereo equalizer. It currently sells
for less than 50.00 new.
Schematic Diagrams
------------------
Figure 1: Schematic Diagram of Modification
graphic equalizer
_____________________
| + + + + + + + + + |
|___________________|
|IN |OUT
____ | |
>----------|____|--------------X---------|
from pre amp out in |
detector | /volume control
/ ___
\ <--------|___|-------[]<|
/ audio speaker
\ output
|
-----
---Receiver/Scanner
_____________________________________________________________________________
Figure 2: Probe for Cassette Recorder Signal Tracer
__ microphone cable .01cap /make shift probe tip
from
---|__}----------------------------{}-------------+-- cap lead
aux/mic <----- 2 ft. -----> ^
input of heatshrink or
cass.rec. sleeving
_____________________________________________________________________________
Figure 3: Example of "Cut" Circuit
circuit trace cut circuit trace
---------------------- ---------------------
\/
scrape and tin
_____________________________________________________________________________
Figure 4: "Phono Plug Jumper"
___ ___
---|___}-------------------{___|---
<--- 2 in. --->
_____________________________________________________________________________
Figure 5: Cascading Left and Right Channels (Inline Equalizer Only)
\\
\\
\\
\\
IN {} {} IN
Left // Right
//
//
//
//
{} {} OUT
OUT \\ Right
Left \\
\\
_____________________________________________________________________________
This article was in the August 89 issue of Monitoring Times, design by Roger
D.Dowd.
This file was brought to you by John Johnson of The Hotline BBS, give us a
call, we have an entire section on Scanner & Radio related topics....
The Hotline BBS 304-736-9169 or 304-743-7909
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