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. 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