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Low-cost audio mixer for camcorders Jazz up your home videos by mixing in some background music from either CD or tape. This simple 2-channel mixer runs off a 9V battery, uses only one common IC & can be built in a couple of hours. By DARREN YATES Although camcorders have made home-movie "blockbusters" easier to make than ever, the soundtrack on most of these recording is usually pretty boring. Most of the time, you find that it's just people talking, while sometimes there is no significant sound there at all. Again the age-old question comes up: "wouldn't it be nice if... ?" This handy 2-channel audio mixer is ideal for adding some "aural interest" to your home videos. It allows you to mix the camera recorded audio with audio from an external source when you copy the camcorder tape to VHS format on the VCR. Generally, the extra audio to be added to the soundtrack will come from a CD player or tape deck, but you can use other audio sources as well. In operation, you simply feed the audio output from the camcorder into one channel and a (mono) signal from the external source into the other. The relative levels of both inputs can then be adjusted using the Channel 1 and Channel 2 level controls, while a Master control sets the overall output level. The maximum output level is about Two of the RCA sockets accept mono input signals from the camcorder & an external music source, while the third provides a mono output for the VCR. The front-panel controls allow the input & output levels to be adjusted. 50 SILICON CHIP 4Vp-p, which is more than ample for any VCR. An on/off switch completes the front-panel line-up, while input/output connections are made via three RCA sockets mounted on one side of the case. The remaining parts are all mounted on a small PC board and this fits neatly inside a small plastic case. These parts are all readily available and, in fact, you may already have most of them sitting in your junkbox. Circuit details Refer now to Fig.1 for the circuit details. As you can see, there's not a lot to it. ICl is a TL064 quad FETinput op amp package. Three of its op amps (ICla, IClb & ICld) are used to mix the audio signals, while the fourth (IClc) is used to provide circuit biasing. At the channel 1 input, the incoming audio signal is AC-coupled via a 2.2µF capacitor to level control VRl. The resulting signal is then fed to pin 3 of ICla via a lkQ stopper resistor to reduce the op amp's sensitivity to RF breakthrough. ICla is configured as a non-inverting amplifier with a gain of 3.2. Its high frequency response is rolled off above 72kHz by the lOOpF feedback capacitor between pins 1 & 2, while the low frequency response is rolled off below 7Hz by the lOkQ resistor and its series 2.2µF capacitor. IClb is used to amplify the channel 2 signal and works in exactly the same fashion as ICla. The outputs from these two op amps (at pins 1 & 7) are then mixed via two lOkQ resistors and applied to pin 12 of ICld via master level control VR3. ICld also functions as a non-inverting amplifier with a gain of 3.2, while VR3 sets the overall signal level at the output. Power supply Power for the circuit is supplied · from a 9V battery, with IClc used to Fig.1: the circuit uses IC1a & IC1b to amplify the input signals which are fed in via VR1 & VR2. The amplified signals are then mixed via 10kQ resistors & applied to op amp IC1d via level control VR3. IC1c is used to derive ±4.5V supply rails from the 9V battery. 10 14 + 100pF ~ · 1&VW ~ 47kI 22k OUTPUT -f 22k 10k 10k 2.2 + 18VW+ 2.2 + 25VW+ POWER S1 ~~-----------~+LSV 100pF 22k J_ ,---....---.... T 9V: ..&.. 10 + 10k 18VW 2.2 + 18VW+ -----------------u-4.SV AUDIO MIXER FOR CAMCORDERS derive ±4.5V supply rails. This op amp acts as a buffer stage, with its inputs and output biased to half the 9V supply from the battery. Its output (pin 8) can sink and source up to 20mA of current and this becomes the circuit ground rail. By using this arrangement, the positive terminal of the battery becomes the +4.5V rail, while its negative terminal becomes the -4.5V rail. All circuit connections to ground are connected to the output of IClc. This configuration is necessary to prevent clipping of the negative-going portions of the audio input signals. The circuit consumes about 4mA, which means that a 9V battery should last for approximately 100 hours. CHANNEL 1 INPUT CHANNEL 2 INPUT OUTPUT ,!' ,,J .t Construction Most of the parts are installed on a small PC board coded 01112921 (103 x 52mm). Fig.2 shows the assembly details. Begin the board assembly by installing the resistors, then fit the capacitors and the IC. Either 5% or 1% resistors can be used here and it's a good idea to check each one using your DMM before mounting it on the board. The resistor colour codes are also shown in the accompanying table to prevent confusion. Most of the capacitors used in this circuit are electrolytic types, so pay special attention to their polarity. Take care also when installing the IC; pin 1 of this device is adjacent to a notch in one end of the plastic body. If you do install the IC the wrong way around, it will self-destruct as soon as power is applied. The board assembly can now be completed by mounting Fig.2: install the wire link under IC1 before installing any of the other parts on the PC board. The wiring to the pots & RCA sockets can be run using rainbow cable. MARCH 1993 51 PARTS LIST 1 PC board, code 01112921, 102 x54mm 1 PC-mount battery holder (to suit 9V battery) 3 RCA panel mount sockets 3 knobs to suit 1 plastic zippy case, 130 x 68 x 41mm 4 rubber feet 1 SPST toggle switch 3 10kn log potentiometers (VR1-VR3) Semiconductors 1 TL064 quad op amp IC Capacitors 4 10µF 16VW electrolytics 5 2.2µF 16VW electrolytics 2 100pF ceramic Resistors (0.25W, 1%) 3 47kQ 5 10kQ 3 22kn 2 1kn Miscellaneous Hookup wire, solder, screws, nuts & washers. The PC board is secured to the bottom of the case using machine screws & nuts, with additional nuts used as spacers. Take care to ensure that the CHANNEL 2 & OUTPUT RCA sockets are spaced far enough apart to clear the battery. o· 0 0 0 0 0 Fig.3: this is the full-size etching pattern for the PC board. Check your board carefully for defects before installing any of the parts. the battery holder. This part is secured to the PC board using three screws and nuts. Don't forget to solder its positive and negative terminals to thei:· respective pads on the board. The PC board can now be used as a template for marking out its mounting holes on the bottom of the case. Drill these holes to 3mm, then fit the adhesive label to the front panel (case lid) and drill holes for the on/off switch and the level controls. In each case, it's best to first drill a 3mm pilot hole and then carefully enlarge the hole using a tapered reamer until the part is an exact fit. The three RCA sockets are mounted along one side of the case. These sockets should be positioned roughly inline with their corresponding front RESISTOR COLOUR CODES 0 0 0 0 0 52 No. Value 4-Band Code (1 %) 5-Band Code (1%) 3 3 5 2 47kQ 22kn 10kQ 1kQ yellow violet orange brown red red orange brown brown black orange brown brown black red brown yellow violet black red brown red red black red brown brown black black red brown brown black black brown brown SILICON CHIP panel controls but make sure that they don't foul the battery when the PC board is later mounted in CAMCORDER ~ position (the battery should fit AUDIO MIXER snugly between two of these sockets). Mount all items of hardware in position when the holes have OFF been drilled. Standard rainbow cable is used for the wiring between the PC board and the front panel conON trols. Complete this wiring as MIN MAX MIN MAX MIN MAX shown in Fig. 2, then fit three short lengths of 2-way rainbow POWER CHANNEL1 CHANNEL2 MASTER cable to the board for the RCA socket wiring (ie, to points 1-6) . This done, the PC board can be Fig.4: this full-size layout can be used as a drilling template for the front panel. secured inside the case using Drill small pilot holes first, then enlarge the holes to size using a tapered reamer. machine screws and nuts, with additional nuts used as spacers. The wiring can now be completed amplifier. This done, connect a 9V put stage (either IC1a or IC1b). by making the connections to the three battery to the circuit and switch on. Similarly, if there's no output, careRCA sockets. Check all wiring care- You can now determine whether or fully check the circuit around IC1d fully when the job is finished to make not the unit is working correctly by and check the supply rails. In particusure that there are no mistakes. varying the three level controls. lar, check the underside of the board If one channel doesn't work, first for missed solder joints and possible Final check-out make sure that an audio signal is be- shorts between the IC pins. To test the unit, feed some audio ing fed in (you can do this be swapThat's it - you can now easily add signals from a couple of sources into ping the two inputs). If there is signal music or voice commentary to your the two inputs and connect the out- coming in, the next step is to check home movies after they have been SC put to an auxiliary input on your stereo the circuitry around the relevant in- recorded. :::flll!I!#- ~ lf!!!!3' QQQ 0 IAN J. TRUSCOTT'S ELECTRONIC WORLD 30 Lacey Street, Croydon, 3136. Phone (03) 723 3344; Fax: (03) 725 9443. Now also at: 200 Smith Street, Collingwood. Phone (03) 419 8208 Mail & Credit Card Orders Welcome We now have a range of 5,6,8,9 & 1O pin Ask about our range of E.A.O. precision push button switches, motor start resistor networks and a comprehensive range of micro's and eproms at ridiculous capacitors and Beckman Industrial prices .. Quote this ad and receive another multimeters. 5% discount. 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