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Do you have a latent urge to produce exotic drum sounds; maybe to accompany the wild gyrations of frenzied dancers? Or maybe you want to investigate a range of percussive instruments? If so, the Syntom is for you. YNTOM - a synthesised tom-tom The Syntom is a very effective percussion instrument synthesiser which can be used by itself or as an add-on to an existing drum kit. It is triggered by rapping it anywhere on the case in which case it gives out a single percussive beat or note. The pitch and decay of the note is variable over a very wide range and the pitch can also be made to fall away as well. The Syntom can also be clamped to a drum kit, in which case it will When you rap the case of the Syntom or the drum kit, the vibrations are picked up by a crystal earpiece and fed to the signal processing circuit. 20 SILICON CHIP be triggered every time the drum is struck. The unit is housed in a black plastic box with four control knobs, for Pitch, Sweep, Decay and Volume. Powered from a single 9 volt battery, it can be used to drive a power amplifier directly or the signal can be fed to a mixer. Circuit description The best way to describe the circuit is to break it up into three sections: an envelope generator, a voltage controlled oscillator (VCO) and a voltage controlled amplifier (VCA). ICl, a 741 op amp, buffers the pulse signal generated by the crystal earpiece pickup and then charges a 2.2J.lF capacitor via diode Dl. So when you rap the case of the Syntom, the earpiece and ICl generate a fast rising pulse. The decay of this pulse voltage is controlled by the lMQ decay potentiometer, VR 1. The pulse voltage is then fed to IC2a which is connected as a voltage follower so that it too acts as a buffer. The output of IC2a feeds pin 5 of IC3 (the voltage controlled amplifier) and VRZ, the 47kn sweep potentiometer. The voltage controlled oscillator comprises IC2c and IC2d, together with transistor Ql and diode DZ. The control voltage for the VCO comes from IC2b which acts as a summing amplifier. It adds the control voltage frorn the pitch control VR3 and the sweep control VRZ. IC2c acts as an integrator while IC2d is a Schmitt trigger. They operate together as follows. The control voltage from the output of IC2b charges the .001!,lF capacitor so that pin 2 of IC2c rises and pin 1 falls. When the voltage at pin 1 falls below the threshold of Schmitt trigger IC2d, the output at pin 7 goes high and turns on Ql. Ql then discharges the .001!,lF capacitor so that the voltage at pin 2 falls . This causes the voltage at pin 1 to rise until the positive threshold of IC2d is exceeded. This causes the output at pin 7 to go low and turn off Ql. Thus the cy- ~t--;,.- ~-.- - - - - , "' "' I· + By LEO SIMPSON cle continues and the result is a sawtooth waveform at pin 2 of IC2c. Provided the pitch control VR3 is not at minimum setting, the VCO runs continuously and if the sweep control VR2 is at the minimum setting, the VCO frequency is constant. The VCO output is fed to pin 3 of IC3, the voltage controlled amplifier (VGA). This can be regarded as a normal op amp but with the added facility that a variable DC voltage fed to pin 5 will control the gain over a wide range. So let's just go through the sequence of operation. When the crystal earpiece picks up a sharp vibration it generates a pulse voltage which is amplified and buffered by ICl and fed to diode Dl to charge the 2.2µF capacitor. The decay of the voltage across the capacitor is controlled by the setting of the Decay control, VRl. Thus, ICl, Dl, VRl and IC2a produce the voltage "envelope" which controls the VGA, IC3. When its pin 5 receives a positive voltage, IC3 rapidly increases its gain and the signal from the VCO is fed through to the output at pin 6. As the envelope voltage at pin 5 falls, the amplitude of the output signal from pin 6 falls away in proportion. So the envelope generator and IC3 control the way in which the VCO signal builds up and then decays away. Remember also that the envelope signal is fed to the Sweep control VR3. This causes the VCO frequency to fall so that not only does the + I ,-. .., C, C, C, C, C, M M Fig.1: the circuit of the Syntom uses three op amp packages but the key component is the voltage controlled amplifier, IC3 . The voltage al its pin 5 controls the gain over a very wide range. MAY1989 21 Fig.2: here's how to assemble the PCB and install the wiring. Warning: don't install Qt for best fit. The PCB was originally designed for a transistor with a BCE pinout while the BC549 supplied has an EBC pinout. This means that you will have to rotate the transistor until the leads match up with the holes in the board. CAPACITORS D D Value IEC EIA 1 .001µF 1 .0047µF 1n 4n7 102K 472K RESISTORS D D D □ D □ □ D D □ D □ 1 1 1 5 8 1 5 1 1 1 1 2 1MO 330k0 220k0 100k0 47k0 22k0 10k0 8.2k0 4.7k0 1.8k0 4700 2200 brown black green gold orange orange yellow gold red red yellow gold brown black yellow gold yellow violet orange gold red red orange gold brown black orange gold grey red red gold yellow violet red gold brown grey red gold yellow violet brown gold red red brown gold OUTPUT -+ ~.._ VO ____ , 9V BATTERY loudness of the signal die away but so does the pitch. By suitably manipulating the Pitch, Sweep and Decay controls, you can produce almost any musical note, from a piano key to a bass drum. The signal from the Syntom is quite sufficient to drive a power amplifier or the line inputs of a mixer. Power is provided by a 9 volt battery which is "split" by two lOkO resistors and a 47µF bypass capacitor to give supply rails of ± 4.5 volts (nominal). The centre point of the DC supply becomes the output signal earth point. Note that ICl, IC2a, IC2b and IC2c are biased from the - 4.5V supply while IC2d and IC3 are biased from the midpoint (ie, OV or earth). Construction Now let's talk about building the unit. It is housed in a standard plastic case with a plastic lid. All of the circuitry is accommodated on a small printed circuit board which 22 SILICON CHIP measures 100 x 40mm. This board looks easy to put together but there is a preferred order of assembly which makes the job a lot easier. The first step is to carefully examine the board for any open circuit tracks or bridges between tracks. The board we assembled was a good quality type with rolled solder finish but a little careful inspection at this stage can prevent heartburn later on. Step 1: insert the PC pins. 14 pins are required and as they are a very tight fit in the holes you'll need a hammer (and preferably a vise) to belt them home. Step 2: insert the resistors. 27 are required and they are all 1/4watt 5% tolerance units. To make it easier, we have listed all the needed values next to the wiring diagram, together with their colour codes. The idea is to do all the resistors of one value first, starting from the top and working down. As you install all the resistors of each value, tick the square box. Be sure to install the resistors so that their colour codes all run in the same direction across the board or down the board. This way, it makes it much easier to check that you have the correct value installed. It looks better too. When all the resistors are installed, you should have one left over. This is a 47k0 unit which is later wired across the output socket. Step 3: install the two diodes and the four capacitors. If you look closely at the diodes, you'll see a black band at one end. Install the diodes so that the black band faces towards the side of the board with all the PC pins. Check the wiring diagram on this point. Two of the capacitors are pigtail electrolytics while the other two are metallised polyester (greencaps ). Make sure that you install the electros with correct polarity. The greencap values are listed with the wiring diagram, together with their alternative markings in the IEC code and the EIA code. Step 4: install the three integrated circuits and the BC549 transistor. Fitting the ICs is simple enough but Hundreds of other items not listed - Send 40c postage stamp for list ~□~□ ©D=[]&~iJ&[m 122 PITT ROAD, NTH CURL CURL MAIL ORDERS: BOX 156. DEE WHY. NSW 2099 TELEPHONE (02) 905 1848 SUPER SPECIAL ) FM STEREO KITS 1;~m i_ .- All three modules supplied are fully assembled and aligned. Circuit diagram supplied. 1! ONLY $22 g ' -~1\~ ~~ - -~ ~ I Osc coils . . . 7Sc Transistors C AD 16 1-162 . . $3 pair I I AD 149 . 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The PC board itself can be used as a template for its four mounting holes and so can the clamp Antenna $1 extra ~ S? make sure they are oriented correctly. Don't fit the transistor simply by inserting it for "best fit ". You have to rotate the transistor so that the emitter, base and collector leads match up with the holes in the same way as shown on the wirmg diagram. This is necessary because the board was originally designed for a transistor with a BCE pinout while the BC549 has an EBC pinout. That's a trap for young players, so watch it. Step 5: carefully inspect your work and compare the completed board against the wiring diagram. Step 6: drill the box . This should be done with care because it affects the overall appearance of the finished product. Four holes need to p&p .2.00 ;1-4-55 _k_H_ z -lF_c_o_ils- fo_ r_v-alv_ e_rad-io-s-. - - -.- .-. -..- .- . -$1~ . [ All the wiring inside the Syntom should be run using ribbon cable to make a neat job. Note that one of the pots has a switch on the rear which is not used. The crystal earpiece is glued in position using contact adhesive. '! ~ Dynamic microphone Desk type S4 Pfl S1 1-----1 ~ TEISCO Ceramic Microphone ~ 1 ~ 1 - -= = = : : :_:-_ _ P_P..-----''-s_2_PP_s_1_---l f 5 mixed rotary switches 12 mixed switches v• ~ $2.50 $4.50 '=~ ~ ~ Touch micro swi tches Microswitches 5A 250V ~ ~ 4 for $1 sac .. 1· Mixed resistors; handy Mixed capacitors: handy types types ~ 100 for $2 100 for $2 ;;;· ~ 1 - - - ~ - - - - + - - - _ ; __ _;__---l~ TV stick rectifiers Transistor earpiece. 0 c $1 each 4 for $1 plug & lead en U'.) 11---------'----------I! Slfder Pots . 250k dual 250k single 50k single 1Ok single 2 5k dual 1 k dual ½ Meg dual VALVES 6V6 8KB 6B06 6B05 EF86 S10 $10 S10 S7 S7 single 50c; dual $1 1 Meg dual 2 Meg dual 2 Meg single $7 S7 S6 S6 S7 S7 5AS4 EF86 68MB 6BL8 6U7 6805 6KB 6U7 6K7 6A7 $7 $10 $7 $7 $10 6V.t SS 12AX7 i? 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"ll .~ ~ ~ ~ ~ .,. i? lnterslate S4.40 ~ 1------------------1 { VALVE SOCKETS OCTAL: 7 PIN , 9 PIN . 4 FOR $1 s .§ Hundreds of other items not listed - Send 40c postage stamp for list PARTS LIST 1 plastic utility case, 158 x 95 x 52mm 1 front panel label 1 aluminium clamp assembly 1 PC board, 1 00 x 40mm 1 crystal earpiece 1 6.5mm mono jack socket 1 miniature SPST toggle switch 4 knobs to suit potentiometers 1 9V battery 1 battery snap connector 1 battery clamp (or secure battery with double-sided tape) 4 plastic PC mounts (or pillars, screws and nuts) 14 PC pins Semiconductors 1 741 opamp(IC1) 1 LM324 quad op amp (IC2) 1 CA3080 transconductance amplifier (IC3) 1 BC549 NPN transistor (01) 2 1 N914, 1 N4148 small signal diodes (D1 , D2) Capacitors 1 4 7 µF 1 6VW pigtail electrolytic 2.2µF 16VW pigtail electrolytic .004 7µF metallised polyester (greencap) .001 µF metallised polyester (greencap) The Syntom has four controls to give a wide variety of percussive effects. The on/off switch and output socket are mounted on a side panel. assembly. The power switch and the jack socket are installed on one side of the case, as shown in the wiring diagram. Step 7: install the PC board using plastic PC mounts or with pillars, screws and nuts. PC mounts do not mate well with the plastic box so we prefer the latter method as it is more secure. Step 9: fix the label to the lid of the case. Future kits may come with the lid already silk screened so this step may not be necessary. Step 10: cut the potentiometer shafts to about 12mm long and then install them on the lid. Install the 24 SILICON CHIP power switch and 6.5mm jack socket in the case. Step 11: glue the crystal earpiece to the inside bottom of the case using a contact adhesive. Don't use an epoxy for this job as it won't stick to the smooth plastic of the case. Step 12: install all the wiring from the PC board to the pots and other hardware. We used ribbon cable as it gives a neat job. The wiring diagram shows all the details. Step 13: connect the 9V battery to the circuit and check the voltages. With the negative prod of your multimeter on the midpoint (0V} of the supply (TPl}, check that + 4.5V Resistors (0.25W, 5%) 1 1 MO 5 1 OkQ 1 330kQ 1 8 .2kQ 1 220kQ 1 4. 7kQ 5 100kQ 1 1.8kQ 8 47kQ 1 4700 1 22kQ 2 2200 1 1 MO log potentiometer 3 50kQ linear potentiometers Miscellaneous 30cm ribbon cable, solder, contact adhesive, screws, nuts, washers . Note: a full kit for this project is available from Jaycar Electronics for $42.50 . is present at pin 7 of ICl, pin 4 of IC2 and pin 7 of IC3. Check that - 4.5V is present at pin 4 of ICl and pin 11 of IC2. Because of the bias currents drawn from the 0V rail, you may find that the + 4.5V rail is closer to + 5.2V while the - 4.5V rail is closer to - 4V. These readings will be dependent on the settings of the controls. Pin 4 of IC3 is supplied via a 1.8kQ resistor from the - 4.5V rail and is likely to be around - 2.3V with respect to TPl. Rotate the Pitch and Sweep controls fully anticlockwise which stops the VCO from operating. The outputs of ICl (pin 6} and IC2 (pins 1 and 14} should now all be low; ie, around - 4V. Pin 7 of IC2 should be high (ie, around + 4V} while pin 8 should be around - 2V. Pin 6 of IC3 should be at 0V. With the pitch control above its minimum anticlockwise setting, the VCO should be operating continuously. If you have a frequency meter or oscilloscope, you can check its range by measuring the output at pin 1 or pin 7. If you want to, you can also listen to the output by connecting the crystal earpiece to pin 7 (the junction of the two 47k0 resistors would be a convenient connection point). To do this you would have to disconnect one side of the crystal earpiece from the input at pin 2 of ICl. Once you have completed all those checks, your Syntom is ready for use. ~