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NEW, IMPROVED THEREMIN This design is an upgrade of our most popular Theremin which was featured in the August 2000 issue. We have added a voicing control, incorporated a larger loudspeaker and increased the power output. We’ve also changed the power supply to avoid problems with switchmode DC plugpacks. By JOHN CLARKE I f you are not familiar with the Theremin, we should and electronically synthesised music in general. Even togive a brief rundown on this most unusual instrument. day that fascination with producing sounds electronically It is an electronic musical instrument that can be altered is still prevalent. If you are interested in the history of the Theremin there is more information in the section headed in pitch and volume using proximity effects. To play the instrument the right hand is moved horizon- “Theremin Origins”. The Theremin invention was not only instrumental in tally toward the antenna to increase the pitch and away from the antenna to reduce the pitch. Left hand movements the development of electronic music; it also had an impact over the horizontal plate provide volume control. The vol- on a free-form style of playing music. The free-gesture ume is reduced as you move your hand closer to the plate. hand control afforded by the Theremin was the harbinger In operation, the pitch change afforded by the antenna of the modern Sensor Chair synthesiser controller where is infinitely variable over several octaves. In some ways the whole body becomes a part of the musical generation this is similar to playing a trombone whereby the slide is process. Before this, Jimi Hendrix was creating new sounds by moved back and forth to vary the pitch. Although most people can play the instrument at first attempt, an ear for generating feedback between his guitar and the amplified pitch and fine hand control are essential requirements to sound and then moving his body to modulate the amplitude. It freed him from the restriction of generating music become proficient at playing the Theremin. Several fine performances by Peter Pringle using a solely by plucking the guitar strings. In recent times there has been quite a renewal of interest Theremin are presented at www.peterpringle.com/thereminmp3s.html. These demonstrate only a fraction of what in the Theremin and there is a lot of information on the can be accomplished with a Theremin in the hands of a Internet. However, none of it is really helpful if you want skilled musician. The same website has links to YouTube to build your own Theremin. This is where the SILICON CHIP Theremin comes into performances, some of which are quite remarkable. The fascination with the instrument, when it was first the picture. It uses just three low cost ICs and a handful of invented by Leon Theremin in 1919, was that it repre- other components. Our Theremin is considerably smaller than the original design too, although you could build it sented a revolutionary change in thinking about how music into a larger timber box could be produced. It if you prefer. challenged traditional Specifications As noted above, this stringed, brass and Power requirements...........12VAC at 250mA minimum or 12V DC at 250mA design is an upgraded percussion musical Current draw: ....................30mA with no sound from loudspeaker, version of our very popuinstruments. Its de-            up to 200mA at full volume. lar Theremin that was sign eventually led to Output frequency range.....Greater than five octaves, from 50Hz to >2kHz published in August the development of Volume control range.........>60dB 2000. Interest in that the Moog Synthesiser Audio output level..............600mV RMS 30  Silicon Chip siliconchip.com.au siliconchip.com.au March 2009  31 LOUDSPEAKER 455kHz REFERENCE OSCILLATOR (Q1, T1) VR2 VOICE ADJUST ROD ANTENNA SIGNAL f1 PRODUCT DETECTOR/MIXER (IC1) CARRIER f2 f2 + f1 f2 – f1 LOW-PASS FILTER (IC2a) f2 – f1 BIAS AMP (IC3) VR1 VOLUME LINE OUTPUT PITCH OSCILLATOR (Q2,T2) DISC ANTENNA AMPLIFIER & LEVEL SHIFTER VOLUME OSCILLATOR (Q3,T3) BAND-PASS FILTER (Q4,T4) DETECTOR AND FILTER (D6) IC2b VR3 RANGE project far exceeded expectations and kits are still being built in Australia and around the world in large numbers. Compared to the August 2000 design, this latest Theremin is easier to construct, with minimal wiring, and it also includes a larger internal loudspeaker. Our new Theremin is built into a medium-sized plastic box with the antenna and volume plate mounted at opposite ends. The internal loudspeaker is for practice sessions and a line output is included for connection to a sound system. The only manual controls are an On/Off switch and Volume control for the loudspeaker. A 12VAC adaptor powers the Theremin. Our original Theremin (August 2000) was designed to run from a 9VDC plugpack. However, DC plugpacks now being sold are switchmode types and these do not work well with a Theremin. They can produce extraneous pitch changes, because a Theremin relies on very small capacitance changes with respect to earth (ground) to alter the pitch of the tone. Switchmode plugpacks effectively ground capacitance either at the switching rate and/or the mains frequency (50 or 60Hz). This will cause a Theremin to misbehave unless the power supply earth is fully grounded. The way around this dilemma is to change the supply to use a plugpack with an AC output. In this case, there is no internal switchmode circuitry and therefore no extraneous pitch changes. Voicing Our August 2000 design produced a fairly pure sine- This shows the Theremin output at 100Hz with the minimum setting for the voice. The sound is relatively pure and similar to the original August 2000 Theremin. 32  Silicon Chip Fig.1: the Theremin circuit comprises three virtually identical oscillators, two to generate the tone and one to control the volume. wave tone that was not fully characteristic of a Theremin, which should have a richer harmonic content. Hence, the new design includes a voicing control. This varies the tone from a pure sinewave to something akin to a cello sound at low frequencies, extending to soprano voice at the higher frequencies. Below and on the facing page, we have included a number of screenshots from Nero Wave Editor (see separate panel to see how we did it!) to show the typical waveforms available from our new design. How it works The block diagram of Fig.l shows the basic arrangement of the Theremin circuit. It comprises three oscillators that all operate at about 455kHz. A beat signal is generated by mixing the reference and pitch oscillators together to produce an audible tone. The volume oscillator is then used to change the level of the tone output. The reference oscillator operates at a fixed frequency and is mixed with the pitch oscillator in the product detector (IC1). The pitch oscillator changes in frequency depending upon the amount of capacitance to earth presented by your hand near the antenna. The product detector essentially mixes the reference oscillator (f1) with the pitch oscillator (f2) to produce sum (f1 + f2) and difference (f2 - f1) frequencies. The sum (f1 + f2) signal is around 900kHz and is removed with a low-pass filter. After filtering we are left with the difference signal of f2 - f1. This normally comprises audio frequencies from 2kHz down to below 10Hz. Now the same 100Hz frequency but with the voicing adjusted to maximum. The sound produced by this waveform is more reminiscent of a cello. siliconchip.com.au So if the pitch oscillator frequency is 456kHz and the reference oscillator is at 455kHz, we will obtain a 1kHz audio output from the low-pass filter. If both the pitch and the reference oscillators are at the same frequency, then there will be no audio output. Varying the coupling between the pitch and reference oscillators provides for voicing. When the pitch oscillator frequency differs from the reference oscillator we obtain an output tone and the difference in frequency between the two oscillators tends to “pull” or distort the f2-f1 wave shape so that it is not a sinewave. Potentiometer VR2 allows adjustment of the coupling and its consequent waveform distortion or voicing. Audio output from the low-pass filter is applied to a power amplifier to drive a loudspeaker. The overall volume from the amplifier is set by the volume control VR1. The sensor plate or disc controls the volume oscillator. As you bring your hand closer to the loop, the frequency of the volume oscillator decreases. This is fed to a bandpass filter that has a centre frequency (fc) that is higher than the volume oscillator frequency. As the frequency of the volume oscillator increases, the level from the bandpass filter will also increase as it approaches the centre frequency. Similarly, as the frequency of the volume oscillator decreases, the level from the bandpass filter will also decrease. Fig.2 shows the output of the bandpass filter in response to the change in volume oscillator frequency. This signal level is detected using a diode and filtered to produce a DC voltage. Amplifier IC2b increases the DC voltage and the level shifter sets the voltage so that it can control the product detector output level over a suitable range via its bias input. Changing the biasing of IC1 alters the gain of this product detector. OUTPUT LEVEL OUTPUT RANGE VOLUME OSCILLATOR FREQUENCY RANGE fc FREQUENCY (CENTRE FREQUENCY) Fig.2: the output of the bandpass filter in response to the change in volume oscillator frequency. As the frequency of the volume oscillator increases, the level from the bandpass filter will also increase as it approaches the centre frequency of the filter. Similarly as the frequency of the volume oscillator decreases, the level from the bandpass filter will also decrease. to ensure oscillation. To make them controllable by hand capacitance, the Pitch and Volume oscillators have the pitch antenna and volume disc attached to the top (ie, active end) of the tuned coils, where they will have the most effect. Diode coupling The full circuit of the Theremin is shown in Fig.3. It comprises three JFETs (Junction Field Effect Transistors), four pre-wound IF (Intermediate Frequency) transformers, three ICs, several diodes, a 3-terminal 9V regulator and associated resistors and capacitors. All three oscillators are essentially identical. Each oscillator comprises a JFET and a standard IF transformer. These IF transformers are commonly used in low-cost AM radio receivers. Each IF transformer comprises a tapped main winding and a parallel connected capacitor to form a tuned circuit. The secondary winding couples the oscillator signal to the following circuitry. Each JFET drives a portion of the primary winding (ie, between the tapped connection at pin 2 and ground) while the signal across the full winding is applied back to the gate via a 68pF capacitor. This provides the positive feedback Diode D5 connects the signal from pin 2 of transformer T1 (reference oscillator) to pin 2 of transformer T2 (pitch oscillator) via 10nF capacitors. The diode is used as part of a capacitive divider with the 10nF capacitors whereby its junction capacitance varies with the applied reverse voltage across it. This reverse voltage is provided by trimpot VR1 and can be adjusted between 0 and 9V. The diode anode (A) connects to ground (0V) via a 100kΩ resistor while the cathode (K) connects to the wiper of VR2 via another 100kΩ resistor. With VR2 wound fully up to the 0V supply, there is no reverse voltage across D2 and this provides the maximum capacitance across the diode and therefore maximum coupling between the two oscillators. When VR2 is wound fully toward the positive 9V supply the diode is reverse biased and provides minimal capacitance. Maximum capacitance of the diode is very small, at around 4pF, but this is sufficient to produce the coupling action required. The reference oscillator is applied to the signal input (pin 1) of an MC1496 balanced mixer, IC1. The pitch oscillator signal is attenuated using a 3.3kΩ resistor and the 1kΩ resistor before being applied to the carrier input at pin 10. At 400Hz the above waveform with minimum setting for the voicing has a near sinusoidal shape and sounds ‘pure’ in tone. While at the maximum voicing setting for 400Hz, the waveform is not so pure and has the tonal characteristic of a soprano voice. Circuit details siliconchip.com.au March 2009  33 34  Silicon Chip siliconchip.com.au 3 3 2 1 10nF 100k G S D S D THEREMIN 100k 68pF Q3 2N5484 G VOLUME OSCILLATOR 100k 68pF Q2 2N5484 1k 1k 100k A 100nF 100nF +9V 100nF D5 1N4148 K S PITCH OSCILLATOR 100nF +9V 100k G D 1k 100k 220pF 680 C Q4 BC548 10 8 4 1k 3 2 1 GAIN CAR– 100nF T4 (BLACK) 4 +9V BIAS 5 A 2.2k 1k 2.2 F K A K A 1 1N4148 100nF 220k IC2a 1N4004 2 3 100k VR1 10k LOG A K A K GND IN 100nF 4 VOLUME A K A K D1– D4: 1N4004 6 1 7809 4 7 8 5 E G S C BC548 D 2N5484 B 10 LINE OUT 12V AC IN 8 SPEAKER CON2 470 F 47nF 10k GND 100 F 470nF CON1 150 POWER S1 470nF 10 F IC3 LM386N OUT 2 3 7 IC2: LM358 8 IC2b 10 F 6 5 +9V 470 F 25V IN 100k GND OUT REG1 7809 220nF 100nF SLOPE DETECTOR 47nF 2.2k 100 F 16V 10 F TP3 RANGE TPG TP2 47nF 6.8k VR3 2k 2.2k 10k K TP1 1k D6 1N4148 +9V 6 Vss 14 12 OUT+ 6 OUT– 3 2 +9V GAIN IC1 MC1496 CAR+ SIG– SIG+ MIXER BANDPASS FILTER 1k 470pF 100 3.3k E 1k 1 100 F 100nF 1k 820 100nF B 220pF 330k 10nF 220pF 1.2k Fig.3: a few subtle but very important changes have been made to the original Theremin circuit. These are explained in the text. SC 2009 4 6 T3 (WHITE) 4 2 1 VOLUME ANTENNA (DISC) T2 (WHITE) 6 3 2 68pF 1 PITCH ANTENNA (ROD) VR2 VOICE 10k 4 6 T1 (WHITE) Q1 2N5484 REFERENCE OSCILLATOR +9V This reduction in signal level is to prevent overloading the mixer stages of IC1. Resistors between the +9V supply and ground set the bias voltages for the inputs of the balanced mixer, while the 1kΩ resistor between pins 2 & 3 sets the gain of the circuit. IC1 provides a balanced output with signals at pins 6 and 12. These complementary outputs are filtered with a 2.2kΩ pullup resistor and 47nF capacitor to produce a rolloff above about 1.5kHz. This rolloff heavily attenuates frequencies at 455kHz. The output from pin 12 is AC-coupled to op amp IC2b. IC2b is biased at half-supply using the two 100kΩ voltage divider resistors across the 9V supply. This biasing allows the op amp to produce an output of 600mV (~850mV peak) above and below 4.5V without clipping. IC2b’s output signal also goes to the line output terminal. IC3 is an LM386 1W amplifier that drives the loudspeaker via a 470μF electrolytic capacitor. The 47nF capacitor and series 10Ω resistor form a Zobel network to prevent spurious oscillation from the amplifier. Volume oscillator Output from the volume oscillator at the secondary winding of T3 is AC-coupled to the base of transistor Q4. This is connected as a common emitter amplifier with the collector load being a parallel-tuned circuit comprising an IF coil with internal capacitor. T4 and the associated capacitor are tuned to a frequency just above the maximum available from the volume oscillator. The emitter resistor is bypassed with a 470pF capacitor to provide a rolloff below about 500kHz. The output level from transformer T4 will vary in proportion to the frequency from the volume oscillator. This is because the filter provides a sharp rolloff below its tuning frequency and small changes in frequency below the centre frequency will cause large changes in the filter response. The action of this circuit is a simple frequency modulation (FM) detector. High frequency signal from T4 is rectified by diode D6 and filtered to provide a DC signal which is amplified by op amp IC2a. Amplification can be up to about 220 times with VR3 set at 0Ω. Typically, the gain is about 100 since VR3 is set so that IC2a’s output sits at about 7V with the hand away from the volume plate or disc. IC2a’s output is then fed via a 6.8kΩ current limiting resistor to the bias input of IC1 at pin 5 to vary the level of the audio signal. Power supply As mentioned above, power for the circuit comes from an AC plugpack. Alternatively the Theremin could be powered from a 12V battery or an earthed DC power supply. As already noted, a 12V DC switchmode plugpack supply is not suitable. Most “plugpack”-type supplies sold these days are switchmode types, so be careful with this one! Switch S1 applies power to the circuit. The 470nF capacitors on each side of the input supply ground the AC connections to swamp any capacitance effects of the plugpack to ground. This ensures there are no spurious sounds from the Theremin due to the plugpack. Diodes D1-D4 rectify the 12VAC voltage and this is then filtered with a 470μF capacitor to provide a relatively siliconchip.com.au Theremin Origins In 1919, a Russian physicist named Lev Termen (or Leon Theremin as he is called in the western world) invented an electronic musical instrument called the “Theremin”. At that time, the Theremin was innovative and unique in the musical world and was essentially the first electronic instrument of its kind. Playing it relied solely on hand movements in the vicinity of two antennas to control two electronic oscillators – one antenna to vary the pitch of the sound and the other to change the volume. In operation, the pitch change afforded by the antenna is infinitely adjustable over several octaves, with the frequency increasing as the hand is brought closer to the antenna. An ear for pitch and fine hand control are essential requirements to become proficient at playing the Theremin. To a large extent, the Theremin was made famous by recitalist Clara Rockmore. Born in Lithuania in 1911, she was an accomplished violinist by 5-years old. She began to learn to play the Theremin after meeting Leon Theremin in 1927 and ultimately developed a unique technique for playing the instrument. This technique involved minute finger movements to capture and modulate the tone of the note and enabled her to play the instrument with great precision. The Theremin was subsequently further developed and manufactured by the Radio Corporation of America (RCA) around 1929. This design consisted of a large box with an attached antenna and wire loop. The antenna provided the control for the pitch while the loop enabled the volume to be adjusted. In practice, the pitch control antenna was mounted vertically while the volume loop sat horizontally, to minimise interaction between them. And of course, the circuit used valves. General Electric (GE) and Westinghouse also made Theremins in the 1920s. However, the number of units produced was quite modest, with only about 500 being made. Today, the Theremin is hailed as the forerunner to modern synthesised music and was instrumental in the development of the famous Moog synthesisers. Because of its unique sound, it has been popular with music producers for both film and live performances. The sound is ideal for background setting the scene for supernatural events and for close encounters with extraterrestrial beings in science fiction movies. Famously, a Theremin was used to produce background music in the feature film “The Ten Commandments” by Cecil B de Mille. Its eerie sounds have also made it ideal for science fiction movies, including the 1951 and the 2008 versions of “The Day the Earth Stood Still” and “It Came From Outer Space”, and in thriller movies such as “Spellbound” and “Lost Weekend”. In addition, Bands such as the Bonzo Dog Band and Led Zeppelin have embraced the Theremin. The Beach Boys used an instrument similar to the Theremin – called an Electro-Theremin (also named a Tannerin) – in their famous “Good Vibrations” hit from the 1960s. The Electro-Theremin differs from the Theremin in that it incorporates a mechanical controller to adjust the pitch rather than hand movements relative to an antenna. The sound, however, is very similar to the Theremin. Many commercial Theremins are available on the market today, including the Etherwave series from Moog Music Inc, PaiA’s Theremax and Wavefront’s Classic and Travel-Case Theremins. SILICON CHIP has also been prolific with three previous designs for home construction – a basic Theremin in August 2000, a MIDI Theremin in April and May 2005 and the Mini Theremin in July and August 2006. March 2009  35 100k TO VOLUME ANTENNA (DISC) 2N5484 4004 4004 4004 POWER 10k 220pF 470nF 470nF CON2 LINE OUT IC3 LM386 220nF HEATSINK 470 F 16V 47nF 470pF 150 REG1 100nF 100k 100k 100 F 10 F 100 F 1 TP1 1k VR3 2k 470 F 25V 10 F 10 100nF 10k Q4 4148 100 Q3 220pF 100k BC548 1 220k TP3 D6 2.2k IC2 LM358 T4 330k 1k 47nF 100nF 100k T3 3.3k 820 1k 6.8k 1k 1k 1k 10nF 100nF 100nF 1k 68pF S1 100nF 100k 100k 4148 100 F 100nF 2N5484 10nF D5 VR2 10k AC INPUT Q2 4004 1 CON1 D1–D4 100nF IC1 MC1496 T2 1k 68pF 2.2k 1.2k 100nF 2.2k 1k 2N5484 Q1 100nF 47nF 220pF 100k 68pF T1 TO PITCH ANTENNA VR1 10k LOG VOLUME TP2 680 TPG 2.2 F 10 F 1 9 0 3 0 2 1 0 NI MERE HT SPEAKER Fig.4 (above): everything except the speaker, volume plate and pitch antenna mount on a single PC board, so construction and wiring should be quite easy. Note the differences in the transformers: three have white cores while one has a black core. This is very easy to see in the matching photo below – so don’t mix them up! smooth 16VDC supply for REG1. REG1 is a 7809 3-terminal regulator that delivers 9V to the circuit. A 220μF capacitor close to the regulator output ensures stability and several 100μF capacitors decouple the supply at positions further away on the PC board. Construction Most of the parts for the Theremin are assembled onto a PC board, coded 01203091 and measuring 147 x 85mm. This is housed in a plastic utility box measuring 158 x 95 x 53mm. While our assembly description revolves around the plastic case with its small speaker, there is no reason why 36  Silicon Chip you couldn’t build it into a much larger case in keeping with a traditional musical instrument. An external power amplifier and loudspeaker would also make a considerable improvement to the overall sound quality. Indeed, fitting it into a large box may also improve the operation – the “playability” if you like – of the Theremin. Separating the pitch (antenna) and volume (plate) controls may give you more control over both. Note that we haven’t tried this idea out but there is nothing to stop you doing it if you want. You can begin construction by checking the PC board for any defects such as shorts between tracks, breaks in the copper tracks and incorrectly drilled holes. Check the siliconchip.com.au Parts List – Theremin 2009 The control end of the Theremin – from left to right the volume control, line output socket, power switch and 12V AC power input. hole sizes for the IF transformers (T1-T4), the PC mount components including the power socket, the RCA socket and potentiometer VR1. Four corner mounting holes should be drilled to 3mm. Holes for the PC stakes should be sized to suit their diameter; they should be a tight fit. Check that the PC board fits into the plastic case and that it has clearance for the corner pillars. The PC board should have its corners shaped to provide this clearance. If this has not been done, a rat-tail file can be used to shape each corner to the outline shown on the PC board pattern. Before proceeding further, mark out the hole positions for the four corner mounting points for the PC board on the base of the case and drill these out to 3mm. The component overlay for the PC board and the wiring details are shown in Fig.4. The long link on the PC board is made using a 80mm length of hookup wire. It is cut and the end stripped so that it is held straight between the two PC pads on the PC board. You can now insert the resistors. Use the resistor table as a guide to selecting each value. In addition, use a digital multimeter to check each resistor value before it is soldered in. The three ICs can be mounted next, taking care with their orientation. Make sure that IC2 & IC3 are placed in their correct positions. Next, the capacitors can be mounted, noting that the electrolytic types are polarised and must Here’s how the pitch antenna mounts: a small cutout in the case lid allows it to be mounted to the side of the case via the screw clearly visible in this photo. siliconchip.com.au 1 PC board, code 01203091, 147 x 85mm 1 plastic utility box, 158 x 95 x 53mm 1 front panel label, 155 x 92mm 1 12VAC 500mA plugpack (do not use a switchmode 12VDC plugpack) 1 telescopic antenna, 6.5mm largest diameter (875mm fully extended) (pitch antenna) 1 80 x 95mm aluminium plate, 1mm thick (for volume) 1 PC-mount DC socket (2.5mm diameter pin) 1 panel-mount RCA socket 1 SPDT miniature PC-mount toggle switch (S1) 1 75mm 8Ω loudspeaker 3 2nd IF coils (white) (T1- T3) 1 3rd IF coil (black) (T4) 1 mini TO-220 heatsink, 19 x 19 x 9.5mm 1 knob to suit potentiometer with 2 nuts 2 solder lug eyelets 1 2-way pin header plug 1 2-way pin header socket 12 M3 x 10mm screws 3 M3 nuts 4 9mm tapped Nylon standoffs 4 stick-on rubber feet 1 260mm length of medium-duty hookup wire 1 80mm length of light duty hookup wire (wire link) 1 100mm cable tie 7 PC stakes Semiconductors 1 MC1496P balanced modulator (lC1) 1 LM358 dual op amp (IC2) 1 LM386N-1 1W audio amplifier (IC3) 1 7809 3-terminal regulator (REG1) 3 2N5484 or 2N5485 N-channel JFETs (Q1-Q3) 1 BC548 NPN transistor (Q4) 4 1N4004 1A diodes (D1-D4) 2 1N4148 signal diodes (D5,D6) Capacitors 1 470μF 25V PC electrolytic 1 470μF 16V PC electrolytic 3 100μF 16V PC electrolytic 3 10μF 16V PC electrolytic 1 2.2μF 16V PC electrolytic 2 470nF MKT polyester 1 220nF MKT polyester 10 100nF MKT polyester 3 47nF MKT polyester 2 10nF MKT polyester 1 470pF ceramic 3 220pF ceramic 3 68pF ceramic Resistors (0.25W, 1%) 1 330kΩ 1 220kΩ 8 100kΩ 2 10kΩ 1 3.3kΩ 3 2.2kΩ 1 1.2kΩ 9 1kΩ 1 680Ω 1 150Ω 1 100Ω 1 10Ω 1 10kΩ log 16mm potentiometer (VR1) 1 10kΩ horizontal trimpot (VR2) 1 2kΩ multi-turn top trim trimpot (VR3) 1 6.8kΩ 1 820Ω March 2009  37 Resistor Colour Codes No. o o o o o o o o o o o o o o 1 1 8 2 1 1 3 1 9 1 1 1 1 1 Value 330kΩ 220kΩ 100kΩ 10kΩ 6.8kΩ 3.3kΩ 2.2kΩ 1.2kΩ 1kΩ 820Ω 680Ω 150Ω 100Ω 10Ω 4-Band Code (1%) orange orange yellow brown red red yellow brown brown black yellow brown brown black orange brown blue grey red brown orange orange red brown red red red brown brown red red brown brown black red brown grey red brown brown blue grey brown brown brown green brown brown brown black brown brown brown black black brown be oriented with the correct polarity, as shown in Fig.4. The MKT and ceramic types are coded and you can crosscheck these codes against the values shown in the table opposite. PC stakes are used for the antenna and volume disc connections, for the test points TP1-TP3 & TP GND and for securing VR1 to the PC board. These can be inserted and soldered in now. In addition, the 2-way pin header for the loudspeaker connection can be inserted now. Transformers T1-T4 are mounted as shown. Be sure to place the ones with the white slugs (the threaded ferrite 5-Band Code (1%) orange orange black orange brown red red black orange brown brown black black orange brown brown black black red brown blue grey black brown brown orange orange black brown brown red red black brown brown brown red black brown brown brown black black brown brown grey red black black brown blue grey black black brown brown green black black brown brown black black black brown brown black black gold brown core) in the Tl-T3 positions and the coil with the black slug in the T4 position. Now mount JFETs Q1-Q3 (2N5484), transistor Q4 (BC548) and the 7809 3-terminal regulator, REG1. 1N4004 diodes DI-D4 and the 1N4148 types for D5 and D6 can be mounted next, taking care with their orientation. REG1 can be installed after the heatsink is attached to the metal tab, using an M3 x 10mm screw and nut. Trimpots VR2 and VR3 can be mounted now. Potentiometer VRl may require the shaft to be cut to length to suit the knob. The potentiometer is mounted in position as shown How it all goes together. The PC board needs to have its corners shaped to fit around the case pillars while the volume plate and pitch antenna fasten to the sides of the case. Note the case lid cutout for the pitch antenna. 38  Silicon Chip siliconchip.com.au CL 95 38 31 21 23 24 29 30 32.5 B C D HOLE B: 6.5mm DIAMETER HOLE C: 10mm DIAMETER E 80 HOLE D: 5mm DIAMETER HOLE E: 6mm DIAMETER END OF UB1 BOX CUTOUTS Fig.5: the “Controls” end of the box showing the cutouts required. Hole B is for the volume pot, C the line out, D is for the power switch and E 12V AC power in. 90  BEND A and is supported using two PC stakes just behind the potentiometer body. Scrape or file off the passivated coating on the potentiometer body just at the positions where the PC stakes make contact. This will allow the PC stakes to be soldered to the potentiometer body. The soldering holds the potentiometer secure and the lower PC stake earths the potentiometer body to the circuit ground. Attach a nut to the pot securing thread. This is used as a spacer between the box and pot. Next, the power socket, switch (S1) and the RCA socket can be inserted and soldered in place. With the PC board complete, you are ready to work on the case. You will need to drill holes in the sides of the box for the DC panel socket, the RCA socket and for the antenna securing screw. Volume plate The volume plate is made from 1mm gauge aluminium, shaped as shown in Fig.6. One end is bent over at right angles so it can be secured to the side of the box using M3 screws and nuts. Mark out and drill the holes required in the side of the box. The aluminium disc is connected via a lead and solder lug to the PC board. The antenna is attached to the side of the case with a A 15 6 14 15 22 15 14 80 HOLES A ARE 3mm DIAMETER MATERIAL 1mm ALUMINIUM ALL DIMENSIONS IN MILLIMETRES Fig.6: the 1mm thick aluminium volume plate. Both these diagrams are reproduced same size. 10mm long M3 screw that screws into the tapped base of the antenna. Mark and drill the hole in the side of the box. An eyelet is clamped between the box and antenna to make the wire connection to the PC board. Note that the lid of the box will require a half-circle cutout out to accommodate the antenna. Mark and drill out the holes in the end of the box for the potentiometer, RCA socket, power switch and power socket as shown in Fig.5. Mount the PC board onto four 9mm tapped spacers using four 10mm M3 screws. Slide the PC board into the box so that the PC-mount components enter the holes and then push the opposite edge of the PC board down into the box. Secure the PC board to the box with four M3 x 10mm screws into the 9mm standoffs from beneath the box. Attach the four rubber stick-on feet. Attach the volume disc using two M3 x 10mm screws Capacitor Codes Looking inside the open case, this photo shows how the volume plate is secured. siliconchip.com.au Value 470nF 220nF 100nF 47nF 10nF 470pF 220pF 68pF μF value 0.47μF 0.22μF 0.1μF .047μF .01μF NA NA NA IEC Code 470n 220n 100n 47n 10n 470p 220p 68p EIA Code 474 224 104 473 103 471 221 68 March 2009  39 Displaying the Theremin waveforms The waveforms on pages 32 and 33 were recorded using Nero Wave Editor. Similar waveforms can be displayed using the NCH Wavepad or Audacity software or similar. The signal from the Theremin was connected to the line input of the computer. An adaptor lead (RCA plug to 3.5mm stereo jack) is required to connect the Theremin output to the computer input. The recording will be in either the left or right channel as the single RCA output will only connect to one channel or the other. In Windows XP, the signal levels are set to prevent clipping of the signal, by selecting <Control Panel>, <Sounds and Audio Devices Properties>, <Sound Recording Volume> and then adjusting the Line In slider. The internal speaker is secured to the lid with silicone sealant or other suitable adhesive. The level meter on the Nero Wave Editor Recording Console shows the signal reading and the volume is set for signal level below the 0dB maximum. Signal is then recorded using a 16-bit 44.1kHz sample rate. and nuts and with the eyelet for the volume disc wire clamped under a nut. The loudspeaker is centrally mounted on the lid which has a pattern of holes to let the sound out. We used 9 x 6.35mm holes with one in the centre and eight spaced evenly on a 16mm radius. The loudspeaker is attached to the lid of the case using contact or other suitable adhesive. Before affixing it, make sure that when the lid is placed in position, the speaker terminals face towards the 2-way connector on the PC board. The loudspeaker is wired to the 2-way header socket on the PC board using two 70mm lengths of hookup wire. A cable tie wrapped around the wire and under the steel speaker frame at the loudspeaker connector will help prevent the wires breaking away from the connector. Setting up The recorded signal can then be expanded out to see the waveform in detail using the zoom in feature. When your Theremin is complete, check your work carefully. Apply power and check that there is 9V between TP1 and TP GND (the voltage could range from between 8.75 and 9.25V). Using a suitable alignment tool, wind the slug for T2 clockwise until there is resistance to movement (do not force it). Then count the number of turns to wind it out anticlockwise completely. Set the slug half way between the two extremes. Volume alignment You must carry out the volume and pitch adjustments away from the effects of metallic objects, otherwise the Theremin will require retuning when removed from these grounding sources. Wind the slug for T3 fully anticlockwise and then out again, counting the number of turns. That done, set T3 about 30% of turns anticlockwise. This will set the frequency of the volume oscillator (T3) to below the frequency of the pitch oscillator (T2) to prevent extraneous sounds that can be emitted if the two oscillators are close in frequency. Now adjust T4’s slug (with your hand and any multimeter wires away from the volume disc) so that there is 2.5V between TP GND and TP2. Move your hand close to the volume disc and the voltage should drop in value. If 40  Silicon Chip siliconchip.com.au This photo of the completed Theremin clearly shows the volume control “plate” on the left side, the pitch antenna at the back (which really is a telescopic whip antenna!) in this case folded down 90° and almost fully contracted, along with the holes drilled in the case top and panel for the sound to get out. the voltage goes up, readjust T4’s slug anticlockwise with your hand away from the disc. Adjust it until the voltage increases and then drops back to 2.5V. Measure the voltage between TP3 and TP GND and set VR3 so that there is 7V, with your hand away from the volume disc. Check that the voltage falls to 0V for a reasonable range of hand movement over the plate. You can change the range of volume control by adjusting the setting of the slug in T4. Setting T4 so there is more than 2.5V at TP2 with your hand away from the plate will reduce the overall volume range while setting the TP2 voltage to less than 2.5V will increase the overall range. Note that the TP3 voltage will have to be set to 7V again using VR3 (with your hand away from the disc) after setting T4’s slug to give a new value at TP2. Pitch Alignment Now you are ready to align the pitch control. Set the volume potentiometer (VR1) slightly away from minimum setting. Set the telescopic antenna so that just the two larger sections are extended. Using a suitable alignment tool, rotate the slug in transformer T1 slowly until a tone is heard in the loudspeaker. Then adjust it to obtain a good frequency range when your hand is brought near to the extended antenna. The note should be at its highest when your hand is close to the antenna and should fall to a very low frequency (just a growl) when your hand is taken away. If the effect is the reverse of this (lower frequency as your hand is brought close to the antenna) then readjust the slug until the effect is correct. Adjust VR2 so you obtain the required sound from your Theremin. Note that adjustment at the fully clockwise setting will cause the pitch to lock to the reference oscillator for some movement of the hand before it snaps into sound. You can now adjust the tuning of the Theremin by carefully adjusting the antenna length from its normal length of two fully extended largest sections. siliconchip.com.au Placing the loudspeaker and lid in position will change the tuning slightly, although adjusting the antenna length should be sufficient to retune correctly. If hand control over volume is affected, then readjust this tuning. Note that if you connect the Theremin to an amplifier, the extra grounding will affect the tuning but adjustment of the antenna length should correct this. SC Radio, Television & Hobbies: the COMPLETE archive on DVD This remarkable collection of PDFs covers every issue of R & H, as it was known from the beginning (April 1939 – price sixpence! right YES! through to the final edition of R,TV&H in NA MORE THA March 1965, before it disappeared Y R U ENT forever with the change of name to EA. QUARTER C ICS ON For the first time ever, complete and in one OF ELECTR handy DVD, every article and every issue ! Y R O T IS H is covered. If you’re an old timer (or even a young timer!) into vintage radio, it doesn’t get much more vintage than this. If you’re a student of history, this archive gives an extraordinary insight into the amazing breakthroughs made in radio and electronics technology following the war years. And speaking of the war years, R&H had some of the best propaganda imaginable! Even if you’re just an electronics dabbler, there’s something here to interest you. • Every issue individually archived, by month and year • Complete with index for each year • A must-have for everyone interested in electronics Please note: this archive is in PDF format on DVD for PC. Your computer will need a DVD-ROM or DVD-recorder (not a CD!) and Acrobat Reader V6 (free download) to enable you to view this archive. This DVD is NOT $ playable through a standard A/V-type DVD player. Exclusive to SILICON CHIP ONLY 62 + $ 00 7 P&P HERE’S HOW TO ORDER YOUR COPY: BY PHONE:* (02) 9939 3295 9-4 Mon-Fri BY FAX:# (02) 9939 2648 24 Hours 7 Days <at> BY EMAIL:# silchip<at>siliconchip.com.au 24 Hours 7 Days BY MAIL:# PO Box 139, Collaroy NSW 2097 * Please have your credit card handy! # Don’t forget to include your name, address, phone no and credit card details. BY INTERNET:^ siliconchip.com.au 24 Hours 7 Days ^ You will be prompted for required information March 2009  41