Build the EM Theremin

1 Build the EM ThereminThis classic electronic instrument gives goodvibrations and Robert Moogost electronic musical instru-ments are sonic chameleonsthat try to sound like a widevariety of other things. How-ever, there is one electronicinstrument that makes nolapologies for its single, immediatelyrecognizable sound: the Theremin . Thismonophonic instrument has added itsdistinctive, melodic character to thescores of many horror and suspensemovies and made its pop debut on theBeach Boys' "Good Vibrations.

2 " It hasalso appeared on many concert stages,including Carnegie Theremin was named after its in-ventor, Russian physicist and musicianLeon Theremin , who developed theinstrument in the 1920s. Unlike mostmusical instruments, the Theremin isplayed with absolutely no physical con-tact. Players wave their hands in the airnear two antennas. As one hand getscloser to the straight vertical tube(called the pitch antenna), the pitchrises; as the other hand gets closer tothe horizontal tubular loop (called thevolume antenna), the volume the Theremin 's pitch andvolume are intimately tied to the play-i86 Electronic Musician February 1996materials that you can buy at your localhardware store or from mail-orderelectronic-parts distributors.

3 If youknow how to read a schematic diagram,solder, and use a voltmeter, and ifyou're comfortable with basic hometools, you should be able to Build andadjust this 's hand motions, the tone has avibrant, wavering quality, not unlike ahuman voice or a the requests for DIY projectsthat EM receives, by far the most com-mon is a do-it-yourself Theremin . Theinstrument presented here is an au-thentic Theremin , with antenna responsecharacteristics, pitch range, and tonecolor that closely emulate LeonTheremin's original designs.

4 However,it is reasonably easy to Build . It usescurrently available components andOVERVIEWWhen you bring your hand near atheremin antenna, you are actuallyforming a variable capacitor: the an-tenna is one "plate" and your hand isthe other. With the high frequenciesand very low currents used by the in-strument, your hand is effectivelygrounded by being attached to yourbody, so the antenna and your handform a variable capacitor to variable capacitance is calledhand capacitance.

5 You increase thehand capacitance by bringing yourhand nearer to the antenna. Duringnormal operation, the hand capacitanceis less than one picofarad, which is avery small capacitance indeed!Each antenna forms a resonantcircuit with a group of inductorscollectively called an antenna coil. Inthis design, the resonant frequenciesare about 260 kHz for the pitchantenna and about 450 kHz for thevolume antenna. At or near theresonant frequency, a tiny change inhand capacitance results in a largerchange in the impedance of theantenna circuit as a to Figure 1, the functional blockdiagram, and Figure 2, the schematic dia-gram of the entire circuit.

6 The variable-pitch oscillator (VPO), fixed-pitch oscillator(FPO), and detector sections form a beat-frequency oscillator. Q1, Q2, and theirassociated components constitute theVPO, the frequency of which is setThe Theremin 's main circuitry is mounted on a single prototyping board, and the two antennacircuits are mounted on their own smaller make fine adjustments to the vol-ume-oscillator frequency audio waveform is applied to pin 3of U3-A at a level high enough to clip has the effect of reshaping thewaveform from a skewed sine to a quasi-rectangular wave.

7 Which is very similar to the waveform of Professor Theremin 'soriginal instruments. P3 varies the inputresistance of U3-A, which influences theamount by which the audio waveform isclipped. P4 shifts the bias at the input ofU3-A, which changes the waveform widthand therefore the output's harmonicspectrum. C24 and C26 roll off thehigh-frequency harmonics to produce apleasant, cello-like tonal the antennas can be should be metallic, rugged,attractively finished, capable of beingrigidly mounted, and easy to fabricateby a home hobbyist.

8 I have found that3/8-inch soft copper tubing of the sortthat plumbers use with bathroom sinksworks well. You can buy preplated,straight, short pieces at your localbuilders' supply or hardware store. Youcan also purchase a simple tubingbender that will allow you to bend thevolume antenna by hand withoutcollapsing the tubing. You'll also need atubing cutter or hacksaw to cut thetubing to finished pitch antenna is astraight, vertical tube eighteen incheslong and 3/8 inch in diameter, and thefinished volume antenna is a horizon-tal, hairpin loop with a total length ofnine inches.

9 The ends of the volumeFIG. 1:This functional block diagram reveals how the Theremin DC voltage appearing at the junc-tion of D1 and C12 is reduced. The re-sulting current flowing through R14 isamplified and level-shifted by the VCAprocessor section (U3-B and associatedcomponents) and then fed throughR30 to control the gain of the voltage-controlled amplifier (U3-A and associatedcomponents). The amplitude-con-trolled audio output is then fed tofront-panel jack J1. The maximum levelis about 0 dBm ( RMS).

10 Q8 and its associated componentsconstitute the volume-tuning circuit,which is nearly identical to thepitchtuning circuit. Potentiometer P2 isusedslightly higher than the resonant fre-quency of the pitch-antenna circuit (es-tablished by adjusting L5). As a playerbrings a hand near the pitch antenna,the changing impedance of the pitchantenna circuit lowers the VPOfrequency by about 3 , Q4, and their associated compo-nents form the FPO, the frequency ofwhich is set equal to the VPO frequen-cy (by adjusting L6) when the player'shand is away from the pitch difference, or beat, frequency isextracted by the detector and appearsas an audio waveform at the junction ofR23 and R24.