Team Theremin 1

Analog Theremin Using Vacuum Tubes and Frequency Detection

via Band-pass Filters

Matt Britt, Ryan Adams, William Findley Jr., Yuri Yelizarov, James Lewis

Georgia Institute of TechnologySchool of Electrical and Computer Engineering

March 4, 2008 and March 6, 2008

Team Theremin 2

Analog Theremin and Frequency Detector

• The Theremin is an electronic musical instrument controlled by interaction with volume and pitch antennae.

• The frequency detector is designed to assist new players in playing on the musical scale.

• The target customers are musicians.• The projected cost of the Theremin is $550.• The projected cost of the frequency detector is

$145.

Team Theremin 3

Theremin: Technical Objectives

• Designed using vacuum tubes in order to suit the preference of musicians.

• Safe to use.

• Dynamic pitch and volume ranges to provide sufficient playability.

• Tuning controls to allow adjustments in player stance.

• Compatible with standard audio equipment.

• Finished case design in a form suitable for a final product.

Team Theremin 4

Theremin: Block Diagram

Team Theremin 5

Theremin: Technical Specifications

• Output of the pitch circuit will cover a 30 Hz to 1 kHz range of sound.

• Volume circuit will produce a control voltage for the voltage-controlled amplifier.

• Power supply circuit will provide two power rails: 6.3 VAC at 2.5 A and 75 VDC at 10 mA.

• The output will use a standard audio equipment line-out, max 1.4 Vrms.

Team Theremin 6

Pitch Control Overview

• RF signals generated by Colpitts oscillators at ~500 kHz

• Mixer is a triode biased in non-linear region• Low-pass filter is a two-pole RC cascade

Team Theremin 7

Pitch Control Circuit

Pitch Reference Oscillator

Pitch Variable Oscillator

Mixer

Low-pass Filter

Team Theremin 8

Pitch Control Performance

• First versions– High harmonic output– Unable to produce tones

below 500 Hz– Smaller effective pitch

range

• Current version– Near-sinusoidal output

from oscillators– Able to produce low

frequencies– Strong RF signal difficult to

isolate

Team Theremin 9

Volume Control Overview

• Uses another RF oscillator at 480 kHz• High-Q band-pass filter using ceramic resonator• Peak detector circuit via level shifter and rectifier• Voltage controlled amplifier in form of a differential

amplifier

Team Theremin 10

Theremin Design Approach

• Research possible circuits

• Hand design using published and simulated I-V curves

• Simulation using phenomenological triode SPICE model– Imperfect– Model better for

“high” plate current Published plate characteristics for 12AU7A twin triode. Circa 1956.

Linear amplifier

Nonlinear mixer

Team Theremin 11

Frequency Detector Overview

• Objective:– Indicate to the user which output is being used by the Theremin.

• Problem:– The Theremin produces a wide range, continuous signal.– With most instruments, a user knows exactly what pitch is

produced.– The Theremin relies on the musician’s ear and muscle memory.

• Solution:– An external system that lights up an LED corresponding to an

output frequency produced by the Theremin.– An LED is provided for each pitch of a chromatic scale.

Team Theremin 12

Frequency Detector Block Diagram

Team Theremin 13

Frequency Detector Building Blocks

• Gain = (Z1 + Z2)/Z1

• Increasing the impedance Z2 or decreasing the impedance Z1 will increase the gain.

• Increasing the impedance Z1 or decreasing the impedance Z2 will decrease the gain.

Team Theremin 14

Frequency Detector Notch Filter Simulation

Team Theremin 15

Frequency Detector: Combining Elements

R1 = 10 kΩ

RL = 1 kΩ

L1 = 3 mH

C1 = 43.6 µF

Op Amp Gain: 1000 V/V

Input Signal: 1 Vrms

Signal Range: 400 Hz – 500 HzNumber of Points: 500

Team Theremin 16

Frequency Detector Simulation

Team Theremin 17

Demonstration Plan

• Measure DC voltage of the antennae, should be less than a few millivolts.

• Demonstrate pitch range, straight antenna.– Keep volume hand steady and change

position of pitch hand.

• Demonstrate volume range, loop antenna.– Keep pitch hand steady and change position

of volume hand.

Team Theremin 18

Problems: RF Propagation

• RF propagation issue:– High power, 5 Vpp, signal coming out of the

oscillators may cause coupling and propagation along power rails.

• Alleviation:– Careful routing of traces and wiring on PCB

and placement of decoupling capacitors.– Reduced signal on DC power rail from 1 V to

10 mV.

Team Theremin 19

Problems: Mixer Design

• Nonlinear design issue:– Nonlinear mixer behavior is difficult to predict.– The practical results differ greatly from

simulation.

• Alleviation:– Trial and error used to get current result.– A curve tracer may be used later.

Team Theremin 20

Project ScheduleTask Owner Status

Evaluation and ordering of tubes

Matt Britt Completed

Building prototype board Ryan Adams Completed

Oscillator design and testing Matt Britt Completed

Antenna design Will Findley Completed

Frequency detector design Yuri Yelizarov In progress

Mixer design Matt Britt In progress

Voltage controlled amplifier design and testing

James Lewis In progress

Power supply design and testing

James Lewis Not started

PCB board design and layout Will Findley Not started

Final construction Ryan Adams Not started

Team Theremin 21

Theremin Cost Analysis

• Projected sales volume of 10,000 over a four year period.

• Three group members working on Theremin portion of project.

• The cost of each Theremin unit is $550.

Description Unit Cost Overall Cost Type

Development Personnel

$36,000 Non-recurring

Development Capital Equipment

$10,000 Non-recurring

Development Overhead 200 %

$92,000 Non-recurring

Total Development Cost $138,000

Parts $200 $2,000,000 Recurring

Production $50 $100,000 Recurring

Market and Sales $100,000 Recurring

Total Recurring Cost $2,200,000

Recurring Cost Overhead, 150% $3,300,000

Adjusted Recurring Cost $5,500,000

Team Theremin 22

Frequency Detector Cost Analysis

• Projected sales volume of 20,000 over a four year period.

• Two group members working on frequency detector portion of project.

• The cost of each frequency detector unit is $145.

Description Unit Cost Overall Cost Type

Development Personnel

$24,000 Non-recurring

Development Capital Equipment

$10,000 Non-recurring

Development Overhead 200 %

$68,000 Non-recurring

Total Development Cost $102,000

Parts $50 $1,000,000 Recurring

Production $30 $60,000 Recurring

Market and Sales $100,000 Recurring

Total Recurring Cost $1,160,000

Recurring Cost Overhead, 150% $1,740,000

Adjusted Recurring Cost $2,900,000

Team Theremin 23

Current Status

• Circuit built on tube “breadboard”.

• Pitch control circuit works up to the low-pass filter.

• Band-pass filter, for volume control circuit, is designed.

• Antennae material is decided on.

Team Theremin 24

Questions?