EC E 490-491 Capstone Design: Analog Drum Machine

Project Overview The purpose of this project was to design and build an analog drum machine with an emphasis on serviceability and avoiding the use of specialized elec-trical componenets. What is an analog drum machine? In its simplest form, the analog drum machine consists of a series of analog circuit boards that in-dividually generate characteristic drum sounds when they are triggered to do so by a controller or sequencer. The sequencer can be manually pro-grammed by the user to generate unqiue rythms and musical compositions.

The project requirements consisted of the following:• The power supply must be linear and made of discrete components.• The chassis should be fabricated from metal and wood.• Circuits should only consist of through-hole components. • Components must be readily available with long life cycles.• The user can program one instrument at a time.• There must be dials for adjusting tempo, tuning instruments, adjusting volume• Instrument outputs should be 2Vpp with minimal noise.• Required instruments include a bass drum, snare, closed hi-hat, open hi-hat,

ride, and crash.

Hardware DesignThe power supply consists of three rails: +12V, -12V, and +5V. The +/-12V rails are primarily used to power the dual rail op amps used on the instrument cards and the mixer board. To generate the +/-12V rails , a 33V center-tapped trans-former was used before passing the power through a rectifier and an LC filter. A darlington BJT paired with an op amp was employed for the voltage regulation. A second 6.3V transformer was used to step down mains voltage for use on the +5V rail, which used a design similar to the +/-12V rails for voltage regulation.

The sequencer primarily consisted of an ATmega328P which was wired to two shift register break-out boards which were mounted to the underside of the top interface panel. The first shift register breakout board is dedicated to receiv-ing the inputs signals from the 16 pushbuttons in addition to start/stop/, re-set, BPM, measure select, loop back, and instrument select controls. The sec-

ond shift register break-out board was dedicated to serially shifting out data related to 5 seven-segement displays and 16 time-step indicator leds.

The sound cards were standardized to 40 pin PATA cables which had enough bandwidth to carry all the necessary signals includig +/-12V, +5V, GND, volume controls, effects controls, trigger signals, etc. The basic structure of each instrument employed the trigger signal from the sequencer as an in-put to an envelope generator which contrlled a voltage controlled ampli-fier (VCA). Several tunable oscillators were used to generate the charac-teristic frequencies of the instruments and these signals were fed into the VCA. The output of the VCA would then pass through a filtering stage be-fore going to the mixer. Each instrument was tuned to output a 2Vpp sig-nal upon recieving the 1ms 5V pulse from the sequencer. Depending on the instrument it would also have additional potentiometers for effect controls.

The drum signals from the instrument cards are passed through 50kΩ logarithmic potentiometers for individual channel gain adjustment.To mix all the drum signals together, a summing amplifier was used with high impednace inputs so as not to degrade signal quality from the instru-ments which employed common collector output stages for low output impedance. A volume unit (VU) meter is connected to the output of the summing amplifier to give visual feedback concerning the master output amplitude which is also controlled by a 50kΩ log pot. Along with all the raw outputs from each of the sound cards, the master signal is sent to a 1/4 inch output jack that can be connected to an external amplifier and speaker.

The enclosure for all the circuit boards consisted of a wooden box made from 1/2” stained and finished oak with glued miter joints. The top and back of the box were made from 14 AWG galvanized steel water-jet for mount-ing the user interface controls. The two sheets are joined by a piano hinge and rivetes were used to secure the piano hinge to the back and top met-al panels. The back metal panel was then secured to the wood with wood screws for which pilot holes were pre-drilled to avoid cracking the wood

AcknowledgementsThank you to the course coordinator, Loren Wyard Scott, our client Jesse Acorn, our technical advisor, Graham Hornig, in addition to Steve Drake, Alan Lim, and the staff in the ECERF machine shop.

Michael Hohnstein | Andrew Schroeder | Koltin Kosik-Harvey | Subhashis Chakraborty Client: Jesse Acorn | Technical Advisor: Graham Hornig

Department of Electrical and Computer Engineering