The Quad Adjustable Tom is minimalistic percussion module that consists of 4 identical analog electronic toms. The main feature of this module is that it is very compact unit built around one Quad Opamp. It is supposed that pitch adjustment is rare operation which may be done with on-board trimmer potentiometers while the front panel has only gate inputs and audio outputs which consume only 4hp of space. Such modules can be joined to kind of compact percussion cluster. The sound of this module is similar to analogue Boss rhythm-boxes.
adjustable_tom_test1
The sound example contains all four toms played in serie. There are dry at the begin and then some reverb effects are presented.

How it works

The incoming Gate pulse is filtered with R1C1. Then, additional filter C2R2, takes out the direct current. Then diode D1 cuts out negative pulse. When pulse is being fed to base of Q1 transistor, it closes and current from R3 goes through the voltage divider R4R5 to the input of OpAmp TL064. The feedback is processed with High-Q bandpass filter (Bridged T-Network). The trimmer R7 adjusts frequency of this filter and R8 limits high bound of available frequency range. The output from the OpAmp goes to filter R9C5, taking out undesirable high frequencies and remains of the negative pulse taking place when Gate is closed. Capacitors C101—C104 filters supply voltage and used once.

3rd version of the module works with positive Gate signal. The signal power on different filter frequency is equal but low-pitched tom stays longer and high-pitched tom is shorter. Because of that, the volume is also different.

Schematics

The one of four channels is shown above. Complete schematics (pdf)

Stripboard

Stripboard design is made with VeeCAD. The designators of some components is messed with same designators from other parts but it is not critical because the values is right. You can design your own PCB using SPICE Netlist. I recommend to use PCB-mounted Minijack sockets like pictured below. Note that space between sockets is smaller then standard space used in OEM Doepfer 8-hole panels. You need to drill holes yourself.

Bill of Materials

Designator    Type, value                   Amount
R1, R2        Resistor, 100K, 1/4W, 5%        2
R3            Resistor, 1K, 1/4W, 5%          1
R4, R9        Resistor, 10K, 1/4W, 5%         2
R5            Resistor, 100R, 1/4W, 5%        1
R6            Resistor, 10M, 1/4W, 5%          1
R7            Trimming Resistor, 10K          1
R8            Resistor, 680R, 1/4W, 5%        1
C1            Film capacitor, 470n            1
C2            Film capacitor, 220n            1
C3, C4        Film capacitor, 6,8n            1
C5            Film capacitor, 22n             1
Q1            Transistor, 2n3904, TO92        1
U1            OpAmp TL064, DIP14              1
C101, C102    Ceramic Capacitor, 100n         1
C103, C104    Electrolytic capacitor, 10u     1

This circuit is published under Creative Commons «Attribution-NonCommercial-NoDerivs» 3.0 license.
Any use beyond this license must be discussed with author.