Barton Musical Circuits BMC018.Analog Drum User manual

BMC018. Analog Drum.
Last updated 12-27-2020
I Features
II Schematics
A.Master Schematic.
B.Input/Decay
C.VC
D.VCA
E.Power Connections.
III Construction
A.Parts List
B.The Board

I. Features
This module is an analog drum sound. It was designed to be built using a minimum of parts
and no difficult to obtain parts. I initially was only trying to build a "disco tom" type sound but I'm
more impressed with it's bass-drum like sounds. It has the following controls:
1.Amplitude Decay
2.Frequency Modulation Decay
3.Depth of Frequency Decay
4.Baseline frequency
5.External Frequency CV Depth
It has just two inputs one for a trigger and one for external CV.
II. Schematics.
On the next page is the master schematic. There are highlighted sections of the master
schematic that indicate what sub-circuit of the module the parts correspond to these sub-circuits are
then described on the following pages.

INPUT/DECAY
On the far left we see the wirepad
marked "IN." A trigger or gate signal should be
input here. The .01uf capacitor and 100K
resistor to ground form a pulse shortener
making the pulse length of the input signal
irrelevant. This is then input to an op-amp
wired as a comparator. The output of the
comparator goes to the two decay sections
through a pair of 1n4148 diodes.
When the output goes high current
flows through these diodes quickly charging the
1uf capacitors. The 220 ohm resistors and
500KA pots in parallel with the capacitors
provide a path for the capacitors to discharge
the voltage. The higher the resistance the more
slowly the capacitors discharge.
Each capacitor is also connected to an
op-amp wired as a buffer which is outputing the
capacitor's voltage onto the next stage as well as lighting up an LED indicator. The output of the
Frequency Decay section goes to the VCO and the output of the Amplitude Decay section goes to
the VCA.
VC
On the far left we see the input from the decay circuit. This voltage is summed with the
voltages from the external frequency control and the baseline frequency control. These voltages are
summed together on the negative input of the op amp wired in conjunction with a 2N3906 to form a
linear voltage controlled current source.
This current source is controlling the current of one half of an LM13700 OTA wired as a
VCO. The design for the VCO is taken from the LM13700 datasheet. The OTA is forming an
Integrator/Schmitt Trigger type oscillator by itself.
IF BUILDING F R +/-15V replace the 10K resistor in the feedback path of the OTA with
a 15K. This is untested if it works for you or you like it more with a different value let me know!

VCA
On the far left we see the output of the decay section for amplitude. A single 2n3906 in series with
a 10k resistor forms the current control. This VCA is incredibly simple the signal is input to the
inverting input through a 22K and 220 ohm resistor voltage divider used to limit the signal on the
input. The output of the VCA goes through the onboard buffer and then a 10uf capacitor to
decouple the DC bias.
Power Connections.
Here we see the two power connecters for MOTM
and Eurorack style systems. The supply is filtered
by a 10 ohm resistor and 10uf capacitor and then .
01uf decoupling capacitors are placed near the
power supply pins of the two ICs.

Parts List
Semiconductors
Value Qty Notes
TL074 1
LM13700 1 or 13600
2N3906 2
1N4148 5
LED 2 3mm
Resistors
Value Qty Notes
10 ohm 2 7.5mm lead spacing
220 ohm 4 " "
470 ohm 2 " "
1K ohm 3 " "
4.7K 2 " "
10K 3 " "
22K 1 " "
33K 1 " "
100K 6 " "
500KA
Potentiometer
2 16mm pot PCB mounted
100KB
Potentiometer
3 " "
Capacitors
Value Qty Notes
.01uf 5 cheap ceramic 2.54mm
0.022uf 1 Poly 5mm lead spacing
1uf 2 Electrolytic
10uf 3 " "
ther
Value Qty Notes
Power
Connecter
1 Either Eurorack or MOTM
14pin DIP
Socket
1
16pin DIP
socket
1

Jacks 3 Either 1/4" or 1/8"
Knobs 5
The Board
The PCB is 100mm x 41mm. The pots are spaced 21mm apart. The mounting holes are spaced
91mm x 29mm.
I've highlighted a few key resistors that a builder may want to modify.
The yellow resistors control the brightness of the LEDs. 1K is appropriate for normal Red Green
or Yellow LEDs. For high effeciency LEDs of these colors or blue/white LEDs try changing to
10K.
The red resistors control the minimum decay time. To increase the minimum time increase the
value of the resistor. If you have a problem where when turning one of the decay pot times down
all the way where the decay controlled by the other pot is also shortened you should increase the
value of these resistors.
The blue resistor controls the minimum frequency on the frequency control pot. Increase it to
increase the minimum frequency.
The green resistor is the 10K that should be replaced with a 15K for +/-15V
WIRING
The PCB has four wirepads they should be connected as follows:
IN → Tip of the Trigger/Gate Input Jack
CV → Tip of the CV Input Jack
OUT → Tip of the Output jack and optionally the switch of the CV jack.
GND → The sleeve of any one jack if using a metal panel. If using a non-conductive panel then
connect the sleeves of the jacks together.

Below is an image of the PCB with traces included to help when troubleshooting. The ground plane
is not depicted so assume pads with no connection are connected to ground:
Installing LEDs Sideways
The PCB indicates that the LEDs should be mounted parallel to the board do not do this.
Leds should be pointing in the same direction as the pots. The leads of the LED should be bent at a
90 degree angle the easiest way to install them is in four steps:
1. Place the LED on the edge of the board facing out with it's leads going over it's pads on the PCB.
Make sure the bottom lip of the LED is flush with the board.
2.Clip the leads 2 or 3 mm past the pads on the PCB.
3.Bend the LED leads 90 degrees 2 or 3mm from the edge.
4.The LED should pop into place easily.