GOD CITY INSTRUMENTS MURDOCK PLUS User manual

God City Instruments –Murdock Plus V1.0 Build guide
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GOD CITY INSTRUMENTS –MURDOCK PLUS V1.0 Build guide
The God City Instruments (GCI) Murdock Plus is an expanded version of the GCI Murdock. Both are tone
shaping pedals derived from the EQ section of a Boss HM-2, but the Murdock Plus adds an output volume
control as well an op amp gain stage with asymmetrical hard clipping inspired by the MXR Distortion+.
The original HM-2 pedal has several gain and clipping stages followed by an EQ section. Despite having only
two EQ controls, it is actually a 3 band gyrator EQ with the high and mid bands voiced fairly close together and
controlled by a single pot. It is arguably this EQ, more than the gain stages, that create the classic HM-2
sound. Murdock Plus’ overdrive stage is simpler and more transparent than that of an HM-2, yet it’s EQ is
capable of even more depth, giving the user new variations on the “old favorite”sound.
This PCB circuit has the original HM-2 “color mix” controls fixed at maximum, however, the amount of EQ can
still be controlled by a single pot labeled “EQ”which controls the boost of all 3 gyrators. When the pot is full
CCW, the signal is not EQ’ed, when the pot is full CW, the signal is fully EQ’ed. Adjust the pot for the desired
amount of HM-2 EQ tone. This EQ can boost more than the original HM-2, so when comparing to other HM-2
circuits, try setting it at about 2 o’clock.
With this type of EQ topology, each band of EQ’s frequency and bandwidth can not be adjusted independently
from each other. However, if more tonal adjustability is desired, R12, R13, and R18 can be replaced by 500
ohm pots and R6, R7, and R8 can be replaced by 100k pots inline with 47k resistors. By adjusting these pots in
pairs, the desired frequency response and bandwidth can be achieved. Check out the AMZ gyrator calculator
to see how changing values affects frequency and bandwidth. This calculator was helpful during the design
phase of this circuit because using stock values produced a tone brighter than the original HM-2. The “high”
gyrator has been adjusted to darken the circuit. Stock HM-2 values are listed in the substitution column of the
BOM.
http://www.muzique.com/lab/gyrator.htm
Headroom is extremely important the achieve the full depth of EQ possible with this circuit. A loud distortion
or boost pedal preceding the EQ section can cause op amp clipping, reducing the apparent effectiveness of the
EQ. TLE2074 is a rail to rail op amp with more headroom than a TL074, but it’s also quite expensive.
Alternately, more headroom can be achieved by adjusting the voltage divider formed by R15 and R16. Think
of these two resistors as a single pot, with the output of the pot being the node between them. If reducing
gain at this stage causes the pedal to be quiet, the gain can be made up by adjusting R17 and R11 which set
the gain of the op amp boost at the output. Be wary that increasing headroom can also increase noise floor.
Finally, the hard clipping diodes, D1, D2, and D3 offer a certain amount of gain protection. Removing theses
diodes or replacing them with higher threshold diodes may result in less headroom. Adjust R15 and R16
accordingly.
This PCB can be build to the same spec as the original Murdock by making the following changes:
•To make the input stage a unity gain buffer, omit C1, R1, and “Drive” pot. Jump pins 1 and 3 of “Drive”
pot.
•To remove hard clipping, omit D1, D2, D3 and C12, jump R9 and C11. (Note: Hard clipping may still be
useful even when configuring this as strictly an EQ circuit as it will offer the EQ op amps some
protection against loud signals.)
•To remove output volume control, jump pins 2 and 3 and insert a 100k resistor between pins 1 and 2.

God City Instruments –Murdock Plus V1.0 Build guide
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This pedal is an easy build, but this guide is intended for people who have some experience building pedals.
Component sourcing, component identification, assembly techniques, wiring stomp switches, etc. is not
covered. The GCI Brutalist Jr. assembly guide has helpful information for less experienced builders. That guide
can be found here:
http://www.kurtballou.com/brutalistjr/
Available separately is the GCI 3PDT utility PCB for PCB pin 3PDT footswitches. This PCB makes footswitch
wiring quick and easy. Not compatible with solder lug style switches.
Don’t forget to connect the ground pad of the PCB to the ground lug of the input, output, and DC power jacks!
Due to the scope of this project, technical support is not available. However, consider joining the GCI DIY PCB
Builders group on Facebook to get advice from and share your work with other builders. We require that all
group members agree to the rules before being accepted into the group.
https://www.facebook.com/groups/2454786551255317/
Component values for the PCB as well as some alternate values are listed below. This is a BOM for the PCB
only. Resistors and diodes are 6.3mm leg spacing, film and ceramic capacitors are 5.08mm leg spacing, and
electrolytic capacitors are 2.54mm leg spacing. I/O jacks, DC jack, switch, enclosure, and knobs are not listed.
The schematic and a drill template for a 125B (1590N1) sized enclosure are also attached.

God City Instruments –Murdock Plus V1.0 Build guide
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Part
Value
Description
Substitute
Substitution Notes
C1
0.1u
Film cap
0.22u
Affects overdrive low end
C2
47n
Film cap
22n-100n
Input cap
C3
2.2n
Film cap
1n-4.7n
Input LPF
C4
270p
MLCC
100p-470p
System LPF. Bigger = darker
C5
1.5u
Film cap
do not substitute
critical for EQ bass frequency
C6
0.15u
Film cap
do not substitute
critical for EQ mid frequency
C7
6.8n
Film cap
C8
68n
Film cap
C9
1u
Film cap
C10
4.7n
Film cap
C11
0.1u
Film cap
C12
270p
MLCC
0-560p
LPF for clipping diodes. Bigger = darker
C13
0.1u
Film cap
C14
560p
MLCC
330p-1n
LPF for EQ stage. Bigger = darker
C15
0.15u
Film cap
0.1u
Stock HM-2
C16
100u
Electrolytic cap
47u-220u
Power filtering
C17
100u
Electrolytic cap
47u-220u
Power filtering
C18
0.1u
MLCC
LED
LED3MM
status LED
IC1
TL072P
Dual Op Amp
LM833, RC4558, MC1458
Any pin compatible dual op amp
IC2
TLE2074
Quad Op Amp
TL074, TLC2274
Any pin compatible quad op amp
D1
1n4148
small signal diode
1n34a, 1n914, 1n456a, 1n5818
Any diode good for hard clipping
D2
1n4149
small signal diode
1n34a, 1n914, 1n456a, 1n5819
Any diode good for hard clipping
D3
1n4150
small signal diode
1n34a, 1n914, 1n456a, 1n5820
Any diode good for hard clipping
D4
1N5818
Schottky diode
1n4001, bat41
Any suitable protection diode for 9v
CLR
4.7k
1/4 watt resistor
1k-10k
Current limiting resistor for LED
R1
2.7k
1/4 watt resistor
R2
1M
1/4 watt resistor
2.2M
Pull down resistor
R3
10k
1/4 watt resistor
1k-33k
Forms input LPF with C3
R4
10k
1/4 watt resistor
R5
10k
1/4 watt resistor
R6
82k
1/4 watt resistor
R7
100k
1/4 watt resistor
R8
120k
1/4 watt resistor
100k
Stock HM-2
R9
1k
1/4 watt resistor
1k-22k
Affects amount of hard clipping, less is more
R10
1M
1/4 watt resistor
470k-2.2M
Bias resistor
R11
47k
1/4 watt resistor
10k-100k
Affects gain of output stage. More is more
R12
330R
1/4 watt resistor
R13
330R
1/4 watt resistor
R14
1k
1/4 watt resistor
R15
10k
1/4 watt resistor
1k-47k
Forms voltage divider with R16. Affects headroom
R16
47k
1/4 watt resistor
47k-100k
Forms voltage divider with R15. Affects headroom
R17
10k
1/4 watt resistor
4.7k-47k
Affects gain of output stage. Less is more
R18
390R
1/4 watt resistor
330R
Stock HM-2
DRIVE
A250K
16mm potentiometer
A100k-500k
Affects gain range of first stage. More is more.
EQ
A5K
16mm potentiometer
B5k
EQ blend pot.
OUTPUT
A100K
16mm potentiometer
A50k
Output volume
S
PAD
Send to PCB
L+
PAD
LED+
L-
PAD
LED-
R
PAD
Return from PCB
V
PAD
9v input
G
PAD
Ground

God City Instruments –Murdock Plus V1.0 Build guide
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God City Instruments –Murdock Plus V1.0 Build guide
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0
0.829
1.929
3.828
4.803
0 0.559 1.309 2.059 2.618
0
0.454
0.914
1.398
0 0.772 1.309 1.847 2.618