God City Instruments – Murdock V1.1 Build guide
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GOD CITY INSTRUMENTS – MURDOCK V1.1 Build guide
The God City Instruments (GCI) Murdock is a tone shaping pedal derived from the EQ section of a Boss HM-‐2. 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 allows the user to impart an HM-‐2 flavor to any other effects pedal by
offering an EQ only version of that circuit. This PCB can be built as a stand-‐alone pedal or installed, space permitting, in existing
pedals to modify them for HM-‐2 EQ. Try it with a GCI Brutalist Jr.!
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. For traditional HM-‐2 signal flow, insert the
EQ after all gain and clipping stages.
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, R7, R8, and R15 can be replaced by 500 ohm pots and R9, R10, and R16 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.
Headroom is extremely important the achieve the full depth of EQ possible with this circuit. A loud distortion or boost pedal
preceding Murdock 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 changing the voltage
divider formed by R5 and R6. 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 R4 and R14 which set the gain of the
op amp boost at the output. Be wary that increasing headroom can also increase noise floor.
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/
For your convenience, complete parts kits including everything you need except the PCB can be purchased through Small Bear
Electronics. Be wary that they may need to make substitutions for work-‐alike components which may or may not influence the tone
of the pedal. Be aware that transistors may have been substituted for work-‐alikes with different pinouts.
http://smallbear-‐electronics.mybigcommerce.com/kit-‐murdock-‐pcb-‐not-‐included/
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. A drill template is not attached, as this PCB is designed for a
variety of installations. When built as a stand-‐alone pedal, the minimum enclosure size is Hammond 1590B.
