God City Instruments
–
Murdock Plus V1.0 Build guide
1
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 Dis.
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 label
ed “
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 full
y 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.
