diy- rackwah



diy- rackwah
Are you one those guitar players that always preferred the sound of an analogue wah wah
device and did you spend hours trying to find a mid controllable device, mountable to an
effect rack?
Well, I did, too... But there were only units out there that needed specific controller pedals
– none of them was willing to fit in a currently existing and well-working guitar rig so
there was only one chance to get such thing – by doing it myself.
This paper was written to give all those wah-seekers a chance to build such nice effect unit
for themselves. Unfortunately some advances skills are needed so it's not a project for
Let's face the basic features of the final device:
The wah effect can be controlled by so called Midi Control Messages which are usually sent
by a midi foot controller such as the Roland FC200 or the Behringer FCB1010. The
controller number of course is programmable such as the threshold for the so called Auto
Engage function. As you know midi pedals do not have any kind of on/off switch for the
effect so the idea is that the effect will be switched on every time the pedal leaves its
maximum position. The effect will be turned off when the pedal is pushed back to the
initial position.
As the wah is to be placed in front of the guitar amplifier we need a bypass function that
will not affect the signal when the effect is inactive. There is only one solution here: true
bypass accomplished by using a relay (or more of them). As all relays will bounce more or
less switching noise will later have to be minimized. Therefore a vactrol will be used.
The wah itself is based on a famous wah circuit from Ibanez, the WH10. This op amp
driven circuit suited my tastes best although some other circuits were tested. It also has the
advantage of a programmable depth. Talking about programming and controlling the
effect: The two effect's parameters (current wah position and effect depth) are controlled
via LEDs illuminating photo resistors and will be programmable, too.
1. The chassis
There are many different chassis available, I really recommend using a high quality chassis
for this project, such as made by Palmer and available at Adam Hall or Thomann (item no.
143108, 50€).
Building the chassis not too hard. Just use one of the templates found on the website
(file: templates.pdf) and send it to your printer (send as is, disable automatic scaling or page
fitting). Each template (one for the front plate and one for the backplate) is separated to
two parts (they won't fit to a single sheet otherwise) which have to be cut out and then
stuck together using some glue or adhesive tape.
Then the left side will have to be geared towards the left side of the blank plate and fixed.
When done all wholes can be center punched and drilled. The hole's drill sizes are shown
on the template as well. Here are the drill sizes used for the prototype. Ensure to use the
correct sizes for your parts!
Drill sizes:
DC jack :
MIDI (mounting hole):
IN/OUT jack:
13.0 mm
15.0 mm
03.0 mm
11.0 mm
Pushbutton (Front plate):
Mounting hole (left side)
Mounting holes (lcd frame)
07.0 mm
03.0 mm
02.0 mm
After having finished drilling it's time for the paint job and labeling. The plates must be
clean and free from grease, ethyl alcohol does a good job here. Then use a (light) color of
choice and – after about one day left for drying - apply the labels using waterslide foil. To
save some costs there's another file that contains only the needed foils (waterslide.pdf).
Leave enough time for drying (min. 1 day!). Then apply some transparent finish to protect
the surface. It is recommended to start with a very thin (almost non-noticeable) covering of
the plate and speed up drying by using standard hair dryer. Using too much coverage
might dissolve the waterslide so be very, very patient and only apply a very small amount
of color. When the plate is completely covered for the first time (this may take 50+ passes!)
apply 2-3 final coatings with more color.
Take a on-day rest or start etching / populating the pcbs (see below).
After having left enough time for drying mount the DC-connector, the input jacks and the
midi jacks (using M3x10 screws and self-locking nuts).
It is absolutely recommended to use isolated jacks here. Otherwise the auto gnd lift circuit
will not work and might lead to a ground loop causing unwanted hum!
Don't mount anything to the front plate at this time. Just make sure that the cutout for the
LCD display exactly reaches to the dashed line.
this line shows the lcd cutout!
Fig. 1: The front plate's waterslide
Then drill a 4 mm hole to the chassis (1.5 cm from the left
side, 4 cm from the back side). Mount a 4x16 mm ground
connection screw here. First build a contact bolt like this:
screw – contact washer – bottom plate – contact washer –
nut. Then mount the eyelet, another washer a nut and
finally a self locking nut.
2. The boards
Fig. 2: pe connector
Start with the front panel pcb. After edging drill the holes
for the vias and the lcd display (0.8 mm) and the holes for the connectors (1 mm). The
switches need 2 mm holes each, finally drill the mounting hole on the left sided (3 mm).
Then continue with soldering some wire to the vias (don't forget the via right to the LCD
connector.)and the lcd display (mind the pin numbers!). The connectors must be solderes
to the bottom side of the board, the thick line marks the connectors high end.
Mount the pushbuttons but do NOT solder them to the board right now as they will be
adjusted correctly when mounting the board to the front plate.
Fig. 3: front panel pcb (top side)
Finally, mount a 10 mm spacer in combination with some 1 mm washer (the lcd's height is
exactly 11 mm). Use a M3x6 screw.
Fig. 4: front pcb (bottom)
Then go on with the wah pcb. After having edged the board drill all holes to 0.8 mm and
then up to the drill sizes shown here:
PCB Connector:
1.0 mm
Wire pads (signal):
1.0 mm
PCB Pots:
1.5 mm
1.5 mm
1.5 mm
Mounting holes:
4.0 mm
Below is the schematics of the wah pcb. It is
easy to see that it is based on the old Ibanez
WH10 with “optomized” controllers. As this is
a one sided pcb with some wires to be placed
on the top side – start with these wires, then
populate the board small parts first. The op
amplifier should be mounted to a socked, all
other parts are soldered directly to the board.
At last solder two shielded wires of about
10 cm to the pads marked as IN/IN_GND and
OUT/OUT_GND and solder a wire of about
10 cm to the pad named CHASSIS_GND.
Note: Some pads (GND, 9V, EXT, D_GND)
will be left unconnected. The wires of LED2
will have to be bended under the LED
otherwise it will not fit to its place. Turn the
pots (TRIM and VOLUME) up to about 50%.
Fig. 6: schematics of the wah pcb
Then move over to the supply pcb. Here
only IC101 should be mounted using a
socked. After having placed the parts
and having soldered them to the board
take the pcb connectors wired ends and
solder them as shown to the board. W1
to W10 are a 10-pin connector leading to
the wah board. It's length should be
about 25 cm – don't cut it, just use it as it
comes out of the box. L1 to L10 are the
wires from the front panel's pcb and E1
to E3 will take the 3-pin connector of the
front panel's switches. EGN is left open.
These wires are also not to be shortened.
Then solder two wires of 15 cm to the
supply pins PWR1/2 and insert a 400mA
fuse. Solder a shielded wire of 45 cm
with 4 leads to the I4/5 and T2/4/5. These
wires will later be connected to the MIDI
jacks. I4 stands for MIDI IN pin 4, T5
marks MIDI THROUGH pin 5. You might notice that the MIDI in jack has only two pins
connected while the through jack additionally gets a ground connection (T2).
Fig. 8: supply pcb
Fig. 9: supply pcb schematics
3. Preparing the 1st test run
A first test run will be done without the
chassis but before doing this the
pushbuttons will have to be soldered to
their final position. Mount the front
panel's pcb to the panel but don't fasten
the screw (you will unmount it again in
a few moments) and solder the switches
to the board in a position where they
exactly reach the front panel. Then
remove the board from the plate. Put
away the chassis and connect the supply
pcb to the lcd pcb (look twice, don't use
the connector of the wah pcb!!!). Solder
the DC jack to it's wires (you'll have to
unsolder it again after testing) or apply
approx. 12V DC directly to the wires.
The LCD's backlight should come up
and you should see the startup message.
An adjustment of the contrast pot
(located on the supply pcb) might be
needed. Test the correct function of the buttons, UP and DOWN should change the
program number. Press select on the current program to enter the program's parameter
setup, move to the last parameter (EXIT) by pressing the UP button, press the SELECT
button to return to the program change mode.
4. Mounting the boards to the chassis
Arrange the boards as shown below. Ensure that there is enough space between the effect's
board on the left side and the controlling board on the right side. The midi cable will be
put such that it can not affect the audio signal (yes, midi message are audible). The pcb
holders can be sticked to the bottom plate and the cables should be bond together get it a
bit tidy.
Solder the cables to the jacks and apply the gnd wire of the midi cable to pin 2 (in the
middle) of the midi through jack. Use a heat shrinking tube to isolate the blank wires.
Don't forget the ground of the chassis (see the black wire on the wah pcb).
Now plug a midi controller, ensure that it is sending on the current program's controller
number using the correct midi channel (default ctrl. number is 7, channel 1). The display
Fig. 11: Almost finished...
should show the effect's on/off status when moving the pedal back and forward.
That's it... Time for a playing. Now cover the leds and ldrs with a dark cloth or something
similar. Adjust the effect volume with P3. You can adjust the wah characteristics with P2. It
will limit the led current and thus can set the maximum wah position.
That's it... have fun...