The Wild Mouse

I described an earlier version of this pedal in Electronics Now magazine some years ago. At the time, a lot of basic stompbox components were still either unreasonably expensive, or not easily available. My problems with building the the WM as I wanted to were part of my motivation then for building a business selling parts. Now that I've done that (!) I've been able to go back and build the pedal that I had intended to.

I first started fiddling with the circuit back in the 70s. It's a tunable active tone boost; if you like playing through a wah pedal that is "cocked" in mid-travel, the WM nails those sounds over a wide tonal range. With it, you can duplicate a number of classic licks (the Beatles "Paperback Writer" comes to mind), and it is also excellent as an driver for a Fuzz Face or other distortion.

Here are a few sound clips. The first one is the unmodified sound (bridge pickup of my Guild Bluesbird.) The others are of the same lick with the effect on, with progressively lower capacitor values in the tank circuit (the Range switch).

   Flat        Range Pos 1        Range Pos 3        Range Pos 6

There are a couple of things going on here that make the WM sound not quite like anything else. Some of the edgy distortion that accompanies each note results because you are playing through a resonant circuit. The closer to the resonant frequency you get, the greater the bite. Some distortion also comes from overdriving the op-amp inputs. The Gain pot controls the distortion level, and the Peak pot sets the sharpness of the filter. While the original WM incorporated trimpots that I set to clamp the gain into a "normal" range, the circuit can easily be driven into some insane oscillation.  I don't particularly like this kind of over-the-top weirdness, but some people clearly do, so I purposely made the trimpots in the original into panel controls with a wide sweep.

How It Works

Here's the schematic:

The two stages of IC1 are connected as regular inverting amplifiers, but with positive feedback at a level set by R14. R10 sets the gain of the first stage, and the tank circuit formed by L1 and a capacitor chosen by switch S1 add the funky resonant peak to the output taken from IC1-a.

S2 is a standard DPDT alternate-action stomp switch. It provides true bypass with control of the in-use indicator LED using what is commonly called the "RAT bypass" circuit. When S3 is in bypass mode, level control R15 grounds the base of Q1, a darlington transistor, and so cuts it off.


How To Build It

The number of components and the tooling requirements make the Wild Mouse an intermediate-level project. Before trying this one, you should have built at least one pedal that required tooling of a metal case and working with perfboard or Veroboard.

Here is a look inside the case:

Begin by following my instructions for building a Shell, but use this drilling template for the 4-knob version. You want to create the tiny pilot hole for the stomp switch, then holes for the pots, rotary switch, LED bezel, input and output jacks and power jack. While it is optional, I did create the depressions for the anti-rotation tabs of all of the pots, and the rotary switch.

Note: The enclosure shown here is a pre-powder-coated white. If you start with an unpainted box, don't do any painting or decorating yet. If you are working with a pre-coated box, make sure that you grind or sand away the color around the holes for the input and output jacks. When you have finished drilling the holes and grinding out any molding artifacts (step 7 in building the shell), temporarily mount all the parts except the LED bezel. The result should look like this:

The next step is to tool the blank perfboard, create cutouts for the jacks and LED bezel, and establish the locations for the mounting studs. The layout drawing and the detail photos show the areas that need to be cut away. Mark these carefully with a Sharpie or similar marker.

Use any combination you want of cutting, scoring or grinding to create the cutouts. (IMORTANT NOTE: Machining epoxy-glass circuit board creates fiberglass dust, which is noxious stuff! When doing any tooling of the board, wear disposable gloves and a face mask!)

After you have done the cuts, bore two 1/8" holes at the left and right top corners for  mounting studs. Also bore 1/8" holes at indexes R25 and J25 for the tabs of the transformer (if you are using that as inductor L1.) Here's a pic of the result:

I added later a 1/8" hole in column 29 between rows C and D to route the leads from the LED assembly. You can see this at the lower left in the detail pic above.

Carefully lay the perfboard in place on the floor of the enclosure. The side with the cutout for the battery should butt against the edge of the enclosure, and the cutouts for the jacks should be clear of the paths of the flanges. Gently hold the board in this position, and mark the position of the LED bezel with a Sharpie:

Grind the cutout for the bezel. Place the board back in position on the floor of the case, and line it up carefully as before. Hold it gently in place with one hand. With your other hand, use a #59 or #60 drill and bore from the top of the case through the pilot hole that you created earlier. (Make sure that the hand holding the board isn't in the path of the drill!) This may create a new hole in the board, or the drill may go through an existing hole. Either way, you now know the starting point for creating the cutout for the stomp switch (or the tactile switch if you are doing Bypass-On -Board.) Mark this point with a Sharpie.

Temporarily install screws and studs in the mounting holes of the board. Note: 1/4" studs are fine if you will be using a standard stomp switch; 3/8" are required for bypass-on-board. Re-position the board, sticking a drill bit through to line up the pilot holes in the perfboard  and the case. Make sure that the flanges of the jacks have room to move freely when you insert a plug. The result should look like this:

You are now at step 12 in the process of building the shell, and you are ready to choose whether you are building with a stomp switch or going bypass-on-board.

If you do bypass-on-board, the general method described in the article on the shell applies. However, the index for the pilot hole on the board will be in the area of O15. The mounting holes for the tactile switch don't come down exactly on indexes, so I positoned the switch temporarily and marked the points to drill with a Sharpie:

 Drill the mounting holes, and continue with installing the switch.

If you use a DPDT or 3PDT stomper, here are detail layouts for making the cutout:

These drawings presume that your pilot hole was at index O15. You may have to slightly adjust the area you tool if the pilot hole didn't come out exactly on the mark.

Steps 19 through 24 cover installing the mounting studs and the battery clip. Glue the studs in place and reinforce with more epoxy in the usual way.

Now disassemble, and paint if you need to or just do your decals. The shell is finished, and you are ready to stuff and wire the board. Here's the layout, and a parts list:


Some suggestions and notes:

The capacitors in the tank circuit, C3-C8, can be any decent poly films.  I used Panasonic ECQ-Bs, values of .0047 mf., .01 mf., .022 mf., .033 mf., .047 mf. and .068 mf. The higher values produce a more "throaty" or "muffed" tone, something like a muted horn at some settings.

R21 should be a minimum of 100 meg to avoid switch pop. I got this by putting five 22 meg resistors in series, though 100 meg resistors are sometimes available as surplus. I installed them radially, soldering the bottoms in place first, and only later connected them when doing the wiring. R3 thru R8 don't eliminate the pop from switching the capacitors, but they keep it tolerable and don't have any effect that I can hear on the tone.

L1 can be a wah inductor, but the primary of a common audio transformer has about the right inductance, works nicely and is very cheap. The original WM used the Radio Shack p/n 273-1380, and I believe that's still available in RS stores. The redesign uses Mouser p/n 42TM013, and it is available on my Stock List.

To help ensure that a build works the first time I power it up, I use a yellow highlighter to mark a connection on the layout drawing as I make it, and then test the connection for continuity:

Here's the stuffed and wired board, installed in the case and ready for wiring to the external components:

The pic at the beginning shows the wired internals. Note that I made the connections to the rotary switch in the same way that I made the connections to the tactile switch: I cut to length sections of single-in-line socket material and soldered leads to the pins to make convenient plugs.


Setup and Testing

Connect a battery or power supply and plug your instrument into J1. Start with the Peak and Gain controls and the Range switch fully counterclockwise, and the level control about half way up. You should see activity from the in-use LED and the boost should be clearly there. If it doesn't kick in after you click the stomp switch, there's an error somewhere; see the troubleshooting tips. If you hear a clear, undistorted treble boost, CONGRATULATIONS!

Using It

With the initial settings, as I noted, you have a useful treble boost. Now run the Range switch through its positions, and you should hear progressively more "throaty" tonalities added to whatever you are playing. If there are any errors in the wiring of this section, they'll show up now. If the Range switch works in all positions, try each one again and vary the Peak and Gain pots at each position to get an idea of how they interact and what tones you might find useful; I find that the pots have a more pronounced effect with higher value caps in the tank--YMMV.


Troubleshooting

If you get no sound at all, first make sure that the chip is getting powered (about 8.75 volts at pin 8). Also make sure that Vref is about half the supply voltage, and that it is getting to the points where it is supposed to be. If the power is OK, use your amp as an audio probe to see if maybe you have made mistake in the wiring to the external components. If you are still stuck, go back to the technique of testing each joint and trace and marking off connections--it works! I built the model you see here from the posted drawings, and I am confident that they are correct. However, if you do come across an error, please let me know so that I can help other builders.


Mods and Substitutions

Obviously, the values of the capacitors in the tank circuit can be varied to taste. Beyond that, some people will want to be able to control the "wildness" externally. I chose to leave my prototype as you see it, but I have done a few experiments and will suggest a route to pursue.

If you put the LDR-side of a high-resistance photocoupler in parallel with the Peak pot, it will be essentially out of the circuit unless power is applied to its associated LED. You can then control the peaking externally, either using a potentiometer and powering the LED from the Wild Mouse's power source, or driving the LED directly from an external control voltage. Here is the idea:

I used a Silonex NSL-32 in my tests, so other similar devices can also be expected to work. To make the external connection, I'd suggest adding an enclosed (Switchcraft #111) 1/4" jack above the Level pot:

If you wire as shown in the schem, the pot is ground referenced, so a simple open-circuit jack is all that's needed.

I hope that you enjoyed building the Wild Mouse, and that it becomes part of your regular lineup of effects. Please direct comments and questions to smallbearelec@ix.netcom.com.