[sdiy] KA-THUMP! wah, wah, wah, wah, KA-THUMP!

[WeAreAs1 at aol.com]

Hello guys and gals,

The guitar player in my band is having an annoying problem with his Morley 
wah pedal, and I'm hoping that someone in here might have a clue as to a 
possible solution.  His pedal is the Morley Classic Wah.  You can download a PDF 
schematic of the pedal from Morley's site:   <A 
HREF="http://www.morleypedals.com/clwes.pdf">http://www.morleypedals.com/clwes.pdf</A>

In some (but not all) gig situations, the pedal makes a very loud popping 
sound when you switch it on or off.  This seems to happen especially if we are 
performing someplace where they run the guitar direct to the P.A. system, 
without using a guitar amp on stage.  I have not really noticed the problem when the 
guitarist plays through a guitar amp.  In these direct-to-P.A. situations, 
there is always a direct box that the guitarist plugs into.  It is almost always 
one of those mid-line passive direct boxes (Whirlwind brand or similar), 
which contain nothing other than an unbalanced in/balanced out impedance-matching 
transformer (usually also with a ground lift switch).  The signal path is thus:

Guitar (Carvin with active pickups and onboard preamp, internally powered by 
one 9 volt battery) > Morley Wah pedal > BOSS multi-effect unit (can't 
remember the model, ME-50 or something like that) > direct box > snake > PA mixer

We play in showrooms in some of the best casinos in Las Vegas, and the sound 
systems in these places are usually very well designed and well maintained.  
Most of these places have fairly modern Yamaha digital mixing boards.  The 
boards always provide phantom power on the inputs, but most of them do not allow 
you to enable/disable phantom power on an individual input channel basis.  
Rather, they will usually have a switch that enables phantom power for a group of 
channels (usually eight channels per group).  Therefore, depending on how the 
mixer input and snake channels are allocated to the various instruments and 
mics on stage, there may or may not be live phantom power on that particular 
snake channel.  

Somehow however, I have a feeling that problem might be more of an issue 
between the Morley and the BOSS effect unit than something that has to do with the 
house P.A. system, since the circuitry of the BOSS should effectively 
electrically isolate the Morley from the P.A. (I would also expect the transformers 
in the direct boxes to give some electrical isolation between the BOSS output 
and the mixer input, too -- at least in the case of blocking phantom power DC).

As you can see from the <A HREF="http://www.morleypedals.com/clwes.pdf">
schematic</A>, the wah pedal has a hard bypass switch (passive, non-electronic 
switching), but it is not a so-called "true bypass".  True bypass would take the 
pedal circuitry completely out of the signal path at both the input and output 
side.  This may or may not have something to do with the problem.  You will 
also note that the pedal is powered by a single 9 volt battery, and uses the 
typical resistor divider (R3 and R4) to simulate the negative rail and provide a 
"virtual ground" for the TLO71 opamp.  It does, however, have coupling 
capacitors at both the input and output of the wah circuit (C2 and C7) -- you'd think 
that these caps would effectively block any DC in the signal path, right?

I have heard of this thing happening with other pedals (such as old passively 
switched MXR and Electro Harmonix pedals), and I have also heard that there 
may be a very simple fix for the problem.  Maybe it was something like tying a 
large-value resistor (1 Meg or thereabouts) from the final output line to the 
signal ground?  Does anybody know anything about this or have any other useful 
suggestions?  We're tired of hearing a big "KA-THUMP!!!!" every time the 
guitarist wants to play a little bit of funky wah wah stuff!

Michael Bacich

P.S. -- There are lot of cool schematics at the Morley site.  Many of them 
show some interesting applications of photoresistors for optical switching and 
"potless" optical control circuits.  Let the flood of SDIY downloading begin...