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

[harrybissell]

I agree... the first suspicion would be a DC level shift from the 'true bypass'.  The Korg

might not have a pull down resistor on the input... or the guitar amp not have one on the
output.   I'd put a 1M on the input to always load the guitar, and one on the
output jack to always load the next device.

Other than that there should be no problem with the Morley, unless there is a flaky
connection
causing a sudden DC level shift.  The photocell tuning should elminate most chance of the
circuit being bad... but a broken solder joint could be another issue.

H^) harry

WeAreAs1 at aol.com wrote:

> 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...